path: root/doc/rfc/rfc9580.txt

Internet Engineering Task Force (IETF)                   P. Wouters, Ed.
Request for Comments: 9580                                         Aiven
Obsoletes: 4880, 5581, 6637                                   D. Huigens
Category: Standards Track                                      Proton AG
ISSN: 2070-1721                                                J. Winter
                                                             Sequoia PGP
                                                                Y. Niibe
                                                                    FSIJ
                                                               July 2024


                                OpenPGP

Abstract

   This document specifies the message formats used in OpenPGP.  OpenPGP
   provides encryption with public key or symmetric cryptographic
   algorithms, digital signatures, compression, and key management.

   This document is maintained in order to publish all necessary
   information needed to develop interoperable applications based on the
   OpenPGP format.  It is not a step-by-step cookbook for writing an
   application.  It describes only the format and methods needed to
   read, check, generate, and write conforming packets crossing any
   network.  It does not deal with storage and implementation questions.
   It does, however, discuss implementation issues necessary to avoid
   security flaws.

   This document obsoletes RFCs 4880 ("OpenPGP Message Format"), 5581
   ("The Camellia Cipher in OpenPGP"), and 6637 ("Elliptic Curve
   Cryptography (ECC) in OpenPGP").

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9580.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction
     1.1.  Terms
   2.  General Functions
     2.1.  Confidentiality via Encryption
     2.2.  Authentication via Digital Signature
     2.3.  Compression
     2.4.  Conversion to Base64
     2.5.  Signature-Only Applications
   3.  Data Element Formats
     3.1.  Scalar Numbers
     3.2.  Multiprecision Integers
       3.2.1.  Using MPIs to Encode Other Data
     3.3.  Key IDs and Fingerprints
     3.4.  Text
     3.5.  Time Fields
     3.6.  Keyrings
     3.7.  String-to-Key (S2K) Specifier
       3.7.1.  S2K Specifier Types
         3.7.1.1.  Simple S2K
         3.7.1.2.  Salted S2K
         3.7.1.3.  Iterated and Salted S2K
         3.7.1.4.  Argon2
       3.7.2.  S2K Usage
         3.7.2.1.  Secret Key Encryption
         3.7.2.2.  Symmetric Key Message Encryption
   4.  Packet Syntax
     4.1.  Overview
     4.2.  Packet Headers
       4.2.1.  OpenPGP Format Packet Lengths
         4.2.1.1.  1-Octet Lengths
         4.2.1.2.  2-Octet Lengths
         4.2.1.3.  5-Octet Lengths
         4.2.1.4.  Partial Body Lengths
       4.2.2.  Legacy Format Packet Lengths
       4.2.3.  Packet Length Examples
     4.3.  Packet Criticality
   5.  Packet Types
     5.1.  Public Key Encrypted Session Key Packet (Type ID 1)
       5.1.1.  Version 3 Public Key Encrypted Session Key Packet
               Format
       5.1.2.  Version 6 Public Key Encrypted Session Key Packet
               Format
       5.1.3.  Algorithm-Specific Fields for RSA Encryption
       5.1.4.  Algorithm-Specific Fields for Elgamal Encryption
       5.1.5.  Algorithm-Specific Fields for ECDH Encryption
       5.1.6.  Algorithm-Specific Fields for X25519 Encryption
       5.1.7.  Algorithm-Specific Fields for X448 Encryption
       5.1.8.  Notes on PKESK
     5.2.  Signature Packet (Type ID 2)
       5.2.1.  Signature Types
         5.2.1.1.  Binary Signature (Type ID 0x00) of a Document
         5.2.1.2.  Text Signature (Type ID 0x01) of a Canonical
                 Document
         5.2.1.3.  Standalone Signature (Type ID 0x02)
         5.2.1.4.  Generic Certification Signature (Type ID 0x10) of a
                 User ID and Public Key Packet
         5.2.1.5.  Persona Certification Signature (Type ID 0x11) of a
                 User ID and Public Key Packet
         5.2.1.6.  Casual Certification Signature (Type ID 0x12) of a
                 User ID and Public Key Packet
         5.2.1.7.  Positive Certification Signature (Type ID 0x13) of
                 a User ID and Public Key Packet
         5.2.1.8.  Subkey Binding Signature (Type ID 0x18)
         5.2.1.9.  Primary Key Binding Signature (Type ID 0x19)
         5.2.1.10. Direct Key Signature (Type ID 0x1F)
         5.2.1.11. Key Revocation Signature (Type ID 0x20)
         5.2.1.12. Subkey Revocation Signature (Type ID 0x28)
         5.2.1.13. Certification Revocation Signature (Type ID 0x30)
         5.2.1.14. Timestamp Signature (Type ID 0x40)
         5.2.1.15. Third-Party Confirmation Signature (Type ID 0x50)
         5.2.1.16. Reserved (Type ID 0xFF)
       5.2.2.  Version 3 Signature Packet Format
       5.2.3.  Versions 4 and 6 Signature Packet Formats
         5.2.3.1.  Algorithm-Specific Fields for RSA Signatures
         5.2.3.2.  Algorithm-Specific Fields for DSA or ECDSA
                 Signatures
         5.2.3.3.  Algorithm-Specific Fields for EdDSALegacy
                 Signatures (Deprecated)
         5.2.3.4.  Algorithm-Specific Fields for Ed25519 Signatures
         5.2.3.5.  Algorithm-Specific Fields for Ed448 Signatures
         5.2.3.6.  Notes on Signatures
         5.2.3.7.  Signature Subpacket Specification
         5.2.3.8.  Signature Subpacket Types
         5.2.3.9.  Notes on Subpackets
         5.2.3.10. Notes on Self-Signatures
         5.2.3.11. Signature Creation Time
         5.2.3.12. Issuer Key ID
         5.2.3.13. Key Expiration Time
         5.2.3.14. Preferred Symmetric Ciphers for v1 SEIPD
         5.2.3.15. Preferred AEAD Ciphersuites
         5.2.3.16. Preferred Hash Algorithms
         5.2.3.17. Preferred Compression Algorithms
         5.2.3.18. Signature Expiration Time
         5.2.3.19. Exportable Certification
         5.2.3.20. Revocable
         5.2.3.21. Trust Signature
         5.2.3.22. Regular Expression
         5.2.3.23. Revocation Key (Deprecated)
         5.2.3.24. Notation Data
         5.2.3.25. Key Server Preferences
         5.2.3.26. Preferred Key Server
         5.2.3.27. Primary User ID
         5.2.3.28. Policy URI
         5.2.3.29. Key Flags
         5.2.3.30. Signer's User ID
         5.2.3.31. Reason for Revocation
         5.2.3.32. Features
         5.2.3.33. Signature Target
         5.2.3.34. Embedded Signature
         5.2.3.35. Issuer Fingerprint
         5.2.3.36. Intended Recipient Fingerprint
       5.2.4.  Computing Signatures
         5.2.4.1.  Notes about Signature Computation
       5.2.5.  Malformed and Unknown Signatures
     5.3.  Symmetric Key Encrypted Session Key Packet (Type ID 3)
       5.3.1.  Version 4 Symmetric Key Encrypted Session Key Packet
               Format
       5.3.2.  Version 6 Symmetric Key Encrypted Session Key Packet
               Format
     5.4.  One-Pass Signature Packet (Type ID 4)
     5.5.  Key Material Packets
       5.5.1.  Key Packet Variants
         5.5.1.1.  Public Key Packet (Type ID 6)
         5.5.1.2.  Public Subkey Packet (Type ID 14)
         5.5.1.3.  Secret Key Packet (Type ID 5)
         5.5.1.4.  Secret Subkey Packet (Type ID 7)
       5.5.2.  Public Key Packet Formats
         5.5.2.1.  Version 3 Public Keys
         5.5.2.2.  Version 4 Public Keys
         5.5.2.3.  Version 6 Public Keys
       5.5.3.  Secret Key Packet Formats
       5.5.4.  Key IDs and Fingerprints
         5.5.4.1.  Version 3 Key ID and Fingerprint
         5.5.4.2.  Version 4 Key ID and Fingerprint
         5.5.4.3.  Version 6 Key ID and Fingerprint
       5.5.5.  Algorithm-Specific Parts of Keys
         5.5.5.1.  Algorithm-Specific Part for RSA Keys
         5.5.5.2.  Algorithm-Specific Part for DSA Keys
         5.5.5.3.  Algorithm-Specific Part for Elgamal Keys
         5.5.5.4.  Algorithm-Specific Part for ECDSA Keys
         5.5.5.5.  Algorithm-Specific Part for EdDSALegacy Keys
                 (Deprecated)
         5.5.5.6.  Algorithm-Specific Part for ECDH Keys
         5.5.5.7.  Algorithm-Specific Part for X25519 Keys
         5.5.5.8.  Algorithm-Specific Part for X448 Keys
         5.5.5.9.  Algorithm-Specific Part for Ed25519 Keys
         5.5.5.10. Algorithm-Specific Part for Ed448 Keys
     5.6.  Compressed Data Packet (Type ID 8)
     5.7.  Symmetrically Encrypted Data Packet (Type ID 9)
     5.8.  Marker Packet (Type ID 10)
     5.9.  Literal Data Packet (Type ID 11)
       5.9.1.  Special Filename _CONSOLE (Deprecated)
     5.10. Trust Packet (Type ID 12)
     5.11. User ID Packet (Type ID 13)
     5.12. User Attribute Packet (Type ID 17)
       5.12.1.  Image Attribute Subpacket
     5.13. Symmetrically Encrypted and Integrity Protected Data Packet
            (Type ID 18)
       5.13.1.  Version 1 Symmetrically Encrypted and Integrity
               Protected Data Packet Format
       5.13.2.  Version 2 Symmetrically Encrypted and Integrity
               Protected Data Packet Format
       5.13.3.  EAX Mode
       5.13.4.  OCB Mode
       5.13.5.  GCM Mode
     5.14. Padding Packet (Type ID 21)
   6.  Base64 Conversions
     6.1.  Optional Checksum
       6.1.1.  An Implementation of the CRC24 in "C"
     6.2.  Forming ASCII Armor
       6.2.1.  Armor Header Line
       6.2.2.  Armor Headers
         6.2.2.1.  "Version" Armor Header
         6.2.2.2.  "Comment" Armor Header
         6.2.2.3.  "Hash" Armor Header
         6.2.2.4.  "Charset" Armor Header
       6.2.3.  Armor Tail Line
   7.  Cleartext Signature Framework
     7.1.  Cleartext Signed Message Structure
     7.2.  Dash-Escaped Text
     7.3.  Issues with the Cleartext Signature Framework
   8.  Regular Expressions
   9.  Constants
     9.1.  Public Key Algorithms
     9.2.  ECC Curves for OpenPGP
       9.2.1.  Curve-Specific Wire Formats
     9.3.  Symmetric Key Algorithms
     9.4.  Compression Algorithms
     9.5.  Hash Algorithms
     9.6.  AEAD Algorithms
   10. Packet Sequence Composition
     10.1.  Transferable Public Keys
       10.1.1.  OpenPGP Version 6 Certificate Structure
       10.1.2.  OpenPGP Version 6 Revocation Certificate
       10.1.3.  OpenPGP Version 4 Certificate Structure
       10.1.4.  OpenPGP Version 3 Key Structure
       10.1.5.  Common Requirements
     10.2.  Transferable Secret Keys
     10.3.  OpenPGP Messages
       10.3.1.  Unwrapping Encrypted and Compressed Messages
       10.3.2.  Additional Constraints on Packet Sequences
         10.3.2.1.  Packet Versions in Encrypted Messages
         10.3.2.2.  Packet Versions in Signatures
     10.4.  Detached Signatures
   11. Elliptic Curve Cryptography
     11.1.  ECC Curves
     11.2.  EC Point Wire Formats
       11.2.1.  SEC1 EC Point Wire Format
       11.2.2.  Prefixed Native EC Point Wire Format
       11.2.3.  Notes on EC Point Wire Formats
     11.3.  EC Scalar Wire Formats
       11.3.1.  EC Octet String Wire Format
       11.3.2.  EC Prefixed Octet String Wire Format
     11.4.  Key Derivation Function
     11.5.  ECDH Algorithm
       11.5.1.  ECDH Parameters
   12. Notes on Algorithms
     12.1.  PKCS#1 Encoding in OpenPGP
       12.1.1.  EME-PKCS1-v1_5-ENCODE
       12.1.2.  EME-PKCS1-v1_5-DECODE
       12.1.3.  EMSA-PKCS1-v1_5
     12.2.  Symmetric Algorithm Preferences
       12.2.1.  Plaintext
     12.3.  Other Algorithm Preferences
       12.3.1.  Compression Preferences
         12.3.1.1.  Uncompressed
       12.3.2.  Hash Algorithm Preferences
     12.4.  RSA
     12.5.  DSA
     12.6.  Elgamal
     12.7.  EdDSA
     12.8.  Reserved Algorithm IDs
     12.9.  CFB Mode
     12.10. Private or Experimental Parameters
     12.11. Meta Considerations for Expansion
   13. Security Considerations
     13.1.  SHA-1 Collision Detection
     13.2.  Advantages of Salted Signatures
     13.3.  Elliptic Curve Side Channels
     13.4.  Risks of a Quick Check Oracle
     13.5.  Avoiding Leaks from PKCS#1 Errors
     13.6.  Fingerprint Usability
     13.7.  Avoiding Ciphertext Malleability
     13.8.  Secure Use of the v2 SEIPD Session-Key-Reuse Feature
     13.9.  Escrowed Revocation Signatures
     13.10. Random Number Generation and Seeding
     13.11. Traffic Analysis
     13.12. Surreptitious Forwarding
     13.13. Hashed vs. Unhashed Subpackets
     13.14. Malicious Compressed Data
   14. Implementation Considerations
     14.1.  Constrained Legacy Fingerprint Storage for Version 6 Keys
   15. IANA Considerations
     15.1.  Renamed Protocol Group
     15.2.  Renamed and Updated Registries
     15.3.  Removed Registry
     15.4.  Added Registries
     15.5.  Registration Policies
       15.5.1.  Registries That Use RFC Required
     15.6.  Designated Experts
       15.6.1.  Key and Signature Versions
       15.6.2.  Encryption Versions
       15.6.3.  Algorithms
         15.6.3.1.  Elliptic Curve Algorithms
         15.6.3.2.  Symmetric Key Algorithms
         15.6.3.3.  Hash Algorithms
   16. References
     16.1.  Normative References
     16.2.  Informative References
   Appendix A.  Test Vectors
     A.1.  Sample Version 4 Ed25519Legacy Key
     A.2.  Sample Version 4 Ed25519Legacy Signature
     A.3.  Sample Version 6 Certificate (Transferable Public Key)
       A.3.1.  Hashed Data Stream for Signature Verification
     A.4.  Sample Version 6 Secret Key (Transferable Secret Key)
     A.5.  Sample Locked Version 6 Secret Key (Transferable Secret
            Key)
       A.5.1.  Intermediate Data for Locked Primary Key
       A.5.2.  Intermediate Data for Locked Subkey
     A.6.  Sample Cleartext Signed Message
     A.7.  Sample Inline-Signed Message
     A.8.  Sample X25519-AEAD-OCB Encryption and Decryption
       A.8.1.  Sample Version 6 Public Key Encrypted Session Key
               Packet
       A.8.2.  X25519 Encryption/Decryption of the Session Key
       A.8.3.  Sample v2 SEIPD Packet
       A.8.4.  Decryption of Data
       A.8.5.  Complete X25519-AEAD-OCB Encrypted Packet Sequence
     A.9.  Sample AEAD-EAX Encryption and Decryption
       A.9.1.  Sample Version 6 Symmetric Key Encrypted Session Key
               Packet
       A.9.2.  Starting AEAD-EAX Decryption of the Session Key
       A.9.3.  Sample v2 SEIPD Packet
       A.9.4.  Decryption of Data
       A.9.5.  Complete AEAD-EAX Encrypted Packet Sequence
     A.10. Sample AEAD-OCB Encryption and Decryption
       A.10.1.  Sample Version 6 Symmetric Key Encrypted Session Key
               Packet
       A.10.2.  Starting AEAD-OCB Decryption of the Session Key
       A.10.3.  Sample v2 SEIPD Packet
       A.10.4.  Decryption of Data
       A.10.5.  Complete AEAD-OCB Encrypted Packet Sequence
     A.11. Sample AEAD-GCM Encryption and Decryption
       A.11.1.  Sample Version 6 Symmetric Key Encrypted Session Key
               Packet
       A.11.2.  Starting AEAD-GCM Decryption of the Session Key
       A.11.3.  Sample v2 SEIPD Packet
       A.11.4.  Decryption of Data
       A.11.5.  Complete AEAD-GCM Encrypted Packet Sequence
     A.12. Sample Messages Encrypted Using Argon2
       A.12.1.  V4 SKESK Using Argon2 with AES-128
       A.12.2.  V4 SKESK Using Argon2 with AES-192
       A.12.3.  V4 SKESK Using Argon2 with AES-256
   Appendix B.  Upgrade Guidance (Adapting Implementations from RFCs
           4880 and 6637)
     B.1.  Terminology Changes
   Appendix C.  Errata Addressed by This Document
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document provides information on the message-exchange packet
   formats used by OpenPGP to provide encryption, decryption, signing,
   and key management functions.  It is a revision of [RFC4880]
   ("OpenPGP Message Format"), which is a revision of [RFC2440], which
   itself replaces [RFC1991] ("PGP Message Exchange Formats").

   This document obsoletes [RFC4880] (OpenPGP), [RFC5581] (Camellia in
   OpenPGP), and [RFC6637] (Elliptic Curves in OpenPGP).  At the time of
   writing, this document incorporates all outstanding verified errata,
   which are listed in Appendix C.

   Software that has already implemented those previous specifications
   may want to review Appendix B for pointers to what has changed.

1.1.  Terms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The key words "Private Use", "Specification Required", and "RFC
   Required" that appear in this document when used to describe
   namespace allocation are to be interpreted as described in [RFC8126].

   Some terminology used in this document has been improved from
   previous versions of the OpenPGP specification.  See Appendix B.1 for
   more details.

2.  General Functions

   OpenPGP provides data confidentiality and integrity for messages and
   data files by using public key and/or symmetric encryption and
   digital signatures.  It provides formats for encoding and
   transferring encrypted and/or signed messages.  In addition, OpenPGP
   provides functionality for encoding and transferring keys and
   certificates, though key storage and management are beyond the scope
   of this document.

2.1.  Confidentiality via Encryption

   OpenPGP combines symmetric key encryption and (optionally) public key
   encryption to provide confidentiality.  When using public keys, first
   the object is encrypted using a symmetric key encryption algorithm.
   Each symmetric key is used only once, for a single object.  A new
   "session key" is generated as a random number for each object
   (sometimes referred to as a "session").  Since it is used only once,
   the session key is bound to the message and transmitted with it.  To
   protect the key, it is encrypted with the receiver's public key.  The
   sequence is as follows:

   1.  The sender creates a message.

   2.  The sending OpenPGP implementation generates a random session key
       for this message.

   3.  The session key is encrypted using each recipient's public key.
       These "encrypted session keys" start the message.

   4.  The sending OpenPGP implementation optionally compresses the
       message and then encrypts it using a message key derived from the
       session key.  The encrypted message forms the remainder of the
       OpenPGP Message.

   5.  The receiving OpenPGP implementation decrypts the session key
       using the recipient's private key.

   6.  The receiving OpenPGP implementation decrypts the message using
       the message key derived from the session key.  If the message was
       compressed, it will be decompressed.

   When using symmetric key encryption, a similar process as described
   above is used, but the session key is encrypted with a symmetric
   algorithm derived from a shared secret.

   Both digital signature and confidentiality services may be applied to
   the same message.  First, a signature is generated for the message
   and attached to the message.  Then, the message plus signature is
   encrypted using a symmetric message key derived from the session key.
   Finally, the session key is encrypted using public key encryption and
   prefixed to the encrypted block.

2.2.  Authentication via Digital Signature

   The digital signature uses a cryptographic hash function and a public
   key algorithm capable of signing.  The sequence is as follows:

   1.  The sender creates a message.

   2.  The sending implementation generates a hash digest of the
       message.

   3.  The sending implementation generates a signature from the hash
       digest using the sender's private key.

   4.  The signature is attached to or transmitted alongside the
       message.

   5.  The receiving implementation obtains a copy of the message and
       the message signature.

   6.  The receiving implementation generates a new hash digest for the
       received message and verifies it using the message's signature.
       If the verification is successful, the message is accepted as
       authentic.

2.3.  Compression

   An OpenPGP implementation MAY support the compression of data.  Many
   existing OpenPGP Messages are compressed.  Implementers, such as
   those working on constrained implementations that do not want to
   support compression, might want to consider at least implementing
   decompression.

2.4.  Conversion to Base64

   OpenPGP's underlying representation for encrypted messages,
   signatures, keys, and certificates is a stream of arbitrary octets.
   Some systems only permit the use of blocks consisting of 7-bit,
   printable text.  For transporting OpenPGP's raw binary octets through
   channels that are not safe to transport raw binary data, a printable
   encoding of these binary octets is defined.  The raw 8-bit binary
   octet stream can be converted to a stream of printable ASCII
   characters using base64 encoding in a format called "ASCII Armor"
   (see Section 6).

   Implementations SHOULD support base64 conversions.

2.5.  Signature-Only Applications

   OpenPGP is designed for applications that use both encryption and
   signatures, but there are a number of use cases that only require a
   signature-only implementation.  Although this specification requires
   both encryption and signatures, it is reasonable for there to be
   subset implementations that are non-conformant only in that they omit
   encryption support.

3.  Data Element Formats

   This section describes the data elements used by OpenPGP.

3.1.  Scalar Numbers

   Scalar numbers are unsigned and always stored in big-endian format.
   Using n[k] to refer to the kth octet being interpreted, the value of
   a 2-octet scalar is ((n[0] << 8) + n[1]).  The value of a 4-octet
   scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) + n[3]).

3.2.  Multiprecision Integers

   Multiprecision Integers (MPIs) are unsigned integers used to hold
   large integers such as the ones used in cryptographic calculations.

   An MPI consists of two pieces: a 2-octet scalar that is the length of
   the MPI in bits, followed by a string of octets that contain the
   actual integer.

   These octets form a big-endian number; a big-endian number can be
   made into an MPI by prefixing it with the appropriate length.

   Examples:

   (Note that all numbers in the octet strings identified by square
   brackets are in hexadecimal.)

      The string of octets [00 00] forms an MPI with the value 0.

      The string of octets [00 01 01] forms an MPI with the value 1.

      The string [00 09 01 FF] forms an MPI with the value 511.

   Additional rules:

   *  The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.

   *  The length field of an MPI describes the length starting from its
      most significant non-zero bit.  Thus, the MPI [00 02 01] is not
      formed correctly.  It should be [00 01 01].  When parsing an MPI
      in a version 6 Key, Signature, or Public Key Encrypted Session Key
      (PKESK) packet, the implementation MUST check that the encoded
      length matches the length starting from the most significant non-
      zero bit; if it doesn't match, reject the packet as malformed.

   *  Unused bits of an MPI MUST be zero.

3.2.1.  Using MPIs to Encode Other Data

   Note that in some places, MPIs are used to encode non-integer data,
   such as an elliptic curve (EC) point (see Section 11.2) or an octet
   string of known, fixed length (see Section 11.3).  The wire
   representation is the same: 2 octets of length in bits counted from
   the first non-zero bit, followed by the smallest series of octets
   that can represent the value while stripping off any leading zero
   octets.

3.3.  Key IDs and Fingerprints

   A Key ID is an 8-octet scalar that identifies a key.  Implementations
   SHOULD NOT assume that Key IDs are unique.  A fingerprint is more
   likely to be unique than a Key ID.  The fingerprint and Key ID of a
   key are calculated differently according to the version of the key.

   Section 5.5.4 describes how Key IDs and Fingerprints are formed.

3.4.  Text

   Unless otherwise specified, the character set for text is the UTF-8
   [RFC3629] encoding of Unicode [ISO10646].

3.5.  Time Fields

   A time field is an unsigned 4-octet number containing the number of
   seconds elapsed since midnight, 1 January 1970 UTC.

3.6.  Keyrings

   A keyring is a collection of one or more keys in a file or database.
   Typically, a keyring is simply a sequential list of keys, but it may
   be any suitable database.  It is beyond the scope of this
   specification to discuss the details of keyrings or other databases.

3.7.  String-to-Key (S2K) Specifier

   A string-to-key (S2K) Specifier is used to convert a passphrase
   string into a symmetric key encryption/decryption key.  Passphrases
   requiring use of S2K conversion are currently used in two places: to
   encrypt the secret part of private keys and for symmetrically
   encrypted messages.

3.7.1.  S2K Specifier Types

   There are four types of S2K Specifiers currently specified and some
   reserved values:

   +=========+==============+===============+==============+===========+
   |      ID | S2K Type     | S2K Field     | Generate?    | Reference |
   |         |              | Size          |              |           |
   |         |              | (Octets)      |              |           |
   +=========+==============+===============+==============+===========+
   |       0 | Simple S2K   | 2             | No           | Section   |
   |         |              |               |              | 3.7.1.1   |
   +---------+--------------+---------------+--------------+-----------+
   |       1 | Salted S2K   | 10            | Only when    | Section   |
   |         |              |               | string is    | 3.7.1.2   |
   |         |              |               | high entropy |           |
   +---------+--------------+---------------+--------------+-----------+
   |       2 | Reserved     | -             | No           |           |
   |         | value        |               |              |           |
   +---------+--------------+---------------+--------------+-----------+
   |       3 | Iterated and | 11            | Yes          | Section   |
   |         | Salted S2K   |               |              | 3.7.1.3   |
   +---------+--------------+---------------+--------------+-----------+
   |       4 | Argon2       | 20            | Yes          | Section   |
   |         |              |               |              | 3.7.1.4   |
   +---------+--------------+---------------+--------------+-----------+
   | 100-110 | Private or   | -             | As           |           |
   |         | Experimental |               | appropriate  |           |
   |         | Use          |               |              |           |
   +---------+--------------+---------------+--------------+-----------+

            Table 1: OpenPGP String-to-Key (S2K) Types Registry

   The S2K Specifier Types are described in the subsections below.  If
   "Yes" is not present in the "Generate?" column, the S2K entry is used
   only for reading in backward-compatibility mode and SHOULD NOT be
   used to generate new output.

3.7.1.1.  Simple S2K

   Simple S2K directly hashes the string to produce the key data.  This
   hashing is done as shown below.

     Octet 0:        0x00
     Octet 1:        hash algorithm

   Simple S2K hashes the passphrase to produce the session key.  The
   manner in which this is done depends on the size of the session key
   (which depends on the cipher the session key will be used with) and
   the size of the hash algorithm's output.  If the hash size is greater
   than the session key size, the high-order (leftmost) octets of the
   hash are used as the key.

   If the hash size is less than the key size, multiple instances of the
   hash context are created -- enough to produce the required key data.
   These instances are preloaded with 0, 1, 2, ... octets of zeros (that
   is, the first instance has no preloading, the second gets preloaded
   with 1 octet of zero, the third is preloaded with 2 octets of zeros,
   and so forth).

   As the data is hashed, it is given independently to each hash
   context.  Since the contexts have been initialized differently, they
   will each produce a different hash output.  Once the passphrase is
   hashed, the output data from the multiple hashes is concatenated,
   first hash leftmost, to produce the key data, and any excess octets
   on the right are discarded.

3.7.1.2.  Salted S2K

   Salted S2K includes a "salt" value in the S2K Specifier -- some
   arbitrary data -- that gets hashed along with the passphrase string
   to help prevent dictionary attacks.

     Octet 0:        0x01
     Octet 1:        hash algorithm
     Octets 2-9:     8-octet salt value

   Salted S2K is exactly like Simple S2K, except that the input to the
   hash function(s) consists of the 8 octets of salt from the S2K
   Specifier, followed by the passphrase.

3.7.1.3.  Iterated and Salted S2K

   Iterated and Salted S2K includes both a salt and an octet count.  The
   salt is combined with the passphrase, and the resulting value is
   repeated and then hashed.  This further increases the amount of work
   an attacker must do to try dictionary attacks.

     Octet  0:        0x03
     Octet  1:        hash algorithm
     Octets 2-9:      8-octet salt value
     Octet  10:       count; a 1-octet coded value

   The count is coded into a 1-octet number using the following formula:

     #define EXPBIAS 6
         count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);

   The above formula is described in [C99], where "Int32" is a type for
   a 32-bit integer, and the variable "c" is the coded count, octet 10.

   Iterated and Salted S2K hashes the passphrase and salt data multiple
   times.  The total number of octets to be hashed is specified in the
   encoded count in the S2K Specifier.  Note that the resulting count
   value is an octet count of how many octets will be hashed, not an
   iteration count.

   Initially, one or more hash contexts are set up the same as the other
   S2K algorithms, depending on how many octets of key data are needed.
   Then the salt, followed by the passphrase data, is repeatedly
   processed as input to each hash context until the number of octets
   specified by the octet count has been hashed.  The input is truncated
   to the octet count, except if the octet count is less than the
   initial size of the salt plus passphrase.  That is, at least one copy
   of the full salt plus passphrase will be provided as input to each
   hash context regardless of the octet count.  After the hashing is
   done, the key data is produced from the hash digest(s), which is the
   same way it is produced for the other S2K algorithms.

3.7.1.4.  Argon2

   This S2K method hashes the passphrase using Argon2, as specified in
   [RFC9106].  This provides memory hardness, further protecting the
   passphrase against brute-force attacks.

     Octet  0:        0x04
     Octets 1-16:     16-octet salt value
     Octet  17:       1-octet number of passes t
     Octet  18:       1-octet degree of parallelism p
     Octet  19:       1-octet encoded_m, specifying the exponent of
                         the memory size

   The salt SHOULD be unique for each passphrase.

   The number of passes t and the degree of parallelism p MUST be non-
   zero.

   The memory size m is 2^(encoded_m) kibibytes (KiB) of RAM.  The
   encoded memory size MUST be a value from 3+ceil(log_2(p)) to 31, such
   that the decoded memory size m is a value from 8*p to 2^31.  Note
   that memory-hardness size is indicated in KiB, not octets.

   Argon2 is invoked with the passphrase as P, the salt as S, the values
   of t, p, and m as described above, the required key size as the tag
   length T, 0x13 as the version v, and Argon2id as the type.

   For the recommended values of t, p, and m, see Section 4 of
   [RFC9106].  If the recommended value of m for a given application is
   not a power of 2, it is RECOMMENDED to round up to the next power of
   2 if the resulting performance would be acceptable; otherwise, round
   down (keeping in mind that m must be at least 8*p).

   As an example, with the first recommended option (t=1, p=4, m=2^21),
   the full S2K Specifier would be:

     04 XX XX XX XX XX XX XX XX XX XX XX XX XX XX XX
     XX 01 04 15

   where XX represents a random octet of salt.

3.7.2.  S2K Usage

   Simple S2K and Salted S2K Specifiers can be brute-forced when used
   with a low-entropy string, such as those typically provided by users.
   In addition, the usage of Simple S2K can lead to key and
   initialization vector (IV) reuse (see Section 5.3).  Therefore, when
   generating an S2K Specifier, an implementation MUST NOT use Simple
   S2K.  Furthermore, an implementation SHOULD NOT generate a Salted S2K
   unless the implementation knows that the input string is high entropy
   (for example, it generated the string itself using a known good
   source of randomness).

   It is RECOMMENDED that implementations use Argon2.  If Argon2 is not
   available, Iterated and Salted S2K MAY be used if care is taken to
   use a high octet count and a strong passphrase.  However, this method
   does not provide memory hardness, unlike Argon2.

3.7.2.1.  Secret Key Encryption

   The first octet following the public key material in a Secret Key
   packet (Section 5.5.3) indicates whether and how the secret key
   material is passphrase protected.  This first octet is known as the
   "S2K usage octet".

   If the S2K usage octet is zero, the secret key data is unprotected.
   If it is non-zero, it describes how to use a passphrase to unlock the
   secret key.

   Implementations predating [RFC2440] indicated a protected key by
   storing a Symmetric Cipher Algorithm ID (see Section 9.3) in the S2K
   usage octet.  In this case, the MD5 hash function was always used to
   convert the passphrase to a key for the specified cipher algorithm.

   Later implementations indicate a protected secret key by storing one
   of the special values 253 (AEAD), 254 (CFB), or 255 (MalleableCFB) in
   the S2K usage octet.  The S2K usage octet is then followed
   immediately by a set of fields that describe how to convert a
   passphrase to a symmetric key that can unlock the secret material,
   plus other parameters relevant to the type of encryption used.

   The wire format fields also differ based on the version of the
   enclosing OpenPGP packet.  The table below, indexed by the S2K usage
   octet, summarizes the specifics described in Section 5.5.3.

   In the table below, check(x) means the "2-octet checksum", which is
   the sum of all octets in x mod 65536.  The info and packetprefix
   parameters are described in detail in Section 5.5.3.  Note that the
   "Generate?" column header has been shortened to "Gen?" here.

   +=========+============+============+==========================+====+
   |S2K Usage|Shorthand   |Encryption  |Encryption                |Gen?|
   |Octet    |            |Parameter   |                          |    |
   |         |            |Fields      |                          |    |
   +=========+============+============+==========================+====+
   |0        |Unprotected |-           |*v3 or v4 keys:*          |Yes |
   |         |            |            |[cleartext secrets ||     |    |
   |         |            |            |check(secrets)]           |    |
   |         |            |            |*v6 keys:* [cleartext     |    |
   |         |            |            |secrets]                  |    |
   +---------+------------+------------+--------------------------+----+
   |Known    |LegacyCFB   |IV          |CFB(MD5(passphrase),      |No  |
   |symmetric|            |            |secrets || check(secrets))|    |
   |cipher   |            |            |                          |    |
   |algo ID  |            |            |                          |    |
   |(see     |            |            |                          |    |
   |Section  |            |            |                          |    |
   |9.3)     |            |            |                          |    |
   +---------+------------+------------+--------------------------+----+
   |253      |AEAD        |params-     |AEAD(HKDF(S2K(passphrase),|Yes |
   |         |            |length      |info), secrets,           |    |
   |         |            |(*v6-only*),|packetprefix)             |    |
   |         |            |cipher-algo,|                          |    |
   |         |            |AEAD-mode,  |                          |    |
   |         |            |S2K-        |                          |    |
   |         |            |specifier-  |                          |    |
   |         |            |length      |                          |    |
   |         |            |(*v6-only*),|                          |    |
   |         |            |S2K-        |                          |    |
   |         |            |specifier,  |                          |    |
   |         |            |nonce       |                          |    |
   +---------+------------+------------+--------------------------+----+
   |254      |CFB         |params-     |CFB(S2K(passphrase),      |Yes |
   |         |            |length      |secrets || SHA1(secrets)) |    |
   |         |            |(*v6-only*),|                          |    |
   |         |            |cipher-algo,|                          |    |
   |         |            |S2K-        |                          |    |
   |         |            |specifier-  |                          |    |
   |         |            |length      |                          |    |
   |         |            |(*v6-only*),|                          |    |
   |         |            |S2K-        |                          |    |
   |         |            |specifier,  |                          |    |
   |         |            |IV          |                          |    |
   +---------+------------+------------+--------------------------+----+
   |255      |MalleableCFB|cipher-algo,|CFB(S2K(passphrase),      |No  |
   |         |            |S2K-        |secrets || check(secrets))|    |
   |         |            |specifier,  |                          |    |
   |         |            |IV          |                          |    |
   +---------+------------+------------+--------------------------+----+

     Table 2: OpenPGP Secret Key Encryption (S2K Usage Octet) Registry

   When emitting a secret key (with or without passphrase protection),
   an implementation MUST only produce data from a row with "Generate?"
   marked as "Yes".  Each row with "Generate?" marked as "No" is
   described for backward compatibility (for reading version 4 and
   earlier keys only) and MUST NOT be used to generate new output.
   Version 6 secret keys using these formats MUST be rejected.

   Note that compared to a version 4 secret key, the parameters of a
   passphrase-protected version 6 secret key are stored with an
   additional pair of length counts, each of which is 1 octet wide.

   Argon2 is only used with Authenticated Encryption with Associated
   Data (AEAD) (S2K usage octet 253).  An implementation MUST NOT create
   and MUST reject as malformed any Secret Key packet where the S2K
   usage octet is not AEAD (253) and the S2K Specifier Type is Argon2.

3.7.2.2.  Symmetric Key Message Encryption

   OpenPGP can create a Symmetric Key Encrypted Session Key (SKESK)
   packet at the front of a message.  This is used to allow S2K
   Specifiers to be used for the passphrase conversion or to create
   messages with a mix of SKESK packets and PKESK packets.  This allows
   a message to be decrypted with either a passphrase or a public key
   pair.

   Implementations predating [RFC2440] always used the International
   Data Encryption Algorithm (IDEA) with Simple S2K conversion when
   encrypting a message with a symmetric algorithm; see Section 5.7.
   IDEA MUST NOT be generated but MAY be consumed for backward
   compatibility.

4.  Packet Syntax

   This section describes the packets used by OpenPGP.

4.1.  Overview

   An OpenPGP Message is constructed from a number of records that are
   typically called packets.  A packet is a chunk of data that has a
   Type ID specifying its meaning.  An OpenPGP Message, keyring,
   certificate, detached signature, and so forth consists of a number of
   packets.  Some of those packets may contain other OpenPGP packets
   (for example, a compressed data packet, when uncompressed, contains
   OpenPGP packets).

   Each packet consists of a packet header, followed by the packet body.
   The packet header is of variable length.

   When handling a stream of packets, the length information in each
   packet header is the canonical source of packet boundaries.  An
   implementation handling a packet stream that wants to find the next
   packet MUST look for it at the precise offset indicated in the
   previous packet header.

   Additionally, some packets contain internal length indicators (for
   example, a subfield within the packet).  In the event that a subfield
   length indicator within a packet implies inclusion of octets outside
   the range indicated in the packet header, a parser MUST abort without
   writing outside the indicated range and MUST treat the packet as
   malformed and unusable.

   An implementation MUST NOT interpret octets outside the range
   indicated in the packet header as part of the contents of the packet.

4.2.  Packet Headers

   The first octet of the packet denotes the format of the rest of the
   header, and it encodes the Packet Type ID, indicating the type of the
   packet (see Section 5).  The remainder of the packet header is the
   length of the packet.

   There are two packet formats: 1) the (current) OpenPGP packet format
   specified by this document and its predecessors [RFC4880] and
   [RFC2440] and 2) the Legacy packet format as used by implementations
   predating any IETF specification of OpenPGP.

   Note that the most significant bit is the leftmost bit, called "bit
   7".  A mask for this bit is 0x80 in hexadecimal.

                             +---------------+
     Encoded Packet Type ID: |7 6 5 4 3 2 1 0|
                             +---------------+
     OpenPGP format:
       Bit 7 -- always one
       Bit 6 -- always one
       Bits 5 to 0 -- Packet Type ID

     Legacy format:
       Bit 7 -- always one
       Bit 6 -- always zero
       Bits 5 to 2 -- Packet Type ID
       Bits 1 to 0 -- length-type

   Bit 6 of the first octet of the packet header indicates whether the
   packet is encoded in the OpenPGP or Legacy packet format.  The Legacy
   packet format MAY be used when consuming packets to facilitate
   interoperability and accessing archived data.  The Legacy packet
   format SHOULD NOT be used to generate new data, unless the recipient
   is known to only support the Legacy packet format.  This latter case
   is extremely unlikely, as the Legacy packet format was obsoleted by
   [RFC2440] in 1998.

   An implementation that consumes and redistributes pre-existing
   OpenPGP data (such as Transferable Public Keys) may encounter packets
   framed with the Legacy packet format.  Such an implementation MAY
   either redistribute these packets in their Legacy format or transform
   them to the current OpenPGP packet format before redistribution.

   Note that Legacy format headers only have 4 bits for the Packet Type
   ID and hence can only encode Packet Type IDs less than 16, whereas
   the OpenPGP format headers can encode IDs as great as 63.

4.2.1.  OpenPGP Format Packet Lengths

   OpenPGP format packets have four possible ways of encoding length:

   1.  A 1-octet Body Length header encodes packet lengths of up to 191
       octets.

   2.  A 2-octet Body Length header encodes packet lengths of 192 to
       8383 octets.

   3.  A 5-octet Body Length header encodes packet lengths of up to
       4,294,967,295 (0xFFFFFFFF) octets in length.  (This actually
       encodes a 4-octet scalar number.)

   4.  When the length of the packet body is not known in advance by the
       issuer, Partial Body Length headers encode a packet of
       indeterminate length, effectively making it a stream.

4.2.1.1.  1-Octet Lengths

   A 1-octet Body Length header encodes a length of 0 to 191 octets.
   This type of length header is recognized because the 1-octet value is
   less than 192.  The body length is equal to:

     bodyLen = 1st_octet;

4.2.1.2.  2-Octet Lengths

   A 2-octet Body Length header encodes a length of 192 to 8383 octets.
   It is recognized because its first octet is in the range 192 to 223.
   The body length is equal to:

     bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

4.2.1.3.  5-Octet Lengths

   A 5-octet Body Length header consists of a single octet holding the
   value 255, followed by a 4-octet scalar.  The body length is equal
   to:

     bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
               (4th_octet << 8)  | 5th_octet

   This basic set of 1-octet, 2-octet, and 5-octet lengths is also used
   internally to some packets.

4.2.1.4.  Partial Body Lengths

   A Partial Body Length header is 1 octet long and encodes the length
   of only part of the data packet.  This length is a power of 2, from 1
   to 1,073,741,824 (2 to the 30th power).  It is recognized by its
   1-octet value that is greater than or equal to 224, and less than
   255.  The Partial Body Length is equal to:

     partialBodyLen = 1 << (1st_octet & 0x1F);

   Each Partial Body Length header is followed by a portion of the
   packet body data; the Partial Body Length header specifies this
   portion's length.  Another length header (1-octet, 2-octet, 5-octet,
   or partial) follows that portion.  The last length header in the
   packet MUST NOT be a Partial Body Length header.  Partial Body Length
   headers may only be used for the non-final parts of the packet.

   Note also that the last Body Length header can be a zero-length
   header.

   An implementation MAY use Partial Body Lengths for data packets,
   whether they are literal, compressed, or encrypted.  The first
   partial length MUST be at least 512 octets long.  Partial Body
   Lengths MUST NOT be used for any other packet types.

4.2.2.  Legacy Format Packet Lengths

   A zero in bit 6 of the first octet of the packet indicates a Legacy
   packet format.  Bits 1 and 0 of the first octet of a Legacy packet
   are the "length-type" field.  The meaning of length-type in Legacy
   format packets is as follows:

   0  The packet has a 1-octet length.  The header is 2 octets long.

   1  The packet has a 2-octet length.  The header is 3 octets long.

   2  The packet has a 4-octet length.  The header is 5 octets long.

   3  The packet is of indeterminate length.  The header is 1 octet
      long, and the implementation must determine how long the packet
      is.  If the packet is in a file, it means that the packet extends
      until the end of the file.  The OpenPGP format headers have a
      mechanism for precisely encoding data of indeterminate length.  An
      implementation MUST NOT generate a Legacy format packet with
      indeterminate length.  An implementation MAY interpret an
      indeterminate length Legacy format packet in order to deal with
      historic data or data generated by a legacy system that predates
      support for [RFC2440].

4.2.3.  Packet Length Examples

   These examples show ways that OpenPGP format packets might encode the
   packet body lengths.

   *  A packet body with length 100 may have its length encoded in one
      octet: 0x64.  This is followed by 100 octets of data.

   *  A packet body with length 1723 may have its length encoded in two
      octets: 0xC5, 0xFB.  This header is followed by the 1723 octets of
      data.

   *  A packet body with length 100000 may have its length encoded in
      five octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.

   It might also be encoded in the following octet stream:

   *  0xEF, first 32768 octets of data;

   *  0xE1, next 2 octets of data;

   *  0xE0, next 1 octet of data;

   *  0xF0, next 65536 octets of data; and

   *  0xC5, 0xDD, last 1693 octets of data.

   This is just one possible encoding, and many variations are possible
   on the size of the Partial Body Length headers, as long as a regular
   Body Length header encodes the last portion of the data.

   Please note that in all of these explanations, the total length of
   the packet is the length of the header(s) plus the length of the
   body.

4.3.  Packet Criticality

   The Packet Type ID space is partitioned into critical packets and
   non-critical packets.  If an implementation encounters a critical
   packet where the packet type is unknown in a packet sequence, it MUST
   reject the whole packet sequence (see Section 10).  On the other
   hand, an unknown non-critical packet MUST be ignored.

   Packets with Type IDs from 0 to 39 are critical.  Packets with Type
   IDs from 40 to 63 are non-critical.

5.  Packet Types

   The defined packet types are as follows:

    +=======+==========+=====================+===========+===========+
    |    ID | Critical | Packet Type         | Shorthand | Reference |
    |       |          | Description         |           |           |
    +=======+==========+=====================+===========+===========+
    |     0 | Yes      | Reserved - this     |           |           |
    |       |          | Packet Type ID MUST |           |           |
    |       |          | NOT be used         |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |     1 | Yes      | Public Key          | PKESK     | Section   |
    |       |          | Encrypted Session   |           | 5.1       |
    |       |          | Key Packet          |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |     2 | Yes      | Signature Packet    | SIG       | Section   |
    |       |          |                     |           | 5.2       |
    +-------+----------+---------------------+-----------+-----------+
    |     3 | Yes      | Symmetric Key       | SKESK     | Section   |
    |       |          | Encrypted Session   |           | 5.3       |
    |       |          | Key Packet          |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |     4 | Yes      | One-Pass Signature  | OPS       | Section   |
    |       |          | Packet              |           | 5.4       |
    +-------+----------+---------------------+-----------+-----------+
    |     5 | Yes      | Secret Key Packet   | SECKEY    | Section   |
    |       |          |                     |           | 5.5.1.3   |
    +-------+----------+---------------------+-----------+-----------+
    |     6 | Yes      | Public Key Packet   | PUBKEY    | Section   |
    |       |          |                     |           | 5.5.1.1   |
    +-------+----------+---------------------+-----------+-----------+
    |     7 | Yes      | Secret Subkey       | SECSUBKEY | Section   |
    |       |          | Packet              |           | 5.5.1.4   |
    +-------+----------+---------------------+-----------+-----------+
    |     8 | Yes      | Compressed Data     | COMP      | Section   |
    |       |          | Packet              |           | 5.6       |
    +-------+----------+---------------------+-----------+-----------+
    |     9 | Yes      | Symmetrically       | SED       | Section   |
    |       |          | Encrypted Data      |           | 5.7       |
    |       |          | Packet              |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |    10 | Yes      | Marker Packet       | MARKER    | Section   |
    |       |          |                     |           | 5.8       |
    +-------+----------+---------------------+-----------+-----------+
    |    11 | Yes      | Literal Data Packet | LIT       | Section   |
    |       |          |                     |           | 5.9       |
    +-------+----------+---------------------+-----------+-----------+
    |    12 | Yes      | Trust Packet        | TRUST     | Section   |
    |       |          |                     |           | 5.10      |
    +-------+----------+---------------------+-----------+-----------+
    |    13 | Yes      | User ID Packet      | UID       | Section   |
    |       |          |                     |           | 5.11      |
    +-------+----------+---------------------+-----------+-----------+
    |    14 | Yes      | Public Subkey       | PUBSUBKEY | Section   |
    |       |          | Packet              |           | 5.5.1.2   |
    +-------+----------+---------------------+-----------+-----------+
    |    17 | Yes      | User Attribute      | UAT       | Section   |
    |       |          | Packet              |           | 5.12      |
    +-------+----------+---------------------+-----------+-----------+
    |    18 | Yes      | Symmetrically       | SEIPD     | Section   |
    |       |          | Encrypted and       |           | 5.13      |
    |       |          | Integrity Protected |           |           |
    |       |          | Data Packet         |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |    19 | Yes      | Reserved (formerly  |           | Section   |
    |       |          | Modification        |           | 5.13.1    |
    |       |          | Detection Code      |           |           |
    |       |          | Packet)             |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |    20 | Yes      | Reserved            |           |           |
    +-------+----------+---------------------+-----------+-----------+
    |    21 | Yes      | Padding Packet      | PADDING   | Section   |
    |       |          |                     |           | 5.14      |
    +-------+----------+---------------------+-----------+-----------+
    | 22-39 | Yes      | Unassigned Critical |           |           |
    |       |          | Packets             |           |           |
    +-------+----------+---------------------+-----------+-----------+
    | 40-59 | No       | Unassigned Non-     |           |           |
    |       |          | Critical Packets    |           |           |
    +-------+----------+---------------------+-----------+-----------+
    | 60-63 | No       | Private or          |           |           |
    |       |          | Experimental Use    |           |           |
    +-------+----------+---------------------+-----------+-----------+

                  Table 3: OpenPGP Packet Types Registry

   The labels in the "Shorthand" column are used for compact reference
   elsewhere in this document, and they may also be used by
   implementations that provide debugging or inspection affordances for
   streams of OpenPGP packets.

5.1.  Public Key Encrypted Session Key Packet (Type ID 1)

   Zero or more PKESK packets and/or SKESK packets (Section 5.3) precede
   an encryption container (that is, a Symmetrically Encrypted and
   Integrity Protected Data (SEIPD) packet or -- for historic data -- a
   Symmetrically Encrypted Data (SED) packet), which holds an Encrypted
   Message.  The message is encrypted with the session key, and the
   session key is itself encrypted and stored in the Encrypted Session
   Key packet(s).  The encryption container is preceded by one Public
   Key Encrypted Session Key packet for each OpenPGP Key to which the
   message is encrypted.  The recipient of the message finds a session
   key that is encrypted to their public key, decrypts the session key,
   and then uses the session key to decrypt the message.

   The body of this packet starts with a 1-octet number giving the
   version number of the packet type.  The currently defined versions
   are 3 and 6.  The remainder of the packet depends on the version.

   The versions differ in how they identify the recipient key and in
   what they encode.  The version of the PKESK packet must align with
   the version of the SEIPD packet (see Section 10.3.2.1).  Any new
   version of the PKESK packet should be registered in the registry
   established in Section 10.3.2.1.

5.1.1.  Version 3 Public Key Encrypted Session Key Packet Format

   A version 3 PKESK packet precedes a v1 SEIPD packet (see
   Section 5.13.1).  In historic data, it is sometimes found preceding a
   deprecated SED packet; see Section 5.7.  A v3 PKESK packet MUST NOT
   precede a v2 SEIPD packet (see Section 10.3.2.1).

   The v3 PKESK packet consists of:

   *  A 1-octet version number with value 3.

   *  An 8-octet number that gives the Key ID of the public key to which
      the session key is encrypted.  If the session key is encrypted to
      a subkey, then the Key ID of this subkey is used here instead of
      the Key ID of the primary key.  The Key ID may also be all zeros,
      for an "anonymous recipient" (see Section 5.1.8).

   *  A 1-octet number giving the public key algorithm used.

   *  A series of values comprising the encrypted session key.  This is
      algorithm specific and described below.

   The public key encryption algorithm (described in subsequent
   sections) is passed two values:

   *  The session key.

   *  The 1-octet algorithm identifier that specifies the symmetric key
      encryption algorithm used to encrypt the v1 SEIPD packet described
      in the following section.

5.1.2.  Version 6 Public Key Encrypted Session Key Packet Format

   A v6 PKESK packet precedes a v2 SEIPD packet (see Section 5.13.2).  A
   v6 PKESK packet MUST NOT precede a v1 SEIPD packet or a deprecated
   SED packet (see Section 10.3.2.1).

   The v6 PKESK packet consists of the following fields:

   *  A 1-octet version number with value 6.

   *  A 1-octet size of the following two fields.  This size may be
      zero, if the key version number field and the fingerprint field
      are omitted for an "anonymous recipient" (see Section 5.1.8).

   *  A 1-octet key version number.

   *  The fingerprint of the public key or subkey to which the session
      key is encrypted.  Note that the length N of the fingerprint for a
      version 4 key is 20 octets; for a version 6 key, N is 32.

   *  A 1-octet number giving the public key algorithm used.

   *  A series of values comprising the encrypted session key.  This is
      algorithm specific and described below.

   The session key is encrypted according to the public key algorithm
   used, as described below.  No symmetric key encryption algorithm
   identifier is passed to the public key algorithm for a v6 PKESK
   packet, as it is included in the v2 SEIPD packet.

5.1.3.  Algorithm-Specific Fields for RSA Encryption

   *  MPI of RSA-encrypted value m^e mod n.

   To produce the value "m" in the above formula, first concatenate the
   following values:

   *  The 1-octet algorithm identifier, if it was passed (in the case of
      a v3 PKESK packet).

   *  The session key.

   *  A 2-octet checksum of the session key, equal to the sum of the
      session key octets, modulo 65536.

   Then, the above values are encoded using the PKCS#1 block encoding
   EME-PKCS1-v1_5, as described in Step 2 in Section 7.2.1 of [RFC8017]
   (see also Section 12.1.1).  When decoding "m" during decryption, an
   implementation should follow Step 3 in Section 7.2.2 of [RFC8017]
   (see also Section 12.1.2).

   Note that when an implementation forms several PKESK packets with one
   session key, forming a message that can be decrypted by several keys,
   the implementation MUST make a new PKCS#1 encoding for each key.
   This defends against attacks such as those discussed in [HASTAD].

5.1.4.  Algorithm-Specific Fields for Elgamal Encryption

   *  MPI of Elgamal (Diffie-Hellman) value g^k mod p.

   *  MPI of Elgamal (Diffie-Hellman) value m * y^k mod p.

   To produce the value "m" in the above formula, first concatenate the
   following values:

   *  The 1-octet algorithm identifier, if it was passed (in the case of
      a v3 PKESK packet).

   *  The session key.

   *  A 2-octet checksum of the session key, equal to the sum of the
      session key octets, modulo 65536.

   Then, the above values are encoded using the PKCS#1 block encoding
   EME-PKCS1-v1_5, as described in Step 2 in Section 7.2.1 of [RFC8017]
   (see also Section 12.1.1).  When decoding "m" during decryption, an
   implementation should follow Step 3 in Section 7.2.2 of [RFC8017]
   (see also Section 12.1.2).

   Note that when an implementation forms several PKESK packets with one
   session key, forming a message that can be decrypted by several keys,
   the implementation MUST make a new PKCS#1 encoding for each key.
   This defends against attacks such as those discussed in [HASTAD].

   An implementation MUST NOT generate ElGamal v6 PKESK packets.

5.1.5.  Algorithm-Specific Fields for ECDH Encryption

   *  MPI of an EC point representing an ephemeral public key in the
      point format associated with the curve as specified in
      Section 9.2.

   *  A 1-octet size, followed by a symmetric key encoded using the
      method described in Section 11.5.

5.1.6.  Algorithm-Specific Fields for X25519 Encryption

   *  32 octets representing an ephemeral X25519 public key.

   *  A 1-octet size of the following fields.

   *  The 1-octet algorithm identifier, if it was passed (in the case of
      a v3 PKESK packet).

   *  The encrypted session key.

   See Section 6.1 of [RFC7748] for more details on the computation of
   the ephemeral public key and the shared secret.  The HMAC-based Key
   Derivation Function (HKDF) [RFC5869] is then used with SHA256
   [RFC6234] and an info parameter of "OpenPGP X25519" and no salt.  The
   input of HKDF is the concatenation of the following three values:

   *  32 octets of the ephemeral X25519 public key from this packet.

   *  32 octets of the recipient public key material.

   *  32 octets of the shared secret.

   The key produced from HKDF is used to encrypt the session key with
   AES-128 key wrap, as defined in [RFC3394].

   Note that unlike Elliptic Curve Diffie-Hellman (ECDH), no checksum or
   padding are appended to the session key before key wrapping.
   Finally, note that unlike the other public key algorithms, in the
   case of a v3 PKESK packet, the symmetric algorithm ID is not
   encrypted.  Instead, it is prepended to the encrypted session key in
   plaintext.  In this case, the symmetric algorithm used MUST be AES-
   128, AES-192, or AES-256 (algorithm IDs 7, 8, or 9, respectively).

5.1.7.  Algorithm-Specific Fields for X448 Encryption

   *  56 octets representing an ephemeral X448 public key.

   *  A 1-octet size of the following fields.

   *  The 1-octet algorithm identifier, if it was passed (in the case of
      a v3 PKESK packet).

   *  The encrypted session key.

   See Section 6.2 of [RFC7748] for more details on the computation of
   the ephemeral public key and the shared secret.  HKDF [RFC5869] is
   then used with SHA512 [RFC6234] and an info parameter of "OpenPGP
   X448" and no salt.  The input of HKDF is the concatenation of the
   following three values:

   *  56 octets of the ephemeral X448 public key from this packet.

   *  56 octets of the recipient public key material.

   *  56 octets of the shared secret.

   The key produced from HKDF is used to encrypt the session key with
   AES-256 key wrap, as defined in [RFC3394].

   Note that unlike ECDH, no checksum or padding are appended to the
   session key before key wrapping.  Finally, note that unlike the other
   public key algorithms, in the case of a v3 PKESK packet, the
   symmetric algorithm ID is not encrypted.  Instead, it is prepended to
   the encrypted session key in plaintext.  In this case, the symmetric
   algorithm used MUST be AES-128, AES-192, or AES-256 (algorithm ID 7,
   8, or 9).

5.1.8.  Notes on PKESK

   An implementation MAY accept or use a Key ID of all zeros, or an
   omitted key fingerprint, to hide the intended decryption key.  In
   this case, the receiving implementation would try all available
   private keys, checking for a valid decrypted session key.  This
   format helps reduce traffic analysis of messages.

5.2.  Signature Packet (Type ID 2)

   A Signature packet describes a binding between some public key and
   some data.  The most common signatures are a signature of a file or a
   block of text and a signature that is a certification of a User ID.

   Three versions of Signature packets are defined.  Version 3 provides
   basic signature information, while versions 4 and 6 provide an
   expandable format with subpackets that can specify more information
   about the signature.

   For historical reasons, versions 1, 2, and 5 of the Signature packet
   are unspecified.  Any new Signature packet version should be
   registered in the registry established in Section 10.3.2.2.

   An implementation MUST generate a version 6 signature when signing
   with a version 6 key.  An implementation MUST generate a version 4
   signature when signing with a version 4 key.  Implementations MUST
   NOT create version 3 signatures; they MAY accept version 3
   signatures.  See Section 10.3.2.2 for more details about packet
   version correspondence between keys and signatures.

5.2.1.  Signature Types

   There are a number of possible meanings for a signature, which are
   indicated by the Signature Type ID in any given signature.  Please
   note that the vagueness of these meanings is not a flaw but rather a
   feature of the system.  Because OpenPGP places final authority for
   validity upon the receiver of a signature, it may be that one
   signer's casual act might be more rigorous than some other
   authority's positive act.  See Section 5.2.4 for detailed information
   on how to compute and verify signatures of each type.

     +======+====================================+==================+
     | ID   | Name                               | Reference        |
     +======+====================================+==================+
     | 0x00 | Binary Signature                   | Section 5.2.1.1  |
     +------+------------------------------------+------------------+
     | 0x01 | Text Signature                     | Section 5.2.1.2  |
     +------+------------------------------------+------------------+
     | 0x02 | Standalone Signature               | Section 5.2.1.3  |
     +------+------------------------------------+------------------+
     | 0x10 | Generic Certification Signature    | Section 5.2.1.4  |
     +------+------------------------------------+------------------+
     | 0x11 | Persona Certification Signature    | Section 5.2.1.5  |
     +------+------------------------------------+------------------+
     | 0x12 | Casual Certification Signature     | Section 5.2.1.6  |
     +------+------------------------------------+------------------+
     | 0x13 | Positive Certification Signature   | Section 5.2.1.7  |
     +------+------------------------------------+------------------+
     | 0x18 | Subkey Binding Signature           | Section 5.2.1.8  |
     +------+------------------------------------+------------------+
     | 0x19 | Primary Key Binding Signature      | Section 5.2.1.9  |
     +------+------------------------------------+------------------+
     | 0x1F | Direct Key Signature               | Section 5.2.1.10 |
     +------+------------------------------------+------------------+
     | 0x20 | Key Revocation Signature           | Section 5.2.1.11 |
     +------+------------------------------------+------------------+
     | 0x28 | Subkey Revocation Signature        | Section 5.2.1.12 |
     +------+------------------------------------+------------------+
     | 0x30 | Certification Revocation Signature | Section 5.2.1.13 |
     +------+------------------------------------+------------------+
     | 0x40 | Timestamp Signature                | Section 5.2.1.14 |
     +------+------------------------------------+------------------+
     | 0x50 | Third-Party Confirmation Signature | Section 5.2.1.15 |
     +------+------------------------------------+------------------+
     | 0xFF | Reserved                           | Section 5.2.1.16 |
     +------+------------------------------------+------------------+

                Table 4: OpenPGP Signature Types Registry

   The meanings of each signature type are described in the subsections
   below.

5.2.1.1.  Binary Signature (Type ID 0x00) of a Document

   This means the signer owns it, created it, or certifies that it has
   not been modified.

5.2.1.2.  Text Signature (Type ID 0x01) of a Canonical Document

   This means the signer owns it, created it, or certifies that it has
   not been modified.  The signature is calculated over the text data
   with its line endings converted to <CR><LF>.

5.2.1.3.  Standalone Signature (Type ID 0x02)

   This signature is a signature of only its own subpacket contents.  It
   is calculated identically to a signature over a zero-length binary
   document.  Version 3 Standalone signatures MUST NOT be generated and
   MUST be ignored.

5.2.1.4.  Generic Certification Signature (Type ID 0x10) of a User ID
          and Public Key Packet

   The issuer of this certification does not make any particular
   assertion as to how well the certifier has checked that the owner of
   the key is in fact the person described by the User ID.

5.2.1.5.  Persona Certification Signature (Type ID 0x11) of a User ID
          and Public Key Packet

   The issuer of this certification has not done any verification of the
   claim that the owner of this key is the User ID specified.

5.2.1.6.  Casual Certification Signature (Type ID 0x12) of a User ID and
          Public Key Packet

   The issuer of this certification has done some casual verification of
   the claim of identity.

5.2.1.7.  Positive Certification Signature (Type ID 0x13) of a User ID
          and Public Key Packet

   The issuer of this certification has done substantial verification of
   the claim of identity.

   Most OpenPGP implementations make their "key signatures" as generic
   (Type ID 0x10) certifications.  Some implementations can issue
   0x11-0x13 certifications, but few differentiate between the types.

5.2.1.8.  Subkey Binding Signature (Type ID 0x18)

   This signature is a statement by the top-level signing key,
   indicating that it owns the subkey.  This signature is calculated
   directly on the primary key and subkey, and not on any User ID or
   other packets.  A signature that binds a signing subkey MUST have an
   Embedded Signature subpacket in this binding signature that contains
   a 0x19 signature made by the signing subkey on the primary key and
   subkey.

5.2.1.9.  Primary Key Binding Signature (Type ID 0x19)

   This signature is a statement by a signing subkey, indicating that it
   is owned by the primary key.  This signature is calculated the same
   way as a Subkey Binding signature (Type ID 0x18): directly on the
   primary key and subkey, and not on any User ID or other packets.

5.2.1.10.  Direct Key Signature (Type ID 0x1F)

   This signature is calculated directly on a key.  It binds the
   information in the Signature subpackets to the key and is appropriate
   to be used for subpackets that provide information about the key,
   such as the Key Flags subpacket or the (deprecated) Revocation Key
   subpacket.  It is also appropriate for statements that non-self
   certifiers want to make about the key itself rather than the binding
   between a key and a name.

5.2.1.11.  Key Revocation Signature (Type ID 0x20)

   This signature is calculated directly on the key being revoked.  A
   revoked key is not to be used.  Only Revocation Signatures by the key
   being revoked, or by a (deprecated) Revocation Key, should be
   considered valid Revocation Signatures.

5.2.1.12.  Subkey Revocation Signature (Type ID 0x28)

   This signature is calculated directly on the primary key and the
   subkey being revoked.  A revoked subkey is not to be used.  Only
   Revocation Signatures by the top-level signature key that is bound to
   this subkey, or by a (deprecated) Revocation Key, should be
   considered valid Revocation Signatures.

5.2.1.13.  Certification Revocation Signature (Type ID 0x30)

   This signature revokes an earlier User ID certification signature
   (Type IDs 0x10 through 0x13) or Direct Key signature (Type ID 0x1F).
   It should be issued by the same key that issued the revoked signature
   or by a (deprecated) Revocation Key. The signature is computed over
   the same data as the certification that it revokes, and it should
   have a later creation date than that certification.

5.2.1.14.  Timestamp Signature (Type ID 0x40)

   This signature is only meaningful for the timestamp contained in it.

5.2.1.15.  Third-Party Confirmation Signature (Type ID 0x50)

   This signature is a signature over another OpenPGP Signature packet.
   It is analogous to a notary seal on the signed data.  A Third-Party
   Confirmation signature SHOULD include a Signature Target subpacket
   that identifies the confirmed signature.

5.2.1.16.  Reserved (Type ID 0xFF)

   An implementation MUST NOT create any signature with this type and
   MUST NOT validate any signature made with this type.  See
   Section 5.2.4.1 for more details.

5.2.2.  Version 3 Signature Packet Format

   The body of a version 3 Signature packet contains:

   *  A 1-octet version number with value 3.

   *  A 1-octet length of the following hashed material; it MUST be 5:

      -  A 1-octet Signature Type ID.

      -  A 4-octet creation time.

   *  An 8-octet Key ID of the signer.

   *  A 1-octet public key algorithm.

   *  A 1-octet hash algorithm.

   *  A 2-octet field holding left 16 bits of the signed hash value.

   *  One or more MPIs comprising the signature.  This portion is
      algorithm specific, as described below.

   The concatenation of the data to be signed, the signature type, and
   the creation time from the Signature packet (5 additional octets) is
   hashed.  The resulting hash value is used in the signature algorithm.
   The high 16 bits (first two octets) of the hash are included in the
   Signature packet to provide a way to reject some invalid signatures
   without performing a signature verification.

   Algorithm-specific fields for RSA signatures:

   *  MPI of RSA signature value m^d mod n.

   Algorithm-specific fields for DSA signatures:

   *  MPI of DSA value r.

   *  MPI of DSA value s.

   The signature calculation is based on a hash of the signed data, as
   described above.  The details of the calculation are different for
   DSA signatures than for RSA signatures; see Sections 5.2.3.1 and
   5.2.3.2.

5.2.3.  Versions 4 and 6 Signature Packet Formats

   The body of a version 4 or version 6 Signature packet contains:

   *  A 1-octet version number.  This is 4 for version 4 signatures and
      6 for version 6 signatures.

   *  A 1-octet Signature Type ID.

   *  A 1-octet public key algorithm.

   *  A 1-octet hash algorithm.

   *  A scalar octet count for the hashed subpacket data that follows
      this field.  For a version 4 signature, this is a 2-octet field.
      For a version 6 signature, this is a 4-octet field.  Note that
      this is the length in octets of all of the hashed subpackets; an
      implementation's pointer incremented by this number will skip over
      the hashed subpackets.

   *  A hashed subpacket data set (zero or more subpackets).

   *  A scalar octet count for the unhashed subpacket data that follows
      this field.  For a version 4 signature, this is a 2-octet field.
      For a version 6 signature, this is a 4-octet field.  Note that
      this is the length in octets of all of the unhashed subpackets; an
      implementation's pointer incremented by this number will skip over
      the unhashed subpackets.

   *  An unhashed subpacket data set (zero or more subpackets).

   *  A 2-octet field holding the left 16 bits of the signed hash value.

   *  Only for version 6 signatures, a variable-length field containing:

      -  A 1-octet salt size.  The value MUST match the value defined
         for the hash algorithm as specified in Table 23.

      -  The salt, which is a random value of the specified size.

   *  One or more MPIs comprising the signature.  This portion is
      algorithm specific.

5.2.3.1.  Algorithm-Specific Fields for RSA Signatures

   *  MPI of RSA signature value m^d mod n.

   With RSA signatures, the hash value is encoded using PKCS#1 encoding
   type EMSA-PKCS1-v1_5, as described in Section 9.2 of [RFC8017] (see
   also Section 12.1.3).  This requires inserting the hash value as an
   octet string into an ASN.1 structure.  The object identifier (OID)
   for the hash algorithm itself is also included in the structure; see
   the OIDs in Table 24.

5.2.3.2.  Algorithm-Specific Fields for DSA or ECDSA Signatures

   *  MPI of DSA or ECDSA value r.

   *  MPI of DSA or ECDSA value s.

   A version 3 signature MUST NOT be created and MUST NOT be used with
   the Elliptic Curve Digital Signature Algorithm (ECDSA).

   A DSA signature MUST use a hash algorithm with a digest size of at
   least the number of bits of q, the group generated by the DSA key's
   generator value.

   If the output size of the chosen hash is larger than the number of
   bits of q, the hash result is truncated to fit by taking the number
   of leftmost bits equal to the number of bits of q.  This (possibly
   truncated) hash function result is treated as a number and used
   directly in the DSA signature algorithm.

   An ECDSA signature MUST use a hash algorithm with a digest size of at
   least the curve's "fsize" value (see Section 9.2), except in the case
   of NIST P-521, for which at least a 512-bit hash algorithm MUST be
   used.

5.2.3.3.  Algorithm-Specific Fields for EdDSALegacy Signatures
          (Deprecated)

   *  Two MPI-encoded values, whose contents and formatting depend on
      the choice of curve used (see Section 9.2.1).

   A version 3 signature MUST NOT be created and MUST NOT be used with
   EdDSALegacy.

   An EdDSALegacy signature MUST use a hash algorithm with a digest size
   of at least the curve's "fsize" value (see Section 9.2).  A verifying
   implementation MUST reject any EdDSALegacy signature that uses a hash
   algorithm with a smaller digest size.

5.2.3.3.1.  Algorithm-Specific Fields for Ed25519Legacy Signatures
            (Deprecated)

   The two MPIs for Ed25519Legacy represent the octet strings R and S of
   the Edwards-curve Digital Signature Algorithm (EdDSA) described in
   [RFC8032].

   *  MPI of an EC point R, represented as a (non-prefixed) native
      (little-endian) octet string up to 32 octets.

   *  MPI of EdDSA value S, also in (non-prefixed) native (little-
      endian) format with a length up to 32 octets.

   Ed25519Legacy MUST NOT be used in Signature packets version 6 or
   above.

5.2.3.4.  Algorithm-Specific Fields for Ed25519 Signatures

   *  64 octets of the native signature.

   For more details, see Section 12.7.

   A version 3 signature MUST NOT be created and MUST NOT be used with
   Ed25519.

   An Ed25519 signature MUST use a hash algorithm with a digest size of
   at least 256 bits.  A verifying implementation MUST reject any
   Ed25519 signature that uses a hash algorithm with a smaller digest
   size.

5.2.3.5.  Algorithm-Specific Fields for Ed448 Signatures

   *  114 octets of the native signature.

   For more details, see Section 12.7.

   A version 3 signature MUST NOT be created and MUST NOT be used with
   Ed448.

   An Ed448 signature MUST use a hash algorithm with a digest size of at
   least 512 bits.  A verifying implementation MUST reject any Ed448
   signature that uses a hash algorithm with a smaller digest size.

5.2.3.6.  Notes on Signatures

   The concatenation of the data being signed, the signature data from
   the version number through the hashed subpacket data (inclusive), and
   (for signature versions later than 3) a 6-octet trailer (see
   Section 5.2.4) is hashed.  The resulting hash value is what is
   signed.  The high 16 bits (first two octets) of the hash are included
   in the Signature packet to provide a way to reject some invalid
   signatures without performing a signature verification.  When
   verifying a version 6 signature, an implementation MUST reject the
   signature if these octets do not match the first two octets of the
   computed hash.

   There are two fields consisting of Signature subpackets.  The first
   field is hashed with the rest of the signature data, while the second
   is not hashed into the signature.  The second set of subpackets (the
   "unhashed section") is not cryptographically protected by the
   signature and should include only advisory information.  See
   Section 13.13 for more information.

   The differences between a version 4 and version 6 signature are two-
   fold: first, a version 6 signature increases the width of the fields
   that indicate the size of the hashed and unhashed subpackets, making
   it possible to include significantly more data in subpackets.
   Second, the hash is salted with random data (see Section 13.2).

   The algorithms for converting the hash function result to a signature
   are described in Section 5.2.4.

5.2.3.7.  Signature Subpacket Specification

   A subpacket data set consists of zero or more Signature subpackets.
   In Signature packets, the subpacket data set is preceded by a 2-octet
   (for version 4 signatures) or 4-octet (for version 6 signatures)
   scalar count of the length in octets of all the subpackets.  A
   pointer incremented by this number will skip over the subpacket data
   set.

   Each subpacket consists of a subpacket header and a body.  The header
   consists of:

   *  The encoded subpacket length (1, 2, or 5 octets).

   *  The encoded Subpacket Type ID (1 octet).

   *  The subpacket-specific data.

   The subpacket length field covers the encoded Subpacket Type ID and
   the subpacket-specific data, and it does not include the subpacket
   length field itself.  It is encoded similarly to a 1-octet, 2-octet,
   or 5-octet OpenPGP format packet header.  The encoded subpacket
   length can be decoded as follows:

   if the 1st octet <  192, then
       lengthOfLength = 1
       subpacketLen = 1st_octet

   if the 1st octet >= 192 and < 255, then
       lengthOfLength = 2
       subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

   if the 1st octet = 255, then
       lengthOfLength = 5
       subpacket length = [4-octet scalar starting at 2nd_octet]

   Bit 7 of the encoded Subpacket Type ID is the "critical" bit.  If
   set, it denotes that the subpacket is one that is critical for the
   evaluator of the signature to recognize.  If a subpacket is
   encountered that is marked critical but is unknown to the evaluating
   implementation, the evaluator SHOULD consider the signature to be in
   error.

   An implementation SHOULD ignore any non-critical subpacket of a type
   that it does not recognize.

   An evaluator may "recognize" a subpacket but not implement it.  The
   purpose of the critical bit is to allow the signer to tell an
   evaluator that it would prefer a new, unknown feature to generate an
   error rather than being ignored.

   The other bits of the encoded Subpacket Type ID (i.e., bits 6-0)
   contain the Subpacket Type ID.

   The following signature subpackets are defined:

        +=========+===========================+==================+
        |      ID | Description               | Reference        |
        +=========+===========================+==================+
        |       0 | Reserved                  |                  |
        +---------+---------------------------+------------------+
        |       1 | Reserved                  |                  |
        +---------+---------------------------+------------------+
        |       2 | Signature Creation Time   | Section 5.2.3.11 |
        +---------+---------------------------+------------------+
        |       3 | Signature Expiration Time | Section 5.2.3.18 |
        +---------+---------------------------+------------------+
        |       4 | Exportable Certification  | Section 5.2.3.19 |
        +---------+---------------------------+------------------+
        |       5 | Trust Signature           | Section 5.2.3.21 |
        +---------+---------------------------+------------------+
        |       6 | Regular Expression        | Section 5.2.3.22 |
        +---------+---------------------------+------------------+
        |       7 | Revocable                 | Section 5.2.3.20 |
        +---------+---------------------------+------------------+
        |       8 | Reserved                  |                  |
        +---------+---------------------------+------------------+
        |       9 | Key Expiration Time       | Section 5.2.3.13 |
        +---------+---------------------------+------------------+
        |      10 | Placeholder for backward  |                  |
        |         | compatibility             |                  |
        +---------+---------------------------+------------------+
        |      11 | Preferred Symmetric       | Section 5.2.3.14 |
        |         | Ciphers for v1 SEIPD      |                  |
        +---------+---------------------------+------------------+
        |      12 | Revocation Key            | Section 5.2.3.23 |
        |         | (deprecated)              |                  |
        +---------+---------------------------+------------------+
        |   13-15 | Reserved                  |                  |
        +---------+---------------------------+------------------+
        |      16 | Issuer Key ID             | Section 5.2.3.12 |
        +---------+---------------------------+------------------+
        |   17-19 | Reserved                  |                  |
        +---------+---------------------------+------------------+
        |      20 | Notation Data             | Section 5.2.3.24 |
        +---------+---------------------------+------------------+
        |      21 | Preferred Hash Algorithms | Section 5.2.3.16 |
        +---------+---------------------------+------------------+
        |      22 | Preferred Compression     | Section 5.2.3.17 |
        |         | Algorithms                |                  |
        +---------+---------------------------+------------------+
        |      23 | Key Server Preferences    | Section 5.2.3.25 |
        +---------+---------------------------+------------------+
        |      24 | Preferred Key Server      | Section 5.2.3.26 |
        +---------+---------------------------+------------------+
        |      25 | Primary User ID           | Section 5.2.3.27 |
        +---------+---------------------------+------------------+
        |      26 | Policy URI                | Section 5.2.3.28 |
        +---------+---------------------------+------------------+
        |      27 | Key Flags                 | Section 5.2.3.29 |
        +---------+---------------------------+------------------+
        |      28 | Signer's User ID          | Section 5.2.3.30 |
        +---------+---------------------------+------------------+
        |      29 | Reason for Revocation     | Section 5.2.3.31 |
        +---------+---------------------------+------------------+
        |      30 | Features                  | Section 5.2.3.32 |
        +---------+---------------------------+------------------+
        |      31 | Signature Target          | Section 5.2.3.33 |
        +---------+---------------------------+------------------+
        |      32 | Embedded Signature        | Section 5.2.3.34 |
        +---------+---------------------------+------------------+
        |      33 | Issuer Fingerprint        | Section 5.2.3.35 |
        +---------+---------------------------+------------------+
        |      34 | Reserved                  |                  |
        +---------+---------------------------+------------------+
        |      35 | Intended Recipient        | Section 5.2.3.36 |
        |         | Fingerprint               |                  |
        +---------+---------------------------+------------------+
        |      37 | Reserved (Attested        |                  |
        |         | Certifications)           |                  |
        +---------+---------------------------+------------------+
        |      38 | Reserved (Key Block)      |                  |
        +---------+---------------------------+------------------+
        |      39 | Preferred AEAD            | Section 5.2.3.15 |
        |         | Ciphersuites              |                  |
        +---------+---------------------------+------------------+
        | 100-110 | Private or Experimental   |                  |
        |         | Use                       |                  |
        +---------+---------------------------+------------------+

           Table 5: OpenPGP Signature Subpacket Types Registry

   Implementations SHOULD implement the four preferred algorithm
   subpackets (11, 21, 22, and 39), as well as the "Features" (30) and
   "Reason for Revocation" (29) subpackets.  To avoid surreptitious
   forwarding (see Section 13.12), implementations SHOULD also implement
   the "Intended Recipients Fingerprint" (35) subpacket.  Note that if
   an implementation chooses not to implement some of the preferences
   subpackets, it MUST default to the mandatory-to-implement algorithms
   to ensure interoperability.  An encrypting implementation that does
   not implement the "Features" (30) subpacket SHOULD select the type of
   encrypted data format based on the versions of the recipient keys or
   external inference (see Section 13.7 for more details).

5.2.3.8.  Signature Subpacket Types

   A number of subpackets are currently defined for OpenPGP signatures.
   Some subpackets apply to the signature itself and some are attributes
   of the key.  Subpackets that are found on a self-signature are placed
   on a certification made by the key itself.  Note that a key may have
   more than one User ID and thus may have more than one self-signature
   and differing subpackets.

   A subpacket may be found in either the hashed or the unhashed
   subpacket sections of a signature.  If a subpacket is not hashed,
   then the information in it cannot be considered definitive because it
   is not covered by the cryptographic signature.  See Section 13.13 for
   more discussion about hashed and unhashed subpackets.

5.2.3.9.  Notes on Subpackets

   It is certainly possible for a signature to contain conflicting
   information in subpackets.  For example, a signature may contain
   multiple copies of a preference or multiple expiration times.  In
   most cases, an implementation SHOULD use the last subpacket in the
   hashed section of the signature, but it MAY use any conflict
   resolution scheme that makes more sense.  Please note that conflict
   resolution is intentionally left to the implementer; most conflicts
   are simply syntax errors, and the ambiguous language here allows a
   receiver to be generous in what they accept, while putting pressure
   on a creator to be stingy in what they generate.

   Some apparent conflicts may actually make sense.  For example,
   suppose a keyholder has a version 3 key and a version 4 key that
   share the same RSA key material.  Either of these keys can verify a
   signature created by the other, and it may be reasonable for a
   signature to contain an Issuer Key ID subpacket (Section 5.2.3.12)
   for each key, as a way of explicitly tying those keys to the
   signature.

5.2.3.10.  Notes on Self-Signatures

   A self-signature is a binding signature made by the key to which the
   signature refers.  There are three types of self-signatures: the
   certification signatures (Type IDs 0x10-0x13), the Direct Key
   signature (Type ID 0x1F), and the Subkey Binding signature (Type ID
   0x18).  A cryptographically valid self-signature should be accepted
   from any primary key, regardless of what Key Flags (Section 5.2.3.29)
   apply to the primary key.  In particular, a primary key does not need
   to have 0x01 set in the first octet of the Key Flags order to make a
   valid self-signature.

   For certification self-signatures, each User ID MAY have a self-
   signature and thus different subpackets in those self-signatures.
   For Subkey Binding signatures, each subkey MUST have a self-
   signature.  Subpackets that appear in a certification self-signature
   apply to the User ID, and subpackets that appear in the subkey self-
   signature apply to the subkey.  Lastly, subpackets on the Direct Key
   signature apply to the entire key.

   An implementation should interpret a self-signature's preference
   subpackets as narrowly as possible.  For example, suppose a key has
   two user names, Alice and Bob. Suppose that Alice prefers the AEAD
   ciphersuite AES-256 with OCB, and Bob prefers Camellia-256 with GCM.
   If the implementation locates this key via Alice's name, then the
   preferred AEAD ciphersuite is AES-256 with OCB; if the implementation
   locates the key via Bob's name, then the preferred algorithm is
   Camellia-256 with GCM.  If the key is located by Key ID, the
   algorithm of the Primary User ID of the key provides the preferred
   AEAD ciphersuite.

   Revoking a self-signature or allowing it to expire has a semantic
   meaning that varies with the signature type.  Revoking the self-
   signature on a User ID effectively retires that user name.  The self-
   signature is a statement, "My name X is tied to my signing key K",
   and it is corroborated by other users' certifications.  If another
   user revokes their certification, they are effectively saying that
   they no longer believe that name and that key are tied together.
   Similarly, if the users themselves revoke their self-signature, then
   the users no longer go by that name, no longer have that email
   address, etc.  Revoking a binding signature effectively retires that
   subkey.  Revoking a Direct Key signature cancels that signature.
   Please see Section 5.2.3.31 for more relevant details.

   Since a self-signature contains important information about the key's
   use, an implementation SHOULD allow the user to rewrite the self-
   signature and important information in it, such as preferences and
   key expiration.

   When an implementation imports a secret key, it SHOULD verify that
   the key's internal self-signatures do not advertise features or
   algorithms that the implementation doesn't support.  If an
   implementation observes such a mismatch, it SHOULD warn the user and
   offer to create new self-signatures that advertise the actual set of
   features and algorithms supported by the implementation.

   An implementation that encounters multiple self-signatures on the
   same object MUST select the most recent valid self-signature and
   ignore all other self-signatures.

   By convention, a version 4 key stores information about the primary
   Public Key (key flags, key expiration, etc.) and the Transferable
   Public Key as a whole (features, algorithm preferences, etc.) in a
   User ID self-signature of type 0x10 or 0x13.  To use a version 4 key,
   some implementations require at least one User ID with a valid self-
   signature to be present.  For this reason, it is RECOMMENDED to
   include at least one User ID with a self-signature in version 4 keys.

   For version 6 keys, it is RECOMMENDED to store information about the
   primary Public Key as well as the Transferable Public Key as a whole
   (key flags, key expiration, features, algorithm preferences, etc.) in
   a Direct Key signature (Type ID 0x1F) over the Public Key, instead of
   placing that information in a User ID self-signature.  An
   implementation MUST ensure that a valid Direct Key signature is
   present before using a version 6 key.  This prevents certain attacks
   where an adversary strips a self-signature specifying a Key
   Expiration Time or certain preferences.

   An implementation SHOULD NOT require a User ID self-signature to be
   present in order to consume or use a key, unless the particular use
   is contingent on the keyholder identifying themselves with the
   textual label in the User ID.  For example, when refreshing a key to
   learn about changes in expiration, advertised features, algorithm
   preferences, revocation, subkey rotation, and so forth, there is no
   need to require a User ID self-signature.  On the other hand, when
   verifying a signature over an email message, an implementation MAY
   choose to only accept a signature from a key that has a valid self-
   signature over a User ID that matches the message's From: header, as
   a way to avoid a signature transplant attack.

5.2.3.11.  Signature Creation Time

   (4-octet time field)

   The time the signature was made.

   This subpacket MUST be present in the hashed area.

   When generating this subpacket, it SHOULD be marked as critical.

5.2.3.12.  Issuer Key ID

   (8-octet Key ID)

   The OpenPGP Key ID of the key issuing the signature.  If the version
   of that key is greater than 4, this subpacket MUST NOT be included in
   the signature.  For these keys, consider the Issuer Fingerprint
   subpacket (Section 5.2.3.35) instead.

   Note: in previous versions of this specification, this subpacket was
   simply known as the "Issuer" subpacket.

5.2.3.13.  Key Expiration Time

   (4-octet time field)

   The validity period of the key.  This is the number of seconds after
   the key creation time that the key expires.  For a direct or
   certification self-signature, the key creation time is that of the
   primary key.  For a Subkey Binding signature, the key creation time
   is that of the subkey.  If this is not present or has a value of
   zero, the key never expires.  This is found only on a self-signature.

   When an implementation generates this subpacket, it SHOULD be marked
   as critical.

5.2.3.14.  Preferred Symmetric Ciphers for v1 SEIPD

   (array of 1-octet values)

   A series of Symmetric Cipher Algorithm IDs indicating how the
   keyholder prefers to receive the version 1 Symmetrically Encrypted
   and Integrity Protected Data packet (Section 5.13.1).  The subpacket
   body is an ordered list of octets with the most preferred listed
   first.  It is assumed that only the algorithms listed are supported
   by the recipient's implementation.  Algorithm IDs are defined in
   Section 9.3.  This is only found on a self-signature.

   When generating a v2 SEIPD packet, this preference list is not
   relevant.  See Section 5.2.3.15 instead.

5.2.3.15.  Preferred AEAD Ciphersuites

   (array of pairs of octets indicating Symmetric Cipher and AEAD
   algorithms)

   A series of paired algorithm IDs indicating how the keyholder prefers
   to receive the version 2 Symmetrically Encrypted and Integrity
   Protected Data packet (Section 5.13.2).  Each pair of octets
   indicates a combination of a symmetric cipher and an AEAD mode that
   the keyholder prefers to use.  The Symmetric Cipher Algorithm ID
   precedes the AEAD algorithm ID in each pair.  The subpacket body is
   an ordered list of pairs of octets with the most preferred algorithm
   combination listed first.

   It is assumed that only the combinations of algorithms listed are
   supported by the recipient's implementation, with the exception of
   the mandatory-to-implement combination of AES-128 and OCB.  If
   AES-128 and OCB are not found in the subpacket, it is implicitly
   listed at the end.

   AEAD algorithm IDs are listed in Section 9.6.  Symmetric Cipher
   Algorithm IDs are listed in Section 9.3.

   For example, a subpacket containing the six octets

   09 02 09 03 13 02

   indicates that the keyholder prefers to receive v2 SEIPD using
   AES-256 with OCB, then AES-256 with GCM, then Camellia-256 with OCB,
   and finally the implicit AES-128 with OCB.

   Note that support for the version 2 Symmetrically Encrypted and
   Integrity Protected Data packet (Section 5.13.2) in general is
   indicated by a Features Flag (Section 5.2.3.32).

   This subpacket is only found on a self-signature.

   When generating a v1 SEIPD packet, this preference list is not
   relevant.  See Section 5.2.3.14 instead.

5.2.3.16.  Preferred Hash Algorithms

   (array of 1-octet values)

   Message digest algorithm IDs that indicate which algorithms the
   keyholder prefers to receive.  Like the Preferred AEAD Ciphersuites,
   the list is ordered.  Algorithm IDs are defined in Section 9.5.  This
   is only found on a self-signature.

5.2.3.17.  Preferred Compression Algorithms

   (array of 1-octet values)

   Compression algorithm IDs that indicate which algorithms the
   keyholder prefers to use.  Like the Preferred AEAD Ciphersuites, the
   list is ordered.  Algorithm IDs are defined in Section 9.4.  A zero,
   or the absence of this subpacket, denotes that uncompressed data is
   preferred; the keyholder's implementation might have no compression
   support available.  This is only found on a self-signature.

5.2.3.18.  Signature Expiration Time

   (4-octet time field)

   The validity period of the signature.  This is the number of seconds
   after the Signature Creation Time that the signature expires.  If
   this is not present or has a value of zero, it never expires.

   When an implementation generates this subpacket, it SHOULD be marked
   as critical.

5.2.3.19.  Exportable Certification

   (1 octet of exportability, 0 for not, 1 for exportable)

   This subpacket denotes whether a certification signature is
   "exportable"; it is intended for use by users other than the
   signature's issuer.  The packet body contains a Boolean flag
   indicating whether the signature is exportable.  If this packet is
   not present, the certification is exportable; it is equivalent to a
   flag containing a 1.

   Non-exportable, or "local", certifications are signatures made by a
   user to mark a key as valid within that user's implementation only.

   Thus, when an implementation prepares a user's copy of a key for
   transport to another user (this is the process of "exporting" the
   key), any local certification signatures are deleted from the key.

   The receiver of a transported key "imports" it and likewise trims any
   local certifications.  In normal operation, there won't be any local
   certifications, assuming the import is performed on an exported key.
   However, there are instances where this can reasonably happen.  For
   example, if an implementation allows keys to be imported from a key
   database in addition to an exported key, then this situation can
   arise.

   Some implementations do not represent the interest of a single user
   (for example, a key server).  Such implementations always trim local
   certifications from any key they handle.

   When an implementation generates this subpacket and denotes the
   signature as non-exportable, the subpacket MUST be marked as
   critical.

5.2.3.20.  Revocable

   (1 octet of revocability, 0 for not, 1 for revocable)

   A Signature's revocability status.  The packet body contains a
   Boolean flag indicating whether the signature is revocable.
   Signatures that are not revocable ignore any later Revocation
   Signatures.  They represent the signer's commitment that its
   signature cannot be revoked for the life of its key.  If this packet
   is not present, the signature is revocable.

5.2.3.21.  Trust Signature

   (1 octet "level" (depth), 1 octet of trust amount)

   The signer asserts that the key is not only valid but also
   trustworthy at the specified level.  Level 0 has the same meaning as
   an ordinary validity signature.  Level 1 means that the signed key is
   asserted to be a valid trusted introducer, with the 2nd octet of the
   body specifying the degree of trust.  Level 2 means that the signed
   key is asserted to be trusted to issue level 1 Trust Signatures; that
   is, the signed key is a "meta introducer".  Generally, a level n
   Trust Signature asserts that a key is trusted to issue level n-1
   Trust Signatures.  The trust amount is in a range from 0-255,
   interpreted such that values less than 120 indicate partial trust and
   values of 120 or greater indicate complete trust.  Implementations
   SHOULD emit values of 60 for partial trust and 120 for complete
   trust.

5.2.3.22.  Regular Expression

   (null-terminated UTF-8 encoded Regular Expression)

   Used in conjunction with Trust Signature packets (of level > 0) to
   limit the scope of trust that is extended.  Only signatures by the
   target key on User IDs that match the Regular Expression in the body
   of this packet have trust extended by the Trust Signature subpacket.
   The Regular Expression uses the same syntax as Henry Spencer's
   "almost public domain" Regular Expression [REGEX] package.  A
   description of the syntax is found in Section 8.  The Regular
   Expression matches (or does not match) a sequence of UTF-8-encoded
   Unicode characters from User IDs.  The expression itself is also
   written with UTF-8 characters.

   For historical reasons, this subpacket includes a null character (an
   octet with value zero) after the Regular Expression.  When an
   implementation parses a Regular Expression subpacket, it MUST remove
   this octet; if it is not present, it MUST reject the subpacket (i.e.,
   ignore the subpacket if it's non-critical and reject the signature if
   it's critical).  When an implementation generates a Regular
   Expression subpacket, it MUST include the null terminator.

   When generating this subpacket, it SHOULD be marked as critical.

5.2.3.23.  Revocation Key (Deprecated)

   (1 octet of class, 1 octet of public key algorithm ID, 20 octets of
   version 4 fingerprint)

   This mechanism is deprecated.  Applications MUST NOT generate such a
   subpacket.

   An application that wants the functionality of delegating revocation
   can use an escrowed Revocation Signature.  See Section 13.9 for more
   details.

   The remainder of this section describes how some implementations
   attempt to interpret this deprecated subpacket.

   This packet was intended to authorize the specified key to issue
   Revocation Signatures for this key.  The class octet must have bit
   0x80 set.  If bit 0x40 is set, it means the revocation information is
   sensitive.  Other bits are for future expansion to other kinds of
   authorizations.  This is only found on a Direct Key self-signature
   (Type ID 0x1F).  The use on other types of self-signatures is
   unspecified.

   If the "sensitive" flag is set, the keyholder feels this subpacket
   contains private trust information that describes a real-world
   sensitive relationship.  If this flag is set, implementations SHOULD
   NOT export this signature to other users except in cases where the
   data needs to be available, i.e., when the signature is being sent to
   the designated revoker or when it is accompanied by a Revocation
   Signature from that revoker.  Note that it may be appropriate to
   isolate this subpacket within a separate signature so that it is not
   combined with other subpackets that need to be exported.

5.2.3.24.  Notation Data

   (4 octets of flags, 2 octets of name length (M), 2 octets of value
   length (N), M octets of name data, N octets of value data)

   This subpacket describes a "notation" on the signature that the
   issuer wishes to make.  The notation has a name and a value, each of
   which are strings of octets.  There may be more than one notation in
   a signature.  Notations can be used for any extension the issuer of
   the signature cares to make.  The "flags" field holds 4 octets of
   flags.

   All undefined flags MUST be zero.  Defined flags are as follows:

        +=======================+================+================+
        | Flag Position         | Shorthand      | Description    |
        +=======================+================+================+
        | 0x80000000 (first bit | human-readable | Notation value |
        | of the first octet)   |                | is UTF-8 text  |
        +-----------------------+----------------+----------------+

             Table 6: OpenPGP Signature Notation Data Subpacket
                          Notation Flags Registry

   Notation names are arbitrary strings encoded in UTF-8.  They reside
   in two namespaces: the IETF namespace and the user namespace.

   The IETF namespace is registered with IANA.  These names MUST NOT
   contain the "@" character (0x40).  This is a tag for the user
   namespace.

              +===============+===========+================+
              | Notation Name | Data Type | Allowed Values |
              +===============+===========+================+
              | No registrations at this time.             |
              +============================================+

                 Table 7: OpenPGP Signature Notation Data
                         Subpacket Types Registry

   This registry is initially empty.

   Names in the user namespace consist of a UTF-8 string tag followed by
   "@", followed by a DNS domain name.  Note that the tag MUST NOT
   contain an "@" character.  For example, the "sample" tag used by
   Example Corporation could be "sample@example.com".

   Names in a user space are owned and controlled by the owners of that
   domain.  Obviously, it's bad form to create a new name in a DNS space
   that you don't own.

   Since the user namespace is in the form of an email address,
   implementers MAY wish to arrange for that address to reach a person
   who can be consulted about the use of the named tag.  Note that due
   to UTF-8 encoding, not all valid user space name tags are valid email
   addresses.

   If there is a critical notation, the criticality applies to that
   specific notation and not to notations in general.

5.2.3.25.  Key Server Preferences

   (N octets of flags)

   This is a list of 1-bit flags that indicates preferences that the
   keyholder has about how the key is handled on a key server.  All
   undefined flags MUST be zero.

    +=========+===========+===========================================+
    | Flag    | Shorthand | Definition                                |
    +=========+===========+===========================================+
    | 0x80... | No-modify | The keyholder requests that this key only |
    |         |           | be modified or updated by the keyholder   |
    |         |           | or an administrator of the key server.    |
    +---------+-----------+-------------------------------------------+

           Table 8: OpenPGP Key Server Preference Flags Registry

   This is found only on a self-signature.

5.2.3.26.  Preferred Key Server

   (String)

   This is a URI of a key server that the keyholder prefers be used for
   updates.  Note that keys with multiple User IDs can have a Preferred
   Key Server for each User ID.  Note also that since this is a URI, the
   key server can actually be a copy of the key retrieved by https, ftp,
   http, etc.

5.2.3.27.  Primary User ID

   (1 octet, Boolean)

   This is a flag in a User ID's self-signature that states whether this
   User ID is the main User ID for this key.  It is reasonable for an
   implementation to resolve ambiguities in preferences, for example, by
   referring to the Primary User ID.  If this flag is absent, its value
   is zero.  If more than one User ID in a key is marked as primary, the
   implementation may resolve the ambiguity in any way it sees fit, but
   it is RECOMMENDED that priority be given to the User ID with the most
   recent self-signature.

   When appearing on a self-signature on a User ID packet, this
   subpacket applies only to User ID packets.  When appearing on a self-
   signature on a User Attribute packet, this subpacket applies only to
   User Attribute packets.  That is, there are two different and
   independent "primaries" -- one for User IDs and one for User
   Attributes.

5.2.3.28.  Policy URI

   (String)

   This subpacket contains a URI of a document that describes the policy
   under which the signature was issued.

5.2.3.29.  Key Flags

   (N octets of flags)

   This subpacket contains a list of binary flags that hold information
   about a key.  It is a string of octets, and an implementation MUST
   NOT assume a fixed size, so that it can grow over time.  If a list is
   shorter than an implementation expects, the unstated flags are
   considered to be zero.  The defined flags are as follows:

   +===========+======================================================+
   | Flag      | Definition                                           |
   +===========+======================================================+
   | 0x01...   | This key may be used to make User ID certifications  |
   |           | (Signature Type IDs 0x10-0x13) or Direct Key         |
   |           | signatures (Signature Type ID 0x1F) over other keys. |
   +-----------+------------------------------------------------------+
   | 0x02...   | This key may be used to sign data.                   |
   +-----------+------------------------------------------------------+
   | 0x04...   | This key may be used to encrypt communications.      |
   +-----------+------------------------------------------------------+
   | 0x08...   | This key may be used to encrypt storage.             |
   +-----------+------------------------------------------------------+
   | 0x10...   | The private component of this key may have been      |
   |           | split by a secret-sharing mechanism.                 |
   +-----------+------------------------------------------------------+
   | 0x20...   | This key may be used for authentication.             |
   +-----------+------------------------------------------------------+
   | 0x80...   | The private component of this key may be in the      |
   |           | possession of more than one person.                  |
   +-----------+------------------------------------------------------+
   | 0x0004... | Reserved (ADSK)                                      |
   +-----------+------------------------------------------------------+
   | 0x0008... | Reserved (timestamping)                              |
   +-----------+------------------------------------------------------+

                   Table 9: OpenPGP Key Flags Registry

   Usage notes:

   The flags in this packet may appear in self-signatures or in
   certification signatures.  They mean different things depending on
   who is making the statement.  For example, a certification signature
   that has the "sign data" flag is stating that the certification is
   for that use.  On the other hand, the "communications encryption"
   flag in a self-signature is stating a preference that a given key be
   used for communications.  However, note that determining what is
   "communications" and what is "storage" is a thorny issue.  This
   decision is left wholly up to the implementation; the authors of this
   document do not claim any special wisdom on the issue and realize
   that accepted opinion may change.

   The "split key" (0x10) and "group key" (0x80) flags are placed on a
   self-signature only; they are meaningless on a certification
   signature.  They SHOULD be placed only on a Direct Key signature
   (Type ID 0x1F) or a Subkey Binding signature (Type ID 0x18), one that
   refers to the key the flag applies to.

   When an implementation generates this subpacket, it SHOULD be marked
   as critical.

5.2.3.30.  Signer's User ID

   (String)

   This subpacket allows a keyholder to state which User ID is
   responsible for the signing.  Many keyholders use a single key for
   different purposes, such as business communications as well as
   personal communications.  This subpacket allows such a keyholder to
   state which of their roles is making a signature.

   This subpacket is not appropriate to use to refer to a User Attribute
   packet.

5.2.3.31.  Reason for Revocation

   (1 octet of revocation code, N octets of reason string)

   This subpacket is used only in Key Revocation and Certification
   Revocation signatures.  It describes the reason why the key or
   certification was revoked.

   The first octet contains a machine-readable code that denotes the
   reason for the revocation:

           +=========+========================================+
           |    Code | Reason                                 |
           +=========+========================================+
           |       0 | No reason specified (Key Revocation or |
           |         | Certification Revocation signatures)   |
           +---------+----------------------------------------+
           |       1 | Key is superseded (Key Revocation      |
           |         | signatures)                            |
           +---------+----------------------------------------+
           |       2 | Key material has been compromised (Key |
           |         | Revocation signatures)                 |
           +---------+----------------------------------------+
           |       3 | Key is retired and no longer used (Key |
           |         | Revocation signatures)                 |
           +---------+----------------------------------------+
           |      32 | User ID information is no longer valid |
           |         | (Certification Revocation signatures)  |
           +---------+----------------------------------------+
           | 100-110 | Private Use                            |
           +---------+----------------------------------------+

                 Table 10: OpenPGP Reason for Revocation
                       (Revocation Octet) Registry

   Following the revocation code is a string of octets that gives
   information about the Reason for Revocation in human-readable form
   (UTF-8).  The string may be null (of zero length).  The length of the
   subpacket is the length of the reason string plus one.  An
   implementation SHOULD implement this subpacket, include it in all
   Revocation Signatures, and interpret revocations appropriately.
   There are important semantic differences between the reasons, and
   there are thus important reasons for revoking signatures.

   If a key has been revoked because of a compromise, all signatures
   created by that key are suspect.  However, if it was merely
   superseded or retired, old signatures are still valid.  If the
   revoked signature is the self-signature for certifying a User ID, a
   revocation denotes that that user name is no longer in use.  Such a
   signature revocation SHOULD include a Reason for Revocation subpacket
   containing code 32.

   Note that any certification may be revoked, including a certification
   on some other person's key.  There are many good reasons for revoking
   a certification signature, such as the case where the keyholder
   leaves the employ of a business with an email address.  A revoked
   certification is no longer a part of validity calculations.

5.2.3.32.  Features

   (N octets of flags)

   The Features subpacket denotes which advanced OpenPGP features a
   user's implementation supports.  This is so that as features are
   added to OpenPGP that cannot be backward compatible, a user can state
   that they can use that feature.  The flags are single bits that
   indicate that a given feature is supported.

   This subpacket is similar to a preferences subpacket and only appears
   in a self-signature.

   An implementation SHOULD NOT use a feature listed when sending to a
   user who does not state that they can use it, unless the
   implementation can infer support for the feature from another
   implementation-dependent mechanism.

   Defined features are as follows:

   First octet:

       +=========+=====================================+===========+
       | Feature | Definition                          | Reference |
       +=========+=====================================+===========+
       | 0x01... | Version 1 Symmetrically Encrypted   | Section   |
       |         | and Integrity Protected Data packet | 5.13.1    |
       +---------+-------------------------------------+-----------+
       | 0x02... | Reserved                            |           |
       +---------+-------------------------------------+-----------+
       | 0x04... | Reserved                            |           |
       +---------+-------------------------------------+-----------+
       | 0x08... | Version 2 Symmetrically Encrypted   | Section   |
       |         | and Integrity Protected Data packet | 5.13.2    |
       +---------+-------------------------------------+-----------+

                 Table 11: OpenPGP Features Flags Registry

   If an implementation implements any of the defined features, it
   SHOULD implement the Features subpacket, too.

   See Section 13.7 for details about how to use the Features subpacket
   when generating encryption data.

5.2.3.33.  Signature Target

   (1 octet public key algorithm, 1 octet hash algorithm, N octets hash)

   This subpacket identifies a specific target signature to which a
   signature refers.  For Revocation Signatures, this subpacket provides
   explicit designation of which signature is being revoked.  For a
   Third-Party Confirmation or Timestamp signature, this designates what
   signature is signed.  All arguments are an identifier of that target
   signature.

   The N octets of hash data MUST be the size of the signature's hash.
   For example, a target signature with a SHA-1 hash MUST have 20 octets
   of hash data.

5.2.3.34.  Embedded Signature

   (1 Signature packet body)

   This subpacket contains a complete Signature packet body as specified
   in Section 5.2.  It is useful when one signature needs to refer to,
   or be incorporated in, another signature.

5.2.3.35.  Issuer Fingerprint

   (1 octet key version number, N octets of fingerprint)

   The OpenPGP Key fingerprint of the key issuing the signature.  This
   subpacket SHOULD be included in all signatures.  If the version of
   the issuing key is 4 and an Issuer Key ID subpacket
   (Section 5.2.3.12) is also included in the signature, the Key ID of
   the Issuer Key ID subpacket MUST match the low 64 bits of the
   fingerprint.

   Note that the length N of the fingerprint for a version 4 key is 20
   octets; for a version 6 key, N is 32.  Since the version of the
   signature is bound to the version of the key, the version octet here
   MUST match the version of the signature.  If the version octet does
   not match the signature version, the receiving implementation MUST
   treat it as a malformed signature (see Section 5.2.5).

5.2.3.36.  Intended Recipient Fingerprint

   (1 octet key version number, N octets of fingerprint)

   The OpenPGP Key fingerprint of the intended recipient primary key.
   If one or more subpackets of this type are included in a signature,
   it SHOULD be considered valid only in an encrypted context, where the
   key it was encrypted to is one of the indicated primary keys or one
   of their subkeys.  This can be used to prevent forwarding a signature
   outside of its intended, encrypted context (see Section 13.12).

   Note that the length N of the fingerprint for a version 4 key is 20
   octets; for a version 6 key, N is 32.

   An implementation SHOULD generate this subpacket when creating a
   signed and encrypted message.

   When generating this subpacket in a version 6 signature, it SHOULD be
   marked as critical.

5.2.4.  Computing Signatures

   All signatures are formed by producing a hash over the signature data
   and then using the resulting hash in the signature algorithm.

   When creating or verifying a version 6 signature, the salt is fed
   into the hash context before any other data.

   For binary document signatures (Type ID 0x00), the document data is
   hashed directly.  For text document signatures (Type ID 0x01), the
   implementation MUST first canonicalize the document by converting
   line endings to <CR><LF> and encoding it in UTF-8 (see [RFC3629]).
   The resulting UTF-8 byte stream is hashed.

   When a version 4 signature is made over a key, the hash data starts
   with the octet 0x99, followed by a 2-octet length of the key,
   followed by the body of the key packet.  When a version 6 signature
   is made over a key, the hash data starts with the salt and then octet
   0x9B, followed by a 4-octet length of the key, followed by the body
   of the key packet.

   A Subkey Binding signature (Type ID 0x18) or Primary Key Binding
   signature (Type ID 0x19) then hashes the subkey using the same format
   as the main key (also using 0x99 or 0x9B as the first octet).
   Primary Key Revocation signatures (Type ID 0x20) hash only the key
   being revoked.  A Subkey Revocation signature (Type ID 0x28) first
   hashes the primary key and then the subkey being revoked.

   A Certification signature (Type IDs 0x10 through 0x13) hashes the
   User ID that is bound to the key into the hash context after the
   above data.  A version 3 certification hashes the contents of the
   User ID or User Attribute packet without the packet header.  A
   version 4 or version 6 certification hashes the constant 0xB4 for
   User ID certifications or the constant 0xD1 for User Attribute
   certifications, followed by a 4-octet number giving the length of the
   User ID or User Attribute data, followed by the User ID or User
   Attribute data.

   A Third-Party Confirmation signature (Type ID 0x50) hashes the salt
   (version 6 signatures only), followed by the octet 0x88, followed by
   the 4-octet length of the signature, and then the body of the
   Signature packet.  (Note that this is a Legacy packet header for a
   Signature packet with the length-of-length field set to zero.)  The
   unhashed subpacket data of the Signature packet being hashed is not
   included in the hash, and the unhashed subpacket data length value is
   set to zero.

   Once the data body is hashed, then a trailer is hashed.  This trailer
   depends on the version of the signature.

   *  A version 3 signature hashes five octets of the packet body,
      starting from the signature type field.  This data is the
      signature type, followed by the 4-octet Signature Creation Time.

   *  A version 4 or version 6 signature hashes the packet body starting
      from its first field, the version number, through the end of the
      hashed subpacket data and a final extra trailer.  Thus, the hashed
      fields are:

      -  an octet indicating the signature version (0x04 for version 4,
         and 0x06 for version 6),

      -  the signature type,

      -  the public key algorithm,

      -  the hash algorithm,

      -  the hashed subpacket length,

      -  the hashed subpacket body,

      -  a second version octet (0x04 for version 4, and 0x06 for
         version 6),

      -  a single octet 0xFF, and

      -  a number representing the length (in octets) of the hashed data
         from the Signature packet through the hashed subpacket body.
         This a 4-octet big-endian unsigned integer of the length modulo
         2^32.

   After all this has been hashed in a single hash context, the
   resulting hash field is used in the signature algorithm, and its
   first two octets are placed in the Signature packet, as described in
   Section 5.2.3.

   For worked examples of the data hashed during a signature, see
   Appendix A.3.1.

5.2.4.1.  Notes about Signature Computation

   The data actually hashed by OpenPGP varies depending on the signature
   version, in order to ensure that a signature made using one version
   cannot be repurposed as a signature with a different version over
   subtly different data.  The hashed data streams differ based on their
   trailer, most critically in the fifth and sixth octets from the end
   of the stream.  In particular:

   *  A version 3 signature uses the fifth octet from the end to store
      its Signature Type ID.  This MUST NOT be Signature Type ID 0xFF.

   *  All signature versions later than version 3 always use a literal
      0xFF in the fifth octet from the end.  For these later signature
      versions, the sixth octet from the end (the octet before the 0xFF)
      stores the signature version number.

5.2.5.  Malformed and Unknown Signatures

   In some cases, a Signature packet (or its corresponding One-Pass
   Signature packet; see Section 5.4) may be malformed or unknown.  For
   example, it might encounter any of the following problems (this is
   not an exhaustive list):

   *  An unknown signature type

   *  An unknown signature version

   *  An unsupported signature version

   *  An unknown "critical" subpacket (see Section 5.2.3.7) in the
      hashed area

   *  A subpacket with a length that diverges from the expected length

   *  A hashed subpacket area with length that exceeds the length of the
      Signature packet itself

   *  A hash algorithm known to be weak (e.g., MD5)

   *  A mismatch between the expected salt length of the hash algorithm
      and the actual salt length

   *  A mismatch between the One-Pass Signature version and the
      Signature version (see Section 10.3.2.2)

   *  A signature with a version other than 6, made by a version 6 key

   When an implementation encounters such a malformed or unknown
   signature, it MUST ignore the signature for validation purposes.  It
   MUST NOT indicate a successful signature validation for such a
   signature.  At the same time, it MUST NOT halt processing on the
   packet stream or reject other signatures in the same packet stream
   just because an unknown or invalid signature exists.

   This requirement is necessary for forward compatibility.  Producing
   an output that indicates that no successful signatures were found is
   preferable to aborting processing entirely.

5.3.  Symmetric Key Encrypted Session Key Packet (Type ID 3)

   The Symmetric Key Encrypted Session Key (SKESK) packet holds the
   symmetric key encryption of a session key used to encrypt a message.
   Zero or more Public Key Encrypted Session Key packets (Section 5.1)
   and/or Symmetric Key Encrypted Session Key packets precede an
   encryption container (that is, a Symmetrically Encrypted and
   Integrity Protected Data packet or -- for historic data -- a
   Symmetrically Encrypted Data packet) that holds an Encrypted Message.
   The message is encrypted with a session key, and the session key is
   itself encrypted and stored in the Encrypted Session Key packet(s).

   If the encryption container is preceded by one or more Symmetric Key
   Encrypted Session Key packets, each specifies a passphrase that may
   be used to decrypt the message.  This allows a message to be
   encrypted to a number of public keys, and also to one or more
   passphrases.

   The body of this packet starts with a 1-octet number giving the
   version number of the packet type.  The currently defined versions
   are 4 and 6.  The remainder of the packet depends on the version.

   The versions differ in how they encrypt the session key with the
   passphrase and in what they encode.  The version of the SKESK packet
   must align with the version of the SEIPD packet (see
   Section 10.3.2.1).  Any new version of the SKESK packet should be
   registered in the registry established in Section 10.3.2.1.

5.3.1.  Version 4 Symmetric Key Encrypted Session Key Packet Format

   A v4 SKESK packet precedes a v1 SEIPD (see Section 5.13.1).  In
   historic data, it is sometimes found preceding a deprecated SED
   packet (see Section 5.7).  A v4 SKESK packet MUST NOT precede a v2
   SEIPD packet (see Section 10.3.2.1).

   A version 4 Symmetric Key Encrypted Session Key packet consists of:

   *  A 1-octet version number with value 4.

   *  A 1-octet number describing the symmetric algorithm used.

   *  An S2K Specifier.  The length of the S2K Specifier depends on its
      type (see Section 3.7.1).

   *  Optionally, the encrypted session key itself, which is decrypted
      with the S2K object.

   If the encrypted session key is not present (which can be detected on
   the basis of packet length and S2K Specifier size), then the S2K
   algorithm applied to the passphrase produces the session key for
   decrypting the message, using the Symmetric Cipher Algorithm ID from
   the Symmetric Key Encrypted Session Key packet.

   If the encrypted session key is present, the result of applying the
   S2K algorithm to the passphrase is used to decrypt just that
   encrypted session key field, using CFB mode with an IV of all zeros.
   The decryption result consists of a 1-octet algorithm identifier that
   specifies the symmetric key encryption algorithm used to encrypt the
   following encryption container, followed by the session key octets
   themselves.

   Note: because an all-zero IV is used for this decryption, the S2K
   Specifier MUST use a salt value, a Salted S2K, an Iterated and Salted
   S2K, or Argon2.  The salt value will ensure that the decryption key
   is not repeated even if the passphrase is reused.

5.3.2.  Version 6 Symmetric Key Encrypted Session Key Packet Format

   A v6 SKESK packet precedes a v2 SEIPD packet (see Section 5.13.2).  A
   v6 SKESK packet MUST NOT precede a v1 SEIPD packet or a deprecated
   Symmetrically Encrypted Data packet (see Section 10.3.2.1).

   A version 6 Symmetric Key Encrypted Session Key packet consists of:

   *  A 1-octet version number with value 6.

   *  A 1-octet scalar octet count for the 5 fields following this
      octet.

   *  A 1-octet Symmetric Cipher Algorithm ID (from Table 21).

   *  A 1-octet AEAD algorithm identifier (from Table 25).

   *  A 1-octet scalar octet count of the following field.

   *  An S2K Specifier.  The length of the S2K Specifier depends on its
      type (see Section 3.7.1).

   *  A starting IV of the size specified by the AEAD algorithm.

   *  The encrypted session key itself.

   *  An authentication tag for the AEAD mode.

   A key-encryption key (KEK) is derived using HKDF [RFC5869] with
   SHA256 [RFC6234] as the hash algorithm.  The Initial Keying Material
   (IKM) for HKDF is the key derived from S2K.  No salt is used.  The
   info parameter is comprised of the Packet Type ID in OpenPGP format
   encoding (bits 7 and 6 are set, and bits 5-0 carry the Packet Type
   ID), the packet version, and the cipher-algo and AEAD-mode used to
   encrypt the key material.

   Then, the session key is encrypted using the resulting key, with the
   AEAD algorithm specified for the version 2 Symmetrically Encrypted
   and Integrity Protected Data packet.  Note that no chunks are used
   and that there is only one authentication tag.  The Packet Type ID
   encoded in OpenPGP format (bits 7 and 6 are set, and bits 5-0 carry
   the Packet Type ID), the packet version number, the cipher algorithm
   ID, and the AEAD algorithm ID are given as additional data.  For
   example, the additional data used with AES-128 with OCB consists of
   the octets 0xC3, 0x06, 0x07, and 0x02.

5.4.  One-Pass Signature Packet (Type ID 4)

   The One-Pass Signature packet precedes the signed data and contains
   enough information to allow the receiver to begin calculating any
   hashes needed to verify the signature.  It allows the Signature
   packet to be placed at the end of the message so that the signer can
   compute the entire signed message in one pass.

   The body of this packet consists of:

   *  A 1-octet version number.  The currently defined versions are 3
      and 6.  Any new One-Pass Signature packet version should be
      registered in the registry established in Section 10.3.2.2.

   *  A 1-octet Signature Type ID.  Signature types are described in
      Section 5.2.1.

   *  A 1-octet number describing the hash algorithm used.

   *  A 1-octet number describing the public key algorithm used.

   *  Only for version 6 packets, a variable-length field containing:

      -  A 1-octet salt size.  The value MUST match the value defined
         for the hash algorithm as specified in Table 23.

      -  The salt; a random value of the specified size.  The value MUST
         match the salt field of the corresponding Signature packet.

   *  Only for v3 packets, an 8-octet number holding the Key ID of the
      signing key.

   *  Only for version 6 packets, 32 octets of the fingerprint of the
      signing key.  Since a version 6 signature can only be made by a
      version 6 key, the length of the fingerprint is fixed.

   *  A 1-octet number holding a flag showing whether the signature is
      nested.  A zero value indicates that the next packet is another
      One-Pass Signature packet that describes another signature to be
      applied to the same message data.

   When generating a one-pass signature, the OPS packet version MUST
   correspond to the version of the associated Signature packet, except
   for the historical accident that version 4 keys use a version 3 One-
   Pass Signature packet (there is no version 4 OPS).  See
   Section 10.3.2.2 for the full correspondence of versions between
   Keys, Signatures, and One-Pass Signatures.

   Note that if a message contains more than one one-pass signature,
   then the Signature packets bracket the message; that is, the first
   Signature packet after the message corresponds to the last One-Pass
   Signature packet and the final Signature packet corresponds to the
   first One-Pass Signature packet.

5.5.  Key Material Packets

   A key material packet contains all the information about a public or
   private key.  There are four variants of this packet type: two major
   versions (versions 4 and 6) and two strongly deprecated versions
   (versions 2 and 3).  Consequently, this section is complex.

   For historical reasons, versions 1 and 5 of the key packets are
   unspecified.

5.5.1.  Key Packet Variants

5.5.1.1.  Public Key Packet (Type ID 6)

   A Public Key packet starts a series of packets that forms an OpenPGP
   Key (sometimes called an OpenPGP certificate).

5.5.1.2.  Public Subkey Packet (Type ID 14)

   A Public Subkey packet (Type ID 14) has exactly the same format as a
   Public Key packet, but it denotes a subkey.  One or more subkeys may
   be associated with a top-level key.  By convention, the top-level key
   offers certification capability, but it does not provide encryption
   services, while a dedicated subkey provides encryption (see
   Section 10.1.5).

5.5.1.3.  Secret Key Packet (Type ID 5)

   A Secret Key packet contains all the information that is found in a
   Public Key packet, including the public key material, but it also
   includes the secret key material after all the public key fields.

5.5.1.4.  Secret Subkey Packet (Type ID 7)

   A Secret Subkey packet (Type ID 7) is the subkey analog of the Secret
   Key packet and has exactly the same format.

5.5.2.  Public Key Packet Formats

   There are four versions of key material packets.  Versions 2 and 3
   have been deprecated since 1998.  Version 4 has been deprecated by
   this document.

   OpenPGP implementations SHOULD create keys with version 6 format.
   Version 4 keys are deprecated; an implementation SHOULD NOT generate
   a version 4 key but SHOULD accept it.  Version 3 keys are deprecated;
   an implementation MUST NOT generate a version 3 key but MAY accept
   it.  Version 2 keys are deprecated; an implementation MUST NOT
   generate a version 2 key but MAY accept it.

   Any new Key Version must be registered in the registry established in
   Section 10.3.2.2.

5.5.2.1.  Version 3 Public Keys

   Version 2 keys are identical to version 3 keys except for the version
   number.  A version 3 Public Key or Public Subkey packet contains:

   *  A 1-octet version number (3).

   *  A 4-octet number denoting the time that the key was created.

   *  A 2-octet number denoting the time in days that the key is valid.
      If this number is zero, then it does not expire.

   *  A 1-octet number denoting the public key algorithm of the key.

   *  A series of multiprecision integers comprising the key material:

      -  MPI of RSA public modulus n.

      -  MPI of RSA public encryption exponent e.

   Version 3 keys are deprecated.  They contain three weaknesses.
   First, it is relatively easy to construct a version 3 key that has
   the same Key ID as any other key because the Key ID is simply the low
   64 bits of the public modulus.  Second, because the fingerprint of a
   version 3 key hashes the key material, but not its length, there is
   an increased opportunity for fingerprint collisions.  Third, there
   are weaknesses in the MD5 hash algorithm that cause developers to
   prefer other algorithms.  See Section 5.5.4 for a fuller discussion
   of Key IDs and fingerprints.

5.5.2.2.  Version 4 Public Keys

   The version 4 format is similar to the version 3 format except for
   the absence of a validity period.  This has been moved to the
   Signature packet.  In addition, fingerprints of version 4 keys are
   calculated differently from version 3 keys, as described in
   Section 5.5.4.

   A version 4 packet contains:

   *  A 1-octet version number (4).

   *  A 4-octet number denoting the time that the key was created.

   *  A 1-octet number denoting the public key algorithm of the key.

   *  A series of values comprising the key material.  This is algorithm
      specific and described in Section 5.5.5.

5.5.2.3.  Version 6 Public Keys

   The version 6 format is similar to the version 4 format except for
   the addition of a count for the key material.  This count helps
   parsing Secret Key packets (which are an extension of the Public Key
   packet format) in the case of an unknown algorithm.  In addition,
   fingerprints of version 6 keys are calculated differently from
   version 4 keys, as described in Section 5.5.4.

   A version 6 packet contains:

   *  A 1-octet version number (6).

   *  A 4-octet number denoting the time that the key was created.

   *  A 1-octet number denoting the public key algorithm of the key.

   *  A 4-octet scalar octet count for the public key material specified
      in the next field.

   *  A series of values comprising the public key material.  This is
      algorithm specific and described in Section 5.5.5.

5.5.3.  Secret Key Packet Formats

   The Secret Key and Secret Subkey packets contain all the data of the
   Public Key and Public Subkey packets, with additional algorithm-
   specific secret key data appended, usually in encrypted form.

   The packet contains:

   *  The fields of a Public Key or Public Subkey packet, as described
      above.

   *  One octet (the "S2K usage octet") indicating whether and how the
      secret key material is protected by a passphrase.  Zero indicates
      that the secret key data is not encrypted.  253 (AEAD), 254 (CFB),
      or 255 (MalleableCFB) indicates that an S2K Specifier and other
      parameters will follow.  Any other value is a symmetric key
      encryption algorithm identifier.  A version 6 packet MUST NOT use
      the value 255 (MalleableCFB).

   *  Only for a version 6 packet where the secret key material is
      encrypted (that is, where the previous octet is not zero), a
      1-octet scalar octet count of the cumulative length of all the
      following conditionally included S2K parameter fields.

   *  Conditionally included S2K parameter fields:

      -  If the S2K usage octet was 253, 254, or 255, a 1-octet
         symmetric key encryption algorithm.

      -  If the S2K usage octet was 253 (AEAD), a 1-octet AEAD
         algorithm.

      -  Only for a version 6 packet, and if the S2K usage octet was 253
         or 254, a 1-octet count of the size of the one field following
         this octet.

      -  If the S2K usage octet was 253, 254, or 255, an S2K Specifier.
         The length of the S2K Specifier depends on its type (see
         Section 3.7.1).

      -  If the S2K usage octet was 253 (AEAD), an IV of a size
         specified by the AEAD algorithm (see Section 5.13.2), which is
         used as the nonce for the AEAD algorithm.

      -  If the S2K usage octet was 254, 255, or a cipher algorithm ID
         (that is, the secret data uses some form of CFB encryption), an
         IV of the same length as the cipher's block size.

   *  Plain or encrypted multiprecision integers comprising the secret
      key data.  This is algorithm specific and described in
      Section 5.5.5.  If the S2K usage octet is 253 (AEAD), then an AEAD
      authentication tag is at the end of that data.  If the S2K usage
      octet is 254 (CFB), a 20-octet SHA-1 hash of the plaintext of the
      algorithm-specific portion is appended to plaintext and encrypted
      with it.  If the S2K usage octet is 255 (MalleableCFB) or another
      non-zero value (that is, a symmetric key encryption algorithm
      identifier), a 2-octet checksum of the plaintext of the algorithm-
      specific portion (sum of all octets, mod 65536) is appended to
      plaintext and encrypted with it.  (This is deprecated and SHOULD
      NOT be used; see below.)

   *  Only for a version 3 or 4 packet where the S2K usage octet is
      zero, a 2-octet checksum of the algorithm-specific portion (sum of
      all octets, mod 65536).

   The details about storing algorithm-specific secrets above are
   summarized in Table 2.

   Note that the version 6 packet format adds two count values to help
   parsing packets with unknown S2K or public key algorithms.

   Secret MPI values can be encrypted using a passphrase.  If an S2K
   Specifier is given, it describes the algorithm for converting the
   passphrase to a key; otherwise, a simple MD5 hash of the passphrase
   is used.  An implementation producing a passphrase-protected Secret
   Key packet MUST use an S2K Specifier; the simple hash is for read-
   only backward compatibility, though implementations MAY continue to
   use existing private keys in the old format.  The cipher for
   encrypting the MPIs is specified in the Secret Key packet.

   Encryption/decryption of the secret data is done using the key
   created from the passphrase and the IV from the packet.  If the S2K
   usage octet is not 253, CFB mode is used.  A different mode is used
   with version 3 keys (which are only RSA) than with other key formats.
   With version 3 keys, the MPI bit count prefix (that is, the first two
   octets) is not encrypted.  Only the MPI non-prefix data is encrypted.
   Furthermore, the CFB state is resynchronized at the beginning of each
   new MPI value so that the CFB block boundary is aligned with the
   start of the MPI data.

   With version 4 and version 6 keys, a simpler method is used.  All
   secret MPI values are encrypted, including the MPI bit count prefix.

   If the S2K usage octet is 253, the KEK is derived using HKDF
   [RFC5869] to provide key separation.  SHA256 [RFC6234] is used as the
   hash algorithm for HKDF.  IKM for HKDF is the key derived from S2K.
   No salt is used.  The info parameter is comprised of the Packet Type
   ID encoded in OpenPGP format (bits 7 and 6 are set, and bits 5-0
   carry the Packet Type ID), the packet version, and the cipher-algo
   and AEAD-mode used to encrypt the key material.

   Then, the encrypted MPI values are encrypted as one combined
   plaintext using one of the AEAD algorithms specified for the version
   2 Symmetrically Encrypted and Integrity Protected Data packet.  Note
   that no chunks are used and that there is only one authentication
   tag.  As additional data, the Packet Type ID in OpenPGP format
   encoding (bits 7 and 6 are set, and bits 5-0 carry the Packet Type
   ID), followed by the Public Key packet fields, starting with the
   packet version number, are passed to the AEAD algorithm.  For
   example, the additional data used with a Secret Key packet of version
   4 consists of the octets 0xC5, 0x04, followed by four octets of
   creation time, one octet denoting the public key algorithm, and the
   algorithm-specific public key parameters.  For a Secret Subkey
   packet, the first octet would be 0xC7.  For a version 6 key packet,
   the second octet would be 0x06, and the 4-octet octet count of the
   public key material would be included as well (see Section 5.5.2).

   The 2-octet checksum that follows the algorithm-specific portion is
   the algebraic sum, mod 65536, of the plaintext of all the algorithm-
   specific octets (including the MPI prefix and data).  With version 3
   keys, the checksum is stored in the clear.  With version 4 keys, the
   checksum is encrypted like the algorithm-specific data.  This value
   is used to check that the passphrase was correct.  However, this
   checksum is deprecated, and an implementation SHOULD NOT use it;
   instead, an implementation should use the SHA-1 hash denoted with a
   usage octet of 254.  The reason for this is that there are some
   attacks that involve modifying the secret key undetected.  If the S2K
   usage octet is 253, no checksum or SHA-1 hash is used, but the
   authentication tag of the AEAD algorithm follows.

   When decrypting the secret key material using any of these schemes
   (that is, where the usage octet is non-zero), the resulting cleartext
   octet stream must be well formed.  In particular, an implementation
   MUST NOT interpret octets beyond the unwrapped cleartext octet stream
   as part of any of the unwrapped MPI objects.  Furthermore, an
   implementation MUST reject any secret key material whose cleartext
   length does not align with the lengths of the unwrapped MPI objects
   as unusable.

5.5.4.  Key IDs and Fingerprints

   Every OpenPGP Key has a fingerprint and a Key ID.  The computation of
   these values differs based on the key version.  The fingerprint
   length varies with the key version, but the Key ID (which is only
   used in v3 PKESK packets; see Section 5.1.1) is always 64 bits.  The
   following registry represents the subsections below:

   +=======+===================+===============+=============+=========+
   |Key    | Fingerprint       | Fingerprint   | Key ID      |Reference|
   |Version|                   | Length        |             |         |
   |       |                   | (Bits)        |             |         |
   +=======+===================+===============+=============+=========+
   |3      | MD5(MPIs without  | 128           | low 64 bits |Section  |
   |       | length octets)    |               | of RSA      |5.5.4.1  |
   |       |                   |               | modulus     |         |
   +-------+-------------------+---------------+-------------+---------+
   |4      | SHA1(normalized   | 160           | last 64     |Section  |
   |       | pubkey packet)    |               | bits of     |5.5.4.2  |
   |       |                   |               | fingerprint |         |
   +-------+-------------------+---------------+-------------+---------+
   |6      | SHA256(normalized | 256           | first 64    |Section  |
   |       | pubkey packet)    |               | bits of     |5.5.4.3  |
   |       |                   |               | fingerprint |         |
   +-------+-------------------+---------------+-------------+---------+

            Table 12: OpenPGP Key IDs and Fingerprints Registry

5.5.4.1.  Version 3 Key ID and Fingerprint

   For a version 3 key, the 8-octet Key ID consists of the low 64 bits
   of the public modulus of the RSA key.

   The fingerprint of a version 3 key is formed by hashing the body (but
   not the 2-octet length) of the MPIs that form the key material
   (public modulus n, followed by exponent e) with MD5.  Note that both
   version 3 keys and MD5 are deprecated.

5.5.4.2.  Version 4 Key ID and Fingerprint

   A version 4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
   followed by the 2-octet packet length, followed by the entire Public
   Key packet starting with the version field.  The Key ID is the low-
   order 64 bits of the fingerprint.  Here are the fields of the hash
   material, including an example of an Ed25519 key:

   a.1)  0x99 (1 octet)

   a.2)  2-octet, big-endian scalar octet count of (b)-(e)

   b)  version number = 4 (1 octet)

   c)  timestamp of key creation (4 octets)

   d)  algorithm (1 octet): 27 = Ed25519 (example)

   e)  algorithm-specific fields

   Algorithm-specific fields for Ed25519 keys (example):

   e.1)  32 octets representing the public key

5.5.4.3.  Version 6 Key ID and Fingerprint

   A version 6 fingerprint is the 256-bit SHA2-256 hash of the octet
   0x9B, followed by the 4-octet packet length, followed by the entire
   Public Key packet starting with the version field.  The Key ID is the
   high-order 64 bits of the fingerprint.  Here are the fields of the
   hash material, including an example of an Ed25519 key:

   a.1)  0x9B (1 octet)

   a.2)  4-octet scalar octet count of (b)-(f)

   b)  version number = 6 (1 octet)

   c)  timestamp of key creation (4 octets)

   d)  algorithm (1 octet): 27 = Ed25519 (example)

   e)  4-octet scalar octet count for the key material specified in the
       next field

   f)  algorithm-specific public key material

   Algorithm-specific fields for Ed25519 keys (example):

   f.1)  32 octets representing the public key

   Note that it is possible for there to be collisions of Key IDs --
   that is, two different keys with the same Key ID.  Note that there is
   a much smaller, but still non-zero, probability that two different
   keys have the same fingerprint.

   Also note that if version 3, version 4, and version 6 format keys
   share the same RSA key material, they will have different Key IDs as
   well as different fingerprints.

   Finally, the Key ID and fingerprint of a subkey are calculated in the
   same way as for a primary key, including the 0x99 (version 4 key) or
   0x9B (version 6 key) as the first octet (even though this is not a
   valid Packet Type ID for a public subkey).

5.5.5.  Algorithm-Specific Parts of Keys

   The public and secret key formats specify algorithm-specific parts of
   a key.  The following sections describe them in detail.

5.5.5.1.  Algorithm-Specific Part for RSA Keys

   For RSA keys, the public key consists of this series of
   multiprecision integers:

   *  MPI of RSA public modulus n,

   *  MPI of RSA public encryption exponent e.

   The secret key consists of this series of multiprecision integers:

   *  MPI of RSA secret exponent d;

   *  MPI of RSA secret prime value p;

   *  MPI of RSA secret prime value q (p < q); and

   *  MPI of u, the multiplicative inverse of p, mod q.

5.5.5.2.  Algorithm-Specific Part for DSA Keys

   For DSA keys, the public key consists of this series of
   multiprecision integers:

   *  MPI of DSA prime p;

   *  MPI of DSA group order q (q is a prime divisor of p-1);

   *  MPI of DSA group generator g; and

   *  MPI of DSA public key value y (= g^x mod p where x is secret).

   The secret key consists of this single multiprecision integer:

   *  MPI of DSA secret exponent x.

5.5.5.3.  Algorithm-Specific Part for Elgamal Keys

   For Elgamal keys, the public key consists of this series of
   multiprecision integers:

   *  MPI of Elgamal prime p;

   *  MPI of Elgamal group generator g; and

   *  MPI of Elgamal public key value y (= g^x mod p where x is secret).

   The secret key consists of this single multiprecision integer:

   *  MPI of Elgamal secret exponent x.

5.5.5.4.  Algorithm-Specific Part for ECDSA Keys

   For ECDSA keys, the public key consists of this series of values:

   *  A variable-length field containing a curve OID, which is formatted
      as follows:

      -  A 1-octet size of the following field; values 0 and 0xFF are
         reserved for future extensions.

      -  The octets representing a curve OID, as defined in Section 9.2.

   *  An MPI of an EC point representing a public key.

   The secret key consists of this single multiprecision integer:

   *  An MPI of an integer representing the secret key, which is a
      scalar of the public EC point.

5.5.5.5.  Algorithm-Specific Part for EdDSALegacy Keys (Deprecated)

   For EdDSALegacy keys (deprecated), the public key consists of this
   series of values:

   *  A variable-length field containing a curve OID, formatted as
      follows:

      -  A 1-octet size of the following field; values 0 and 0xFF are
         reserved for future extensions.

      -  The octets representing a curve OID, as defined in Section 9.2.

   *  An MPI of an EC point representing a public key Q in prefixed
      native form (see Section 11.2.2).

   The secret key consists of this single multiprecision integer:

   *  An MPI-encoded octet string representing the native form of the
      secret key in the curve-specific format, as described in
      Section 9.2.1.

   Note that the native form for an EdDSA secret key is a fixed-width
   sequence of unstructured random octets, with size corresponding to
   the specific curve.  That sequence of random octets is used with a
   cryptographic digest to produce both a curve-specific secret scalar
   and a prefix used when making a signature.  See Section 5.1.5 of
   [RFC8032] for more details about how to use the native octet strings
   for Ed25519Legacy.  The value stored in an OpenPGP EdDSALegacy Secret
   Key packet is the original sequence of random octets.

   Note that the only curve defined for use with EdDSALegacy is the
   Ed25519Legacy OID.

5.5.5.6.  Algorithm-Specific Part for ECDH Keys

   For ECDH keys, the public key consists of this series of values:

   *  A variable-length field containing a curve OID, which is formatted
      as follows:

      -  A 1-octet size of the following field; values 0 and 0xFF are
         reserved for future extensions.

      -  The octets representing a curve OID, as defined in Section 9.2.

   *  An MPI of an EC point representing a public key, in the point
      format associated with the curve, as specified in Section 9.2.1.

   *  A variable-length field containing key derivation function (KDF)
      parameters, which is formatted as follows:

      -  A 1-octet size of the following fields; values 0 and 0xFF are
         reserved for future extensions.

      -  A 1-octet value 1, reserved for future extensions.

      -  A 1-octet hash function ID used with a KDF.

      -  A 1-octet algorithm ID for the symmetric algorithm that is used
         to wrap the symmetric key for message encryption; see
         Section 11.5 for details.

   The secret key consists of this single multiprecision integer:

   *  An MPI representing the secret key, in the curve-specific format
      described in Section 9.2.1.

5.5.5.6.1.  ECDH Secret Key Material

   When curve NIST P-256, NIST P-384, NIST P-521, brainpoolP256r1,
   brainpoolP384r1, or brainpoolP512r1 are used in ECDH, their secret
   keys are represented as a simple integer in standard MPI form.  Other
   curves are presented on the wire differently (though still as a
   single MPI), as described below and in Section 9.2.1.

5.5.5.6.1.1.  Curve25519Legacy ECDH Secret Key Material (Deprecated)

   A Curve25519Legacy secret key is stored as a standard integer in big-
   endian MPI form.  Curve25519Legacy MUST NOT be used in key packets
   version 6 or above.  Note that this form is in reverse octet order
   from the little-endian "native" form found in [RFC7748].

   Note also that the integer for a Curve25519Legacy secret key for
   OpenPGP MUST have the appropriate form; that is, it MUST be divisible
   by 8, MUST be at least 2^254, and MUST be less than 2^255.  The
   length of this MPI in bits is by definition always 255, so the two
   leading octets of the MPI will always be 00 FF, and reversing the
   following 32 octets from the wire will produce the "native" form.

   When generating a new Curve25519Legacy secret key from 32 fully
   random octets, the following pseudocode produces the MPI wire format
   (note the similarity to decodeScalar25519 as described in [RFC7748]):

   def curve25519Legacy_MPI_from_random(octet_list):
       octet_list[0] &= 248
       octet_list[31] &= 127
       octet_list[31] |= 64
       mpi_header = [ 0x00, 0xFF ]
       return mpi_header || reversed(octet_list)

5.5.5.7.  Algorithm-Specific Part for X25519 Keys

   For X25519 keys, the public key consists of this single value:

   *  32 octets of the native public key.

   The secret key consists of this single value:

   *  32 octets of the native secret key.

   See Section 6.1 of [RFC7748] for more details about how to use the
   native octet strings.  The value stored in an OpenPGP X25519 Secret
   Key packet is the original sequence of random octets.  The value
   stored in an OpenPGP X25519 Public Key packet is the value
   X25519(secretKey, 9).

5.5.5.8.  Algorithm-Specific Part for X448 Keys

   For X448 keys, the public key consists of this single value:

   *  56 octets of the native public key.

   The secret key consists of this single value:

   *  56 octets of the native secret key.

   See Section 6.2 of [RFC7748] for more details about how to use the
   native octet strings.  The value stored in an OpenPGP X448 Secret Key
   packet is the original sequence of random octets.  The value stored
   in an OpenPGP X448 Public Key packet is the value X448(secretKey, 5).

5.5.5.9.  Algorithm-Specific Part for Ed25519 Keys

   For Ed25519 keys, the public key consists of this single value:

   *  32 octets of the native public key.

   The secret key consists of this single value:

   *  32 octets of the native secret key.

   See Section 5.1.5 of [RFC8032] for more details about how to use the
   native octet strings.  The value stored in an OpenPGP Ed25519 Secret
   Key packet is the original sequence of random octets.

5.5.5.10.  Algorithm-Specific Part for Ed448 Keys

   For Ed448 keys, the public key consists of this single value:

   *  57 octets of the native public key.

   The secret key consists of this single value:

   *  57 octets of the native secret key.

   See Section 5.2.5 of [RFC8032] for more details about how to use the
   native octet strings.  The value stored in an OpenPGP Ed448 Secret
   Key packet is the original sequence of random octets.

5.6.  Compressed Data Packet (Type ID 8)

   The Compressed Data packet contains compressed data.  Typically, this
   packet is found as the contents of an encrypted packet, or following
   a Signature or One-Pass Signature packet, and contains a Literal Data
   packet.

   The body of this packet consists of:

   *  One octet specifying the algorithm used to compress the packet.

   *  Compressed data, which makes up the remainder of the packet.

   A Compressed Data packet's body contains data that is a compression
   of a series of OpenPGP packets.  See Section 10 for details on how
   messages are formed.

   A ZIP-compressed series of packets is compressed into raw DEFLATE
   blocks [RFC1951].

   A ZLIB-compressed series of packets is compressed with raw ZLIB-style
   blocks [RFC1950].

   A BZip2-compressed series of packets is compressed using the BZip2
   [BZ2] algorithm.

   An implementation that generates a Compressed Data packet MUST use
   the OpenPGP format for packet framing (see Section 4.2.1).  It MUST
   NOT generate a Compressed Data packet with Legacy format
   (Section 4.2.2).

   An implementation that deals with either historic data or data
   generated by legacy implementations predating support for [RFC2440]
   MAY interpret Compressed Data packets that use the Legacy format for
   packet framing.

5.7.  Symmetrically Encrypted Data Packet (Type ID 9)

   The Symmetrically Encrypted Data packet contains data encrypted with
   a symmetric key algorithm.  When it has been decrypted, it contains
   other packets (usually a Literal Data packet or compressed data
   packet, but in theory, it could be another sequence of packets that
   forms a valid OpenPGP Message).

   This packet is obsolete.  An implementation MUST NOT create this
   packet.  An implementation SHOULD reject such a packet and stop
   processing the message.  If an implementation chooses to process the
   packet anyway, it MUST return a clear warning that a non-integrity-
   protected packet has been processed.

   This packet format is impossible to handle safely in general because
   the ciphertext it provides is malleable.  See Section 13.7 about
   selecting a better OpenPGP encryption container that does not have
   this flaw.

   The body of this packet consists of:

   *  A random prefix, containing block-size random octets (for example,
      16 octets for a 128-bit block length) followed by a copy of the
      last two octets, encrypted together using Cipher Feedback (CFB)
      mode, with an IV of all zeros.

   *  Data encrypted using CFB mode, with the last block-size octets of
      the first ciphertext as the IV.

   The symmetric cipher used may be specified in a Public Key or
   Symmetric Key Encrypted Session Key packet that precedes the
   Symmetrically Encrypted Data packet.  In that case, the cipher
   algorithm ID is prefixed to the session key before it is encrypted.
   If no packets of these types precede the encrypted data, the IDEA
   algorithm is used with the session key calculated as the MD5 hash of
   the passphrase, though this use is deprecated.

   The data is encrypted in CFB mode (see Section 12.9).  For the random
   prefix, the IV is specified as all zeros.  Instead of achieving
   randomized encryption through an IV, a string of length equal to the
   block size of the cipher plus two is encrypted for this purpose.  The
   first block-size octets (for example, 16 octets for a 128-bit block
   length) are random, and the following two octets are copies of the
   last two octets of the first block-size random octets.  For example,
   for a 16-octet block length, octet 17 is a copy of octet 15, and
   octet 18 is a copy of octet 16.  For a cipher of block length 8,
   octet 9 is a copy of octet 7, and octet 10 is a copy of octet 8.  (In
   both of these examples, we consider the first octet to be numbered
   1.)

   After encrypting these block-size-plus-two octets, a new CFB context
   is created for the encryption of the data, with the last block-size
   octets of the first ciphertext as the IV.  (Alternatively and
   equivalently, the CFB state is resynchronized: the last block-size
   octets of ciphertext are passed through the cipher, and the block
   boundary is reset.)

   The repetition of two octets in the random prefix allows the receiver
   to immediately check whether the session key is incorrect.  See
   Section 13.4 for hints on the proper use of this "quick check".

5.8.  Marker Packet (Type ID 10)

   The body of the Marker packet consists of:

   *  The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).

   Such a packet MUST be ignored when received.

5.9.  Literal Data Packet (Type ID 11)

   A Literal Data packet contains the body of a message; that is, data
   that is not to be further interpreted.

   The body of this packet consists of:

   *  A 1-octet field that describes how the data is formatted.

      If it is a b (0x62), then the Literal Data packet contains binary
      data.  If it is a u (0x75), then the Literal Data packet contains
      UTF-8-encoded text data and thus may need line ends converted to
      local form or other text mode changes.

      Previous versions of the OpenPGP specification used t (0x74) to
      indicate textual data but did not specify the character encoding.
      Implementations SHOULD NOT emit this value.  An implementation
      that receives a Literal Data packet with this value in the format
      field SHOULD interpret the packet data as UTF-8 encoded text,
      unless reliable (not attacker-controlled) context indicates a
      specific alternate text encoding.  This mode is deprecated due to
      its ambiguity.

      Some implementations predating [RFC2440] also defined a value of l
      as a "local" mode for machine-local conversions.  [RFC1991]
      incorrectly states that this local mode flag is 1 (ASCII numeral
      one).  Both of these local modes are deprecated.

   *  The file name as a string (1-octet length, followed by a file
      name).  This may be a zero-length string.  Commonly, if the source
      of the encrypted data is a file, it will be the name of the
      encrypted file.  An implementation MAY consider the file name in
      the Literal Data packet to be a more authoritative name than the
      actual file name.

   *  A 4-octet number that indicates a date associated with the literal
      data.  Commonly, the date might be the modification date of a
      file, or the time the packet was created, or a zero that indicates
      no specific time.

   *  The remainder of the packet is literal data.

      Text data MUST be encoded with UTF-8 (see [RFC3629]) and stored
      with <CR><LF> text endings (that is, network-normal line endings).
      These should be converted to native line endings by the receiving
      implementation.

   Note that OpenPGP signatures do not include the formatting octet, the
   file name, and the date field of the Literal Data packet in a
   signature hash; therefore, those fields are not protected against
   tampering in a signed document.  A receiving implementation MUST NOT
   treat those fields as though they were cryptographically secured by
   the surrounding signature when either representing them to the user
   or acting on them.

   Due to their inherent malleability, an implementation that generates
   a Literal Data packet SHOULD avoid storing any significant data in
   these fields.  If the implementation is certain that the data is
   textual and is encoded with UTF-8 (for example, if it will follow
   this Literal Data packet with a Signature packet of type 0x01 (see
   Section 5.2.1), it MAY set the format octet to u.  Otherwise, it MUST
   set the format octet to b.  It SHOULD set the filename to the empty
   string (encoded as a single zero octet) and the timestamp to zero
   (encoded as four zero octets).

   An application that wishes to include such filesystem metadata within
   a signature is advised to sign an encapsulated archive (for example,
   [PAX]).

   An implementation that generates a Literal Data packet MUST use the
   OpenPGP format for packet framing (see Section 4.2.1).  It MUST NOT
   generate a Literal Data packet with Legacy format (Section 4.2.2).

   An implementation that deals with either historic data or data
   generated by an implementation that predates support for [RFC2440]
   MAY interpret Literal Data packets that use the Legacy format for
   packet framing.

5.9.1.  Special Filename _CONSOLE (Deprecated)

   The Literal Data packet's filename field has a historical special
   case for the special name _CONSOLE.  When the filename field is
   _CONSOLE, the message is considered to be "for your eyes only".  This
   advises that the message data is unusually sensitive, and the
   receiving program should process it more carefully, perhaps avoiding
   storing the received data to disk, for example.

   An OpenPGP deployment that generates Literal Data packets MUST NOT
   depend on this indicator being honored in any particular way.  It
   cannot be enforced, and the field itself is not covered by any
   cryptographic signature.

   It is NOT RECOMMENDED to use this special filename in a newly
   generated Literal Data packet.

5.10.  Trust Packet (Type ID 12)

   The Trust packet is used only within keyrings and is not normally
   exported.  Trust packets contain data that record the user's
   specifications of which keyholders are trustworthy introducers, along
   with other information that implementation uses for trust
   information.  The format of Trust packets is defined by a given
   implementation.

   Trust packets SHOULD NOT be emitted to output streams that are
   transferred to other users, and they SHOULD be ignored on any input
   other than local keyring files.

5.11.  User ID Packet (Type ID 13)

   A User ID packet consists of UTF-8 text that is intended to represent
   the name and email address of the keyholder.  By convention, it
   includes a mail name-addr as described in [RFC2822], but there are no
   restrictions on its content.  The packet length in the header
   specifies the length of the User ID.

5.12.  User Attribute Packet (Type ID 17)

   The User Attribute packet is a variation of the User ID packet.  It
   is capable of storing more types of data than the User ID packet,
   which is limited to text.  Like the User ID packet, a User Attribute
   packet may be certified by the key owner ("self-signed") or any other
   key owner who cares to certify it.  Except as noted, a User Attribute
   packet may be used anywhere that a User ID packet may be used.

   While User Attribute packets are not a required part of the OpenPGP
   specification, implementations SHOULD provide at least enough
   compatibility to properly handle a certification signature on the
   User Attribute packet.  A simple way to do this is by treating the
   User Attribute packet as a User ID packet with opaque contents, but
   an implementation may use any method desired.

   The User Attribute packet is made up of one or more attribute
   subpackets.  Each subpacket consists of a subpacket header and a
   body.  The header consists of:

   *  the subpacket length (1, 2, or 5 octets)

   *  the Subpacket Type ID (1 octet)

   and is followed by the subpacket specific data.

   The following table lists the currently known subpackets:

        +=========+=============================+================+
        |      ID | Attribute Subpacket         | Reference      |
        +=========+=============================+================+
        |       0 | Reserved                    |                |
        +---------+-----------------------------+----------------+
        |       1 | Image Attribute Subpacket   | Section 5.12.1 |
        +---------+-----------------------------+----------------+
        | 100-110 | Private or Experimental Use |                |
        +---------+-----------------------------+----------------+

        Table 13: OpenPGP User Attribute Subpacket Types Registry

   An implementation SHOULD ignore any subpacket of a type that it does
   not recognize.

5.12.1.  Image Attribute Subpacket

   The Image Attribute subpacket is used to encode an image, presumably
   (but not required to be) that of the key owner.

   The Image Attribute subpacket begins with an image header.  The first
   two octets of the image header contain the length of the image
   header.  Note that unlike other multi-octet numerical values in this
   document, due to a historical accident, this value is encoded as a
   little-endian number.  The image header length is followed by a
   single octet for the image header version.  The only currently
   defined version of the image header is 1, which is a 16-octet image
   header.  The first three octets of a version 1 image header are thus
   0x10, 0x00, 0x01.

                       +=========+================+
                       | Version | Reference      |
                       +=========+================+
                       |       1 | Section 5.12.1 |
                       +---------+----------------+

                         Table 14: OpenPGP Image
                            Attribute Versions
                                 Registry

   The fourth octet of a version 1 image header designates the encoding
   format of the image.  The only currently defined encoding format is
   the value 1 to indicate JPEG.  Image format IDs 100 through 110 are
   reserved for Private or Experimental Use. The rest of the version 1
   image header is made up of 12 reserved octets, all of which MUST be
   set to 0.

                 +=========+=============================+
                 |      ID | Encoding                    |
                 +=========+=============================+
                 |       0 | Reserved                    |
                 +---------+-----------------------------+
                 |       1 | JPEG [JFIF]                 |
                 +---------+-----------------------------+
                 | 100-110 | Private or Experimental Use |
                 +---------+-----------------------------+

                     Table 15: OpenPGP Image Attribute
                          Encoding Format Registry

   The rest of the image subpacket contains the image itself.  As the
   only currently defined image type is JPEG, the image is encoded in
   the JPEG File Interchange Format (JFIF), a standard file format for
   JPEG images [JFIF].

   An implementation MAY try to determine the type of an image by
   examination of the image data if it is unable to handle a particular
   version of the image header or if a specified encoding format value
   is not recognized.

5.13.  Symmetrically Encrypted and Integrity Protected Data Packet (Type
       ID 18)

   The SEIPD packet contains integrity-protected and encrypted data.
   When it has been decrypted, it will contain other packets forming an
   OpenPGP Message (see Section 10.3).

   The first octet of this packet is always used to indicate the version
   number, but different versions contain ciphertext that is structured
   differently.  Version 1 of this packet contains data encrypted with a
   symmetric key algorithm and is thus protected against modification by
   the SHA-1 hash algorithm.  This mechanism was introduced in [RFC4880]
   and offers some protections against ciphertext malleability.

   Version 2 of this packet contains data encrypted with an AEAD
   construction.  This offers a more cryptographically rigorous defense
   against ciphertext malleability.  See Section 13.7 for more details
   on choosing between these formats.

   Any new version of the SEIPD packet should be registered in the
   registry established in Section 10.3.2.1.

5.13.1.  Version 1 Symmetrically Encrypted and Integrity Protected Data
         Packet Format

   A version 1 Symmetrically Encrypted and Integrity Protected Data
   packet consists of:

   *  A 1-octet version number with value 1.

   *  Encrypted data -- the output of the selected symmetric key cipher
      operating in CFB mode.

   The symmetric cipher used MUST be specified in a Public Key or
   Symmetric Key Encrypted Session Key packet that precedes the
   Symmetrically Encrypted and Integrity Protected Data packet.  In
   either case, the cipher algorithm ID is prefixed to the session key
   before it is encrypted.

   The data is encrypted in CFB mode (see Section 12.9).  The IV is
   specified as all zeros.  Instead of achieving randomized encryption
   through an IV, OpenPGP prefixes an octet string to the data before it
   is encrypted for this purpose.  The length of the octet string equals
   the block size of the cipher in octets, plus two.  The first octets
   in the group, of length equal to the block size of the cipher, are
   random; the last two octets are each copies of their 2nd preceding
   octet.  For example, with a cipher whose block size is 128 bits or 16
   octets, the prefix data will contain 16 random octets, then two more
   octets, which are copies of the 15th and 16th octets, respectively.
   Unlike the deprecated Symmetrically Encrypted Data packet
   (Section 5.7), this prefix data is encrypted in the same CFB context,
   and no special CFB resynchronization is done.

   The repetition of 16 bits in the random data prefixed to the message
   allows the receiver to immediately check whether the session key is
   incorrect.  See Section 13.4 for hints on the proper use of this
   "quick check".

   Two constant octets with the values 0xD3 and 0x14 are appended to the
   plaintext.  Then, the plaintext of the data to be encrypted is passed
   through the SHA-1 hash function.  The input to the hash function is
   comprised of the prefix data described above and all of the
   plaintext, including the trailing constant octets 0xD3, 0x14.  The 20
   octets of the SHA-1 hash are then appended to the plaintext (after
   the constant octets 0xD3, 0x14) and encrypted along with the
   plaintext using the same CFB context.  This trailing checksum is
   known as the Modification Detection Code (MDC).

   During decryption, the plaintext data should be hashed with SHA-1,
   including the prefix data as well as the trailing constant octets
   0xD3, 0x14, but excluding the last 20 octets containing the SHA-1
   hash.  The computed SHA-1 hash is then compared with the last 20
   octets of plaintext.  A mismatch of the hash indicates that the
   message has been modified and MUST be treated as a security problem.
   Any failure SHOULD be reported to the user.

      NON-NORMATIVE EXPLANATION

      The MDC system, as the integrity protection mechanism of the
      version 1 Symmetrically Encrypted and Integrity Protected Data
      packet is called, was created to provide an integrity mechanism
      that is less strong than a signature, yet stronger than bare CFB
      encryption.

      CFB encryption has a limitation as damage to the ciphertext will
      corrupt the affected cipher blocks and the block following.
      Additionally, if data is removed from the end of a CFB-encrypted
      block, that removal is undetectable.  (Note also that CBC mode has
      a similar limitation, but data removed from the front of the block
      is undetectable.)

      The obvious way to protect or authenticate an encrypted block is
      to digitally sign it.  However, many people do not wish to
      habitually sign data for a large number of reasons that are beyond
      the scope of this document.  Suffice it to say that many people
      consider properties such as deniability to be as valuable as
      integrity.

      OpenPGP addresses this desire to have more security than raw
      encryption and yet preserve deniability with the MDC system.  An
      MDC is intentionally not a Message Authentication Code (MAC).  Its
      name was not selected by accident.  It is analogous to a checksum.

      Despite the fact that it is a relatively modest system, it has
      proved itself in the real world.  It is an effective defense to
      several attacks that have surfaced since it has been created.  It
      has met its modest goals admirably.

      Consequently, because it is a modest security system, it has
      modest requirements on the hash function(s) it employs.  It does
      not rely on a hash function being collision-free; it relies on a
      hash function being one-way.  If a forger, Frank, wishes to send
      Alice a (digitally) unsigned message that says, "I've always
      secretly loved you, signed Bob", it is far easier for him to
      construct a new message than it is to modify anything intercepted
      from Bob.  (Note also that if Bob wishes to communicate secretly
      with Alice, but without authentication or identification and with
      a threat model that includes forgers, he has a problem that
      transcends mere cryptography.)

      Note also that unlike nearly every other OpenPGP subsystem, there
      are no parameters in the MDC system.  It hard-defines SHA-1 as its
      hash function.  This is not an accident.  It is an intentional
      choice to avoid downgrade and cross-grade attacks while making a
      simple, fast system.  (A downgrade attack is an attack that would
      replace SHA2-256 with SHA-1, for example.  A cross-grade attack
      would replace SHA-1 with another 160-bit hash, such as RIPEMD-160,
      for example.)

      However, no update will be needed because the MDC has been
      replaced by the AEAD encryption described in this document.

5.13.2.  Version 2 Symmetrically Encrypted and Integrity Protected Data
         Packet Format

   A version 2 Symmetrically Encrypted and Integrity Protected Data
   packet consists of:

   *  A 1-octet version number with value 2.

   *  A 1-octet cipher algorithm ID.

   *  A 1-octet AEAD algorithm identifier.

   *  A 1-octet chunk size.

   *  32 octets of salt.  The salt is used to derive the message key and
      MUST be securely generated (see Section 13.10).

   *  Encrypted data; that is, the output of the selected symmetric key
      cipher operating in the given AEAD mode.

   *  A final summary authentication tag for the AEAD mode.

   The decrypted session key and the salt are used to derive an M-bit
   message key and N-64 bits used as the IV, where M is the key size of
   the symmetric algorithm and N is the nonce size of the AEAD
   algorithm.  M + N - 64 bits are derived using HKDF (see [RFC5869]).
   The leftmost M bits are used as a symmetric algorithm key, and the
   remaining N - 64 bits are used as an IV.  HKDF is used with SHA256
   [RFC6234] as hash algorithm.  The session key is used as IKM and the
   salt as salt.  The Packet Type ID in OpenPGP format encoding (bits 7
   and 6 are set, and bits 5-0 carry the Packet Type ID), version
   number, cipher algorithm ID, AEAD algorithm ID, and chunk size octet
   are used as info parameter.

   The KDF mechanism provides key separation between cipher and AEAD
   algorithms.  Furthermore, an implementation can securely reply to a
   message even if a recipient's certificate is unknown by reusing the
   Encrypted Session Key packets and replying with a different salt that
   yields a new, unique message key.  See Section 13.8 for guidance on
   how applications can securely implement this feature.

   A v2 SEIPD packet consists of one or more chunks of data.  The
   plaintext of each chunk is of a size specified by the chunk size
   octet using the method specified below.

   The encrypted data consists of the encryption of each chunk of
   plaintext, followed immediately by the relevant authentication tag.
   If the last chunk of plaintext is smaller than the chunk size, the
   ciphertext for that data may be shorter; nevertheless, it is followed
   by a full authentication tag.

   For each chunk, the AEAD construction is given the Packet Type ID
   encoded in OpenPGP format (bits 7 and 6 are set, and bits 5-0 carry
   the Packet Type ID), version number, cipher algorithm ID, AEAD
   algorithm ID, and chunk size octet as additional data.  For example,
   the additional data of the first chunk using EAX and AES-128 with a
   chunk size of 2^22 octets consists of the octets 0xD2, 0x02, 0x07,
   0x01, and 0x10.

   After the final chunk, the AEAD algorithm is used to produce a final
   authentication tag encrypting the empty string.  This AEAD instance
   is given the additional data specified above, plus an 8-octet, big-
   endian value specifying the total number of plaintext octets
   encrypted.  This allows detection of a truncated ciphertext.

   The chunk size octet specifies the size of chunks using the following
   formula (in C [C99]), where c is the chunk size octet:

     chunk_size = (uint32_t) 1 << (c + 6)

   An implementation MUST accept chunk size octets with values from 0 to
   16.  An implementation MUST NOT create data with a chunk size octet
   value larger than 16 (4 MiB chunks).

   The nonce for AEAD mode consists of two parts.  Let N be the size of
   the nonce.  The leftmost N - 64 bits are the IV derived using HKDF.
   The rightmost 64 bits are the chunk index as a big-endian value.  The
   index of the first chunk is zero.

5.13.3.  EAX Mode

   The EAX AEAD algorithm used in this document is defined in [EAX].

   The EAX algorithm can only use block ciphers with 16-octet blocks.
   The nonce is 16 octets long.  EAX authentication tags are 16 octets
   long.

5.13.4.  OCB Mode

   The OCB AEAD algorithm used in this document is defined in [RFC7253].

   The OCB algorithm can only use block ciphers with 16-octet blocks.
   The nonce is 15 octets long.  OCB authentication tags are 16 octets
   long.

5.13.5.  GCM Mode

   The GCM AEAD algorithm used in this document is defined in
   [SP800-38D].

   The GCM algorithm can only use block ciphers with 16-octet blocks.
   The nonce is 12 octets long.  GCM authentication tags are 16 octets
   long.

5.14.  Padding Packet (Type ID 21)

   The Padding packet contains random data and can be used to defend
   against traffic analysis (see Section 13.11) on v2 SEIPD messages
   (see Section 5.13.2) and Transferable Public Keys (see Section 10.1).

   Such a packet MUST be ignored when received.

   Its contents SHOULD be random octets to make the length obfuscation
   it provides more robust even when compressed.

   An implementation adding padding to an OpenPGP stream SHOULD place
   such a packet:

   *  At the end of a version 6 Transferable Public Key that is
      transferred over an encrypted channel (see Section 10.1).

   *  As the last packet of an Optionally Padded Message within a
      version 2 Symmetrically Encrypted and Integrity Protected Data
      packet (see Section 10.3.1).

   An implementation MUST be able to process Padding packets anywhere
   else in an OpenPGP stream so that future revisions of this document
   may specify further locations for padding.

   Policy about how large to make such a packet to defend against
   traffic analysis is beyond the scope of this document.

6.  Base64 Conversions

   As stated in the introduction, OpenPGP's underlying representation
   for objects is a stream of arbitrary octets, and some systems desire
   these objects to be immune to damage caused by character set
   translation, data conversions, etc.

   In principle, any printable encoding scheme that met the requirements
   of the unsafe channel would suffice, since it would not change the
   underlying binary bit streams of the OpenPGP data structures.  The
   OpenPGP specification specifies one such printable encoding scheme to
   ensure interoperability; see Section 6.2.

   The encoding is composed of two parts: a base64 encoding of the
   binary data and an optional checksum.  The base64 encoding used is
   described in Section 4 of [RFC4648], and it is wrapped into lines of
   no more than 76 characters each.

   When decoding base64, an OpenPGP implementation MUST ignore all
   whitespace.

6.1.  Optional Checksum

   The optional checksum is a 24-bit Cyclic Redundancy Check (CRC)
   converted to four characters of base64 encoding by the same MIME
   base64 transformation, preceded by an equal sign (=).  The CRC is
   computed by using the generator 0x864CFB and an initialization of
   0xB704CE.  The accumulation is done on the data before it is
   converted to base64 rather than on the converted data.  A sample
   implementation of this algorithm is in Section 6.1.1.

   If present, the checksum with its leading equal sign MUST appear on
   the next line after the base64-encoded data.

   An implementation MUST NOT reject an OpenPGP object when the CRC24
   footer is present, missing, malformed, or disagrees with the computed
   CRC24 sum.  When forming ASCII Armor, the CRC24 footer SHOULD NOT be
   generated, unless interoperability with implementations that require
   the CRC24 footer to be present is a concern.

   The CRC24 footer MUST NOT be generated if it can be determined by the
   context or by the OpenPGP object being encoded that the consuming
   implementation accepts base64-encoded blocks without a CRC24 footer.
   Notably:

   *  An ASCII-armored Encrypted Message packet sequence that ends in a
      v2 SEIPD packet MUST NOT contain a CRC24 footer.

   *  An ASCII-armored sequence of Signature packets that only includes
      version 6 Signature packets MUST NOT contain a CRC24 footer.

   *  An ASCII-armored Transferable Public Key packet sequence of a
      version 6 key MUST NOT contain a CRC24 footer.

   *  An ASCII-armored keyring consisting of only version 6 keys MUST
      NOT contain a CRC24 footer.

   Rationale: Previous draft versions of this document stated that the
   CRC24 footer is optional, but the text was ambiguous.  In practice,
   very few implementations require the CRC24 footer to be present.
   Computing the CRC24 incurs a significant cost, while providing no
   meaningful integrity protection.  Therefore, generating it is now
   discouraged.

6.1.1.  An Implementation of the CRC24 in "C"

   The following code is written in [C99].

   #define CRC24_INIT 0xB704CEL
   #define CRC24_GENERATOR 0x864CFBL

   typedef unsigned long crc24;
   crc24 crc_octets(unsigned char *octets, size_t len)
   {
       crc24 crc = CRC24_INIT;
       int i;
       while (len--) {
           crc ^= (*octets++) << 16;
           for (i = 0; i < 8; i++) {
               crc <<= 1;
               if (crc & 0x1000000) {
                   crc &= 0XFFFFFF; /* Clear bit 25 to avoid overflow */
                   crc ^= CRC24_GENERATOR;
               }
           }
       }
       return crc & 0xFFFFFFL;
   }

6.2.  Forming ASCII Armor

   When OpenPGP encodes data into ASCII Armor, it puts specific headers
   around the base64-encoded data, so OpenPGP can reconstruct the data
   later.  An OpenPGP implementation MAY use ASCII Armor to protect raw
   binary data.  OpenPGP informs the user what kind of data is encoded
   in the ASCII Armor through the use of the headers.

   Concatenating the following data creates ASCII Armor:

   *  An Armor Header Line, appropriate for the type of data

   *  Armor Headers

   *  A blank (zero length or containing only whitespace) line

   *  The ASCII-Armored data

   *  An optional Armor Checksum (discouraged; see Section 6.1)

   *  The Armor Tail, which depends on the Armor Header Line

6.2.1.  Armor Header Line

   An Armor Header Line consists of the appropriate header line text
   surrounded by five (5) dashes (-, 0x2D) on either side of the header
   line text.  The header line text is chosen based on the type of data
   being encoded in Armor and how it is being encoded.  Header line
   texts include the following strings:

    +===================+============================================+
    | Armor Header      | Use                                        |
    +===================+============================================+
    | BEGIN PGP MESSAGE | Used for signed, encrypted, or compressed  |
    |                   | files.                                     |
    +-------------------+--------------------------------------------+
    | BEGIN PGP PUBLIC  | Used for armoring public keys.             |
    | KEY BLOCK         |                                            |
    +-------------------+--------------------------------------------+
    | BEGIN PGP PRIVATE | Used for armoring private keys.            |
    | KEY BLOCK         |                                            |
    +-------------------+--------------------------------------------+
    | BEGIN PGP         | Used for detached signatures, OpenPGP/MIME |
    | SIGNATURE         | signatures, and cleartext signatures.      |
    +-------------------+--------------------------------------------+

              Table 16: OpenPGP Armor Header Lines Registry

   Note that all of these Armor Header Lines are to consist of a
   complete line.  Therefore, the header lines MUST start at the
   beginning of a line and MUST NOT have text other than whitespace
   following them on the same line.

6.2.2.  Armor Headers

   The Armor Headers are pairs of strings that can give the user or the
   receiving OpenPGP implementation some information about how to decode
   or use the message.  The Armor Headers are a part of the armor, not
   the message, and hence are not protected by any signatures applied to
   the message.

   The format of an Armor Header is that of a key-value pair.  A colon
   (: 0x3A) and a single space (0x20) separate the key and value.  An
   OpenPGP implementation may consider improperly formatted Armor
   Headers to be a corruption of the ASCII Armor, but it SHOULD make an
   effort to recover.  Unknown keys should be silently ignored, and an
   OpenPGP implementation SHOULD continue to process the message.

   Note that some transport methods are sensitive to line length.  For
   example, the SMTP protocol that transports email messages has a line
   length limit of 998 characters (see Section 2.1.1 of [RFC5322]).

   While there is a limit of 76 characters for the base64 data
   (Section 6), there is no limit for the length of Armor Headers.  Care
   should be taken to ensure that the Armor Headers are short enough to
   survive transport.  One way to do this is to repeat an Armor Header
   Key multiple times with different values for each so that no one line
   is overly long.

   Currently defined Armor Header Keys are as follows:

       +=========+==============================+=================+
       | Key     | Summary                      | Reference       |
       +=========+==============================+=================+
       | Version | Implementation information   | Section 6.2.2.1 |
       +---------+------------------------------+-----------------+
       | Comment | Arbitrary text               | Section 6.2.2.2 |
       +---------+------------------------------+-----------------+
       | Hash    | Hash algorithms used in some | Section 6.2.2.3 |
       |         | v4 cleartext signed messages |                 |
       +---------+------------------------------+-----------------+
       | Charset | Character set                | Section 6.2.2.4 |
       +---------+------------------------------+-----------------+

               Table 17: OpenPGP Armor Header Keys Registry

6.2.2.1.  "Version" Armor Header

   The Armor Header Key Version describes the OpenPGP implementation and
   version used to encode the message.  To minimize metadata,
   implementations SHOULD NOT emit this key and its corresponding value
   except for debugging purposes with explicit user consent.

6.2.2.2.  "Comment" Armor Header

   The Armor Header Key Comment supplies a user-defined comment.
   OpenPGP defines all text to be in UTF-8.  A comment may be any UTF-8
   string.  However, the whole point of armoring is to provide 7-bit
   clean data.  Consequently, if a comment has characters that are
   outside the ASCII range of UTF-8, they may very well not survive
   transport.

6.2.2.3.  "Hash" Armor Header

   The Armor Header Key Hash is deprecated, but some older
   implementations expect it in messages using the Cleartext Signature
   Framework (Section 7).  When present, this Armor Header Key contains
   a comma-separated list of hash algorithms used in the signatures on
   message, with digest names as specified in the "Text Name" column in
   Table 23.  These headers SHOULD NOT be emitted unless:

   *  the cleartext signed message contains a version 4 signature made
      using a SHA2-based digest (SHA224, SHA256, SHA384, or SHA512), and

   *  the cleartext signed message might be verified by a legacy OpenPGP
      implementation that requires this header.

   A verifying application MUST decline to validate any signature in a
   message with a non-conformant Hash header (that is, a Hash header
   that contains anything other than a comma-separated list of hash
   algorithms).  When a conformant Hash header is present, a verifying
   application MUST ignore its contents, deferring to the hash algorithm
   indicated in the Embedded Signature.

6.2.2.4.  "Charset" Armor Header

   The Armor Header Key Charset contains a description of the character
   set that the plaintext is in (see [RFC2978]).  Please note that
   OpenPGP defines text to be in UTF-8.  An implementation will get the
   best results by translating into and out of UTF-8.  However, there
   are many instances where this is easier said than done.  Also, there
   are communities of users who have no need for UTF-8 because they are
   all happy with a character set like ISO Latin-5 or a Japanese
   character set.  In such instances, an implementation MAY override the
   UTF-8 default by using this header key.  An implementation MAY
   implement this key and any translations it cares to; an
   implementation MAY ignore it and assume all text is UTF-8.

6.2.3.  Armor Tail Line

   The Armor Tail Line is composed in the same manner as the Armor
   Header Line, except the string "BEGIN" is replaced by the string
   "END".

7.  Cleartext Signature Framework

   It is desirable to be able to sign a textual octet stream without
   ASCII armoring the stream itself, so the signed text is still
   readable with any tool capable of rendering text.  In order to bind a
   signature to such a cleartext, the Cleartext Signature Framework is
   used, which follows the same basic format and restrictions as the
   ASCII armoring described in Section 6.2.  (Note that this framework
   is not intended to be reversible.  [RFC3156] defines another way to
   sign cleartext messages for environments that support MIME.)

7.1.  Cleartext Signed Message Structure

   An OpenPGP cleartext signed message consists of:

   *  The cleartext header -----BEGIN PGP SIGNED MESSAGE----- on a
      single line.

   *  One or more legacy Hash Armor Headers that MAY be included by some
      implementations and MUST be ignored when well formed (see
      Section 6.2.2.3).

   *  An empty line (not included in the message digest).

   *  The dash-escaped cleartext.

   *  A line ending separating the cleartext and following armored
      signature (not included in the message digest).

   *  The ASCII-armored signature(s), including the -----BEGIN PGP
      SIGNATURE----- Armor Header and Armor Tail Lines.

   As with any other Text signature (Section 5.2.1.2), a cleartext
   signature is calculated on the text using canonical <CR><LF> line
   endings.  As described above, the line ending before the -----BEGIN
   PGP SIGNATURE----- Armor Header Line of the armored signature is not
   considered part of the signed text.

   Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
   the end of any line is removed before signing or verifying a
   cleartext signed message.

   Between the -----BEGIN PGP SIGNED MESSAGE----- line and the first
   empty line, the only Armor Header permitted is a well-formed Hash
   Armor Header (see Section 6.2.2.3).  To reduce the risk of confusion
   about what has been signed, a verifying implementation MUST decline
   to validate any signature in a cleartext message if that message has
   any other Armor Header present in this location.

7.2.  Dash-Escaped Text

   The cleartext content of the message must also be dash-escaped.

   Dash-escaped cleartext is the ordinary cleartext where every line
   starting with a "-" (HYPHEN-MINUS, U+002D) is prefixed by the
   sequence "-" (HYPHEN-MINUS, U+002D) and " " (SPACE, U+0020).  This
   prevents the parser from recognizing Armor Headers of the cleartext
   itself.  An implementation MAY dash-escape any line, SHOULD dash-
   escape lines commencing in "From" followed by a space, and MUST dash-
   escape any line commencing in a dash.  The message digest is computed
   using the cleartext itself, not the dash-escaped form.

   When reversing dash-escaping, an implementation MUST strip the string
   - if it occurs at the beginning of a line, and it SHOULD provide a
   warning for - and any character other than a space at the beginning
   of a line.

7.3.  Issues with the Cleartext Signature Framework

   Since creating a cleartext signed message involves trimming trailing
   whitespace on every line, the Cleartext Signature Framework will fail
   to safely round-trip any textual stream that may include semantically
   meaningful whitespace.

   For example, the Unified Diff format [UNIFIED-DIFF] contains
   semantically meaningful whitespace: an empty line of context will
   consist of a line with a single " " (SPACE, U+0020) character, and
   any line that has trailing whitespace added or removed will represent
   such a change with semantically meaningful whitespace.

   Furthermore, a Cleartext Signature Framework message that is very
   large is unlikely to work well.  In particular, it will be difficult
   for any human reading the message to know which part is covered by
   the signature because they can't understand the whole message at
   once, especially in the case where an Armor Header line is placed
   somewhere in the body.  And, very large Cleartext Signature Framework
   messages cannot be processed in a single pass, since the signature
   salt and digest algorithms are only discovered at the end.

   An implementation that knows it is working with a textual stream with
   any of the above characteristics SHOULD NOT use the Cleartext
   Signature Framework.  Safe alternatives for a semantically meaningful
   OpenPGP signature over such a file format are:

   *  A signed message, as described in Section 10.3.

   *  A detached signature, as described in Section 10.4.

   Either of these alternatives may be ASCII-armored (see Section 6.2)
   if they need to be transmitted across a text-only (or 7-bit clean)
   channel.

   Finally, when a Cleartext Signature Framework message is presented to
   the user as is, an attacker can include additional text in the Hash
   header, which may mislead the user into thinking it is part of the
   signed text.  The signature validation constraints described in
   Sections 6.2.2.3 and 7.1 help to mitigate the risk of arbitrary or
   misleading text in the Armor Headers.

8.  Regular Expressions

   This section describes Regular Expressions.

   Regular Expression:  Zero or more branches, separated by |. It
      matches anything that matches one of the branches.

   Branch:  Zero or more pieces, concatenated.  It matches a match for
      the first, followed by a match for the second, etc.

   Piece:  An atom possibly followed by *, +, or ?. An atom followed by
      * matches a sequence of 0 or more matches of the atom.  An atom
      followed by + matches a sequence of 1 or more matches of the atom.
      An atom followed by ? matches a match of the atom or the null
      string.

   Atom:  A Regular Expression in parentheses (matching a match for the
      Regular Expression), a range (see below), a . (matching any single
      Unicode character), a ^ (matching the null string at the beginning
      of the input string), a $ (matching the null string at the end of
      the input string), a \ followed by a single Unicode character
      (matching that character), or a single Unicode character with no
      other significance (matching that character).

   Range:  A sequence of characters enclosed in [].  It normally matches
      any single character from the sequence.  If the sequence begins
      with ^, it matches any single Unicode character not from the rest
      of the sequence.  If two characters in the sequence are separated
      by -, this is shorthand for the full list of Unicode characters
      between them in codepoint order (for example, [0-9] matches any
      decimal digit).  To include a literal ] in the sequence, make it
      the first character (following a possible ^).  To include a
      literal -, make it the first or last character.

9.  Constants

   This section describes the constants used in OpenPGP.

   Note that these tables are not exhaustive lists; an implementation
   MAY implement an algorithm that is not on these lists, as long as the
   algorithm IDs are chosen from the Private or Experimental Use
   algorithm range.

   See Section 12 for more discussion of the algorithms.

9.1.  Public Key Algorithms

   +===+==============+=========+============+===========+=============+
   | ID| Algorithm    |Public   | Secret Key | Signature | PKESK       |
   |   |              |Key      | Format     | Format    | Format      |
   |   |              |Format   |            |           |             |
   +===+==============+=========+============+===========+=============+
   |  0| Reserved     |         |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   |  1| RSA (Encrypt |MPI(n),  | MPI(d),    | MPI(m^d   | MPI(m^e     |
   |   | or Sign)     |MPI(e)   | MPI(p),    | mod n)    | mod n)      |
   |   | [FIPS186]    |[Section | MPI(q),    | [Section  | [Section    |
   |   |              |5.5.5.1] | MPI(u)     | 5.2.3.1]  | 5.1.3]      |
   +---+--------------+---------+------------+-----------+-------------+
   |  2| RSA Encrypt- |MPI(n),  | MPI(d),    | N/A       | MPI(m^e     |
   |   | Only         |MPI(e)   | MPI(p),    |           | mod n)      |
   |   | [FIPS186]    |[Section | MPI(q),    |           | [Section    |
   |   |              |5.5.5.1] | MPI(u)     |           | 5.1.3]      |
   +---+--------------+---------+------------+-----------+-------------+
   |  3| RSA Sign-    |MPI(n),  | MPI(d),    | MPI(m^d   | N/A         |
   |   | Only         |MPI(e)   | MPI(p),    | mod n)    |             |
   |   | [FIPS186]    |[Section | MPI(q),    | [Section  |             |
   |   |              |5.5.5.1] | MPI(u)     | 5.2.3.1]  |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 16| Elgamal      |MPI(p),  | MPI(x)     | N/A       | MPI(g^k     |
   |   | (Encrypt-    |MPI(g),  |            |           | mod p),     |
   |   | Only)        |MPI(y)   |            |           | MPI(m *     |
   |   | [ELGAMAL]    |[Section |            |           | y^k mod     |
   |   |              |5.5.5.3] |            |           | p)          |
   |   |              |         |            |           | [Section    |
   |   |              |         |            |           | 5.1.4]      |
   +---+--------------+---------+------------+-----------+-------------+
   | 17| DSA (Digital |MPI(p),  | MPI(x)     | MPI(r),   | N/A         |
   |   | Signature    |MPI(q),  |            | MPI(s)    |             |
   |   | Algorithm)   |MPI(g),  |            | [Section  |             |
   |   | [FIPS186]    |MPI(y)   |            | 5.2.3.2]  |             |
   |   |              |[Section |            |           |             |
   |   |              |5.5.5.2] |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 18| ECDH public  |OID,     | MPI(value  | N/A       | MPI(point   |
   |   | key          |MPI(point| in curve-  |           | in curve-   |
   |   | algorithm    |in curve-| specific   |           | specific    |
   |   |              |specific | format)    |           | point       |
   |   |              |point    | [Section   |           | format),    |
   |   |              |format), | 9.2.1]     |           | size        |
   |   |              |KDFParams|            |           | octet,      |
   |   |              |[Sections|            |           | encoded     |
   |   |              |9.2.1 and|            |           | key         |
   |   |              |5.5.5.6] |            |           | [Sections   |
   |   |              |         |            |           | 9.2.1,      |
   |   |              |         |            |           | 5.1.5,      |
   |   |              |         |            |           | and 11.5]   |
   +---+--------------+---------+------------+-----------+-------------+
   | 19| ECDSA public |OID,     | MPI(value) | MPI(r),   | N/A         |
   |   | key          |MPI(point|            | MPI(s)    |             |
   |   | algorithm    |in SEC1  |            | [Section  |             |
   |   | [FIPS186]    |format)  |            | 5.2.3.2]  |             |
   |   |              |[Section |            |           |             |
   |   |              |5.5.5.4] |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 20| Reserved     |         |            |           |             |
   |   | (formerly    |         |            |           |             |
   |   | Elgamal      |         |            |           |             |
   |   | Encrypt or   |         |            |           |             |
   |   | Sign)        |         |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 21| Reserved for |         |            |           |             |
   |   | Diffie-      |         |            |           |             |
   |   | Hellman      |         |            |           |             |
   |   | (X9.42, as   |         |            |           |             |
   |   | defined for  |         |            |           |             |
   |   | IETF-S/MIME) |         |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 22| EdDSALegacy  |OID,     | MPI(value  | MPI, MPI  | N/A         |
   |   | (deprecated) |MPI(point| in curve-  | [Sections |             |
   |   |              |in       | specific   | 9.2.1 and |             |
   |   |              |prefixed | format)    | 5.2.3.3]  |             |
   |   |              |native   | [Section   |           |             |
   |   |              |format)  | 9.2.1]     |           |             |
   |   |              |[Sections|            |           |             |
   |   |              |11.2.2   |            |           |             |
   |   |              |and      |            |           |             |
   |   |              |5.5.5.5] |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 23| Reserved     |         |            |           |             |
   |   | (AEDH)       |         |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 24| Reserved     |         |            |           |             |
   |   | (AEDSA)      |         |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 25| X25519       |32 octets| 32 octets  | N/A       | 32          |
   |   |              |[Section |            |           | octets,     |
   |   |              |5.5.5.7] |            |           | size        |
   |   |              |         |            |           | octet,      |
   |   |              |         |            |           | encoded     |
   |   |              |         |            |           | key         |
   |   |              |         |            |           | [Section    |
   |   |              |         |            |           | 5.1.6]      |
   +---+--------------+---------+------------+-----------+-------------+
   | 26| X448         |56 octets| 56 octets  | N/A       | 56          |
   |   |              |[Section |            |           | octets,     |
   |   |              |5.5.5.8] |            |           | size        |
   |   |              |         |            |           | octet,      |
   |   |              |         |            |           | encoded     |
   |   |              |         |            |           | key         |
   |   |              |         |            |           | [Section    |
   |   |              |         |            |           | 5.1.7]      |
   +---+--------------+---------+------------+-----------+-------------+
   | 27| Ed25519      |32 octets| 32 octets  | 64 octets |             |
   |   |              |[Section |            | [Section  |             |
   |   |              |5.5.5.9] |            | 5.2.3.4]  |             |
   +---+--------------+---------+------------+-----------+-------------+
   | 28| Ed448        |57 octets| 57 octets  | 114       |             |
   |   |              |[Section |            | octets    |             |
   |   |              |5.5.5.10]|            | [Section  |             |
   |   |              |         |            | 5.2.3.5]  |             |
   +---+--------------+---------+------------+-----------+-------------+
   |100| Private or   |         |            |           |             |
   | to| Experimental |         |            |           |             |
   |110| Use          |         |            |           |             |
   +---+--------------+---------+------------+-----------+-------------+

              Table 18: OpenPGP Public Key Algorithms Registry

   Implementations MUST implement Ed25519 (27) for signatures and X25519
   (25) for encryption.  Implementations SHOULD implement Ed448 (28) and
   X448 (26).

   RSA (1) keys are deprecated and SHOULD NOT be generated but may be
   interpreted.  RSA Encrypt-Only (2) and RSA Sign-Only (3) are
   deprecated and MUST NOT be generated (see Section 12.4).  Elgamal
   (16) keys are deprecated and MUST NOT be generated (see
   Section 12.6).  DSA (17) keys are deprecated and MUST NOT be
   generated (see Section 12.5).  For notes on Elgamal Encrypt or Sign
   (20) and X9.42 (21), see Section 12.8.  Implementations MAY implement
   any other algorithm.

   Note that an implementation conforming to the previous version of
   this specification [RFC4880] has only DSA (17) and Elgamal (16) as
   the algorithms that MUST be implemented.

   A compatible specification of ECDSA is given in [RFC6090] (as "KT-I
   Signatures") and in [SEC1]; ECDH is defined in Section 11.5 of this
   document.

9.2.  ECC Curves for OpenPGP

   The parameter curve OID is an array of octets that defines a named
   curve.

   The table below specifies the exact sequence of octets for each named
   curve referenced in this document.  It also specifies which public
   key algorithms the curve can be used with, as well as the size of
   expected elements in octets.  Note that there is a break in
   "EdDSALegacy" for display purposes only.

   +======================+===+========+================+======+=======+
   |ASN.1 Object          |OID| Curve  |Curve Name      |Usage |Field  |
   |Identifier            |Len| OID    |                |      |Size   |
   |                      |   | Octets |                |      |(fsize)|
   +======================+===+========+================+======+=======+
   |1.2.840.10045.3.1.7   |8  | 2A 86  |NIST P-256      |ECDSA,|32     |
   |                      |   | 48 CE  |                |ECDH  |       |
   |                      |   | 3D 03  |                |      |       |
   |                      |   | 01 07  |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.132.0.34          |5  | 2B 81  |NIST P-384      |ECDSA,|48     |
   |                      |   | 04 00  |                |ECDH  |       |
   |                      |   | 22     |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.132.0.35          |5  | 2B 81  |NIST P-521      |ECDSA,|66     |
   |                      |   | 04 00  |                |ECDH  |       |
   |                      |   | 23     |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.36.3.3.2.8.1.1.7  |9  | 2B 24  |brainpoolP256r1 |ECDSA,|32     |
   |                      |   | 03 03  |                |ECDH  |       |
   |                      |   | 02 08  |                |      |       |
   |                      |   | 01 01  |                |      |       |
   |                      |   | 07     |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.36.3.3.2.8.1.1.11 |9  | 2B 24  |brainpoolP384r1 |ECDSA,|48     |
   |                      |   | 03 03  |                |ECDH  |       |
   |                      |   | 02 08  |                |      |       |
   |                      |   | 01 01  |                |      |       |
   |                      |   | 0B     |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.36.3.3.2.8.1.1.13 |9  | 2B 24  |brainpoolP512r1 |ECDSA,|64     |
   |                      |   | 03 03  |                |ECDH  |       |
   |                      |   | 02 08  |                |      |       |
   |                      |   | 01 01  |                |      |       |
   |                      |   | 0D     |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.6.1.4.1.11591.15.1|9  | 2B 06  |Ed25519Legacy   |EdDSA |32     |
   |                      |   | 01 04  |                |Legacy|       |
   |                      |   | 01 DA  |                |      |       |
   |                      |   | 47 0F  |                |      |       |
   |                      |   | 01     |                |      |       |
   +----------------------+---+--------+----------------+------+-------+
   |1.3.6.1.4.1.3029.1.5.1|10 | 2B 06  |Curve25519Legacy|ECDH  |32     |
   |                      |   | 01 04  |                |      |       |
   |                      |   | 01 97  |                |      |       |
   |                      |   | 55 01  |                |      |       |
   |                      |   | 05 01  |                |      |       |
   +----------------------+---+--------+----------------+------+-------+

            Table 19: OpenPGP ECC Curve OIDs and Usage Registry

   The "Field Size (fsize)" column represents the field size of the
   group in number of octets, rounded up, such that x or y coordinates
   for a point on the curve or native point representations for the
   curve can be represented in that many octets.  The curves specified
   here, and scalars such as the base point order and the private key,
   can be represented in fsize octets.  However, note that curves exist
   outside this specification where the representation of scalars
   requires an additional octet.

   The sequence of octets in the third column is the result of applying
   the Distinguished Encoding Rules (DER) to the ASN.1 Object Identifier
   with subsequent truncation.  The truncation removes the two fields of
   encoded Object Identifier.  The first omitted field is 1 octet
   representing the Object Identifier tag, and the second omitted field
   is the length of the Object Identifier body.  For example, the
   complete ASN.1 DER encoding for the NIST P-256 curve OID is "06 08 2A
   86 48 CE 3D 03 01 07", from which the first entry in the table above
   is constructed by omitting the first two octets.  Only the truncated
   sequence of octets is the valid representation of a curve OID.

   The deprecated OIDs for Ed25519Legacy and Curve25519Legacy are used
   only in version 4 keys and signatures.  Implementations MAY implement
   these variants for compatibility with existing version 4 key material
   and signatures.  Implementations MUST NOT accept or generate version
   6 key material using the deprecated OIDs.

9.2.1.  Curve-Specific Wire Formats

   Some elliptic curve public key algorithms use different conventions
   for specific fields depending on the curve in use.  Each field is
   always formatted as an MPI, but with a curve-specific framing.  This
   table summarizes those distinctions.

   +================+========+============+=======+=========+==========+
   |Curve           |ECDH    |ECDH Secret |EdDSA  |EdDSA    |EdDSA     |
   |                |Point   |Key MPI     |Secret |Signature|Signature |
   |                |Format  |            |Key MPI|first MPI|second    |
   |                |        |            |       |         |MPI       |
   +================+========+============+=======+=========+==========+
   |NIST P-256      |SEC1    |integer     |N/A    |N/A      |N/A       |
   +----------------+--------+------------+-------+---------+----------+
   |NIST P-384      |SEC1    |integer     |N/A    |N/A      |N/A       |
   +----------------+--------+------------+-------+---------+----------+
   |NIST P-521      |SEC1    |integer     |N/A    |N/A      |N/A       |
   +----------------+--------+------------+-------+---------+----------+
   |brainpoolP256r1 |SEC1    |integer     |N/A    |N/A      |N/A       |
   +----------------+--------+------------+-------+---------+----------+
   |brainpoolP384r1 |SEC1    |integer     |N/A    |N/A      |N/A       |
   +----------------+--------+------------+-------+---------+----------+
   |brainpoolP512r1 |SEC1    |integer     |N/A    |N/A      |N/A       |
   +----------------+--------+------------+-------+---------+----------+
   |Ed25519Legacy   |N/A     |N/A         |32     |32 octets|32 octets |
   |                |        |            |octets |of R     |of S      |
   |                |        |            |of     |         |          |
   |                |        |            |secret |         |          |
   +----------------+--------+------------+-------+---------+----------+
   |Curve25519Legacy|prefixed|integer     |N/A    |N/A      |N/A       |
   |                |native  |(Section    |       |         |          |
   |                |        |5.5.5.6.1.1)|       |         |          |
   +----------------+--------+------------+-------+---------+----------+

         Table 20: OpenPGP ECC Curve-Specific Wire Formats Registry

   For the native octet-string forms of Ed25519Legacy values, see
   [RFC8032].  For the native octet-string forms of Curve25519Legacy
   secret scalars and points, see [RFC7748].

9.3.  Symmetric Key Algorithms

         +=========+============================================+
         |      ID | Algorithm                                  |
         +=========+============================================+
         |       0 | Plaintext or unencrypted data              |
         +---------+--------------------------------------------+
         |       1 | IDEA [IDEA]                                |
         +---------+--------------------------------------------+
         |       2 | TripleDES (or DES-EDE) [SP800-67] with     |
         |         | 168-bit key derived from 192               |
         +---------+--------------------------------------------+
         |       3 | CAST5 with 128-bit key [RFC2144]           |
         +---------+--------------------------------------------+
         |       4 | Blowfish with 128-bit key, 16 rounds       |
         |         | [BLOWFISH]                                 |
         +---------+--------------------------------------------+
         |       5 | Reserved                                   |
         +---------+--------------------------------------------+
         |       6 | Reserved                                   |
         +---------+--------------------------------------------+
         |       7 | AES with 128-bit key [AES]                 |
         +---------+--------------------------------------------+
         |       8 | AES with 192-bit key                       |
         +---------+--------------------------------------------+
         |       9 | AES with 256-bit key                       |
         +---------+--------------------------------------------+
         |      10 | Twofish with 256-bit key [TWOFISH]         |
         +---------+--------------------------------------------+
         |      11 | Camellia with 128-bit key [RFC3713]        |
         +---------+--------------------------------------------+
         |      12 | Camellia with 192-bit key                  |
         +---------+--------------------------------------------+
         |      13 | Camellia with 256-bit key                  |
         +---------+--------------------------------------------+
         | 100-110 | Private or Experimental Use                |
         +---------+--------------------------------------------+
         | 253-255 | Reserved to avoid collision with Secret    |
         |         | Key Encryption (Table 2 and Section 5.5.3) |
         +---------+--------------------------------------------+

           Table 21: OpenPGP Symmetric Key Algorithms Registry

   Implementations MUST implement AES-128.  Implementations SHOULD
   implement AES-256.  Implementations MUST NOT encrypt data with IDEA,
   TripleDES, or CAST5.  Implementations MAY decrypt data that uses
   IDEA, TripleDES, or CAST5 for the sake of reading older messages or
   new messages from implementations predating support for [RFC2440].
   An Implementation that decrypts data using IDEA, TripleDES, or CAST5
   SHOULD generate a deprecation warning about the symmetric algorithm,
   indicating that message confidentiality is suspect.  Implementations
   MAY implement any other algorithm.

9.4.  Compression Algorithms

                 +=========+=============================+
                 |      ID | Algorithm                   |
                 +=========+=============================+
                 |       0 | Uncompressed                |
                 +---------+-----------------------------+
                 |       1 | ZIP [RFC1951]               |
                 +---------+-----------------------------+
                 |       2 | ZLIB [RFC1950]              |
                 +---------+-----------------------------+
                 |       3 | BZip2 [BZ2]                 |
                 +---------+-----------------------------+
                 | 100-110 | Private or Experimental Use |
                 +---------+-----------------------------+

                       Table 22: OpenPGP Compression
                            Algorithms Registry

   Implementations MUST implement uncompressed data.  Implementations
   SHOULD implement ZLIB.  For interoperability reasons, implementations
   SHOULD be able to decompress using ZIP.  Implementations MAY
   implement any other algorithm.

9.5.  Hash Algorithms

   +=========+==================+=============+========================+
   |      ID | Algorithm        | Text Name   | V6 Signature           |
   |         |                  |             | Salt Size              |
   +=========+==================+=============+========================+
   |       0 | Reserved         |             |                        |
   +---------+------------------+-------------+------------------------+
   |       1 | MD5 [RFC1321]    | "MD5"       | N/A                    |
   +---------+------------------+-------------+------------------------+
   |       2 | SHA-1 [FIPS180]  | "SHA1"      | N/A                    |
   +---------+------------------+-------------+------------------------+
   |       3 | RIPEMD-160       | "RIPEMD160" | N/A                    |
   |         | [RIPEMD-160]     |             |                        |
   +---------+------------------+-------------+------------------------+
   |       4 | Reserved         |             |                        |
   +---------+------------------+-------------+------------------------+
   |       5 | Reserved         |             |                        |
   +---------+------------------+-------------+------------------------+
   |       6 | Reserved         |             |                        |
   +---------+------------------+-------------+------------------------+
   |       7 | Reserved         |             |                        |
   +---------+------------------+-------------+------------------------+
   |       8 | SHA2-256         | "SHA256"    | 16                     |
   |         | [FIPS180]        |             |                        |
   +---------+------------------+-------------+------------------------+
   |       9 | SHA2-384         | "SHA384"    | 24                     |
   |         | [FIPS180]        |             |                        |
   +---------+------------------+-------------+------------------------+
   |      10 | SHA2-512         | "SHA512"    | 32                     |
   |         | [FIPS180]        |             |                        |
   +---------+------------------+-------------+------------------------+
   |      11 | SHA2-224         | "SHA224"    | 16                     |
   |         | [FIPS180]        |             |                        |
   +---------+------------------+-------------+------------------------+
   |      12 | SHA3-256         | "SHA3-256"  | 16                     |
   |         | [FIPS202]        |             |                        |
   +---------+------------------+-------------+------------------------+
   |      13 | Reserved         |             |                        |
   +---------+------------------+-------------+------------------------+
   |      14 | SHA3-512         | "SHA3-512"  | 32                     |
   |         | [FIPS202]        |             |                        |
   +---------+------------------+-------------+------------------------+
   | 100-110 | Private or       |             |                        |
   |         | Experimental Use |             |                        |
   +---------+------------------+-------------+------------------------+

                 Table 23: OpenPGP Hash Algorithms Registry

    +============+=========================+=========================+
    | Hash       | OID                     | Full Hash Prefix        |
    | Algorithm  |                         |                         |
    +============+=========================+=========================+
    | MD5        | 1.2.840.113549.2.5      | 0x30, 0x20, 0x30, 0x0C, |
    |            |                         | 0x06, 0x08, 0x2A, 0x86, |
    |            |                         | 0x48, 0x86, 0xF7, 0x0D, |
    |            |                         | 0x02, 0x05, 0x05, 0x00, |
    |            |                         | 0x04, 0x10              |
    +------------+-------------------------+-------------------------+
    | SHA-1      | 1.3.14.3.2.26           | 0x30, 0x21, 0x30, 0x09, |
    |            |                         | 0x06, 0x05, 0x2B, 0x0E, |
    |            |                         | 0x03, 0x02, 0x1A, 0x05, |
    |            |                         | 0x00, 0x04, 0x14        |
    +------------+-------------------------+-------------------------+
    | RIPEMD-160 | 1.3.36.3.2.1            | 0x30, 0x21, 0x30, 0x09, |
    |            |                         | 0x06, 0x05, 0x2B, 0x24, |
    |            |                         | 0x03, 0x02, 0x01, 0x05, |
    |            |                         | 0x00, 0x04, 0x14        |
    +------------+-------------------------+-------------------------+
    | SHA2-256   | 2.16.840.1.101.3.4.2.1  | 0x30, 0x31, 0x30, 0x0D, |
    |            |                         | 0x06, 0x09, 0x60, 0x86, |
    |            |                         | 0x48, 0x01, 0x65, 0x03, |
    |            |                         | 0x04, 0x02, 0x01, 0x05, |
    |            |                         | 0x00, 0x04, 0x20        |
    +------------+-------------------------+-------------------------+
    | SHA2-384   | 2.16.840.1.101.3.4.2.2  | 0x30, 0x41, 0x30, 0x0D, |
    |            |                         | 0x06, 0x09, 0x60, 0x86, |
    |            |                         | 0x48, 0x01, 0x65, 0x03, |
    |            |                         | 0x04, 0x02, 0x02, 0x05, |
    |            |                         | 0x00, 0x04, 0x30        |
    +------------+-------------------------+-------------------------+
    | SHA2-512   | 2.16.840.1.101.3.4.2.3  | 0x30, 0x51, 0x30, 0x0D, |
    |            |                         | 0x06, 0x09, 0x60, 0x86, |
    |            |                         | 0x48, 0x01, 0x65, 0x03, |
    |            |                         | 0x04, 0x02, 0x03, 0x05, |
    |            |                         | 0x00, 0x04, 0x40        |
    +------------+-------------------------+-------------------------+
    | SHA2-224   | 2.16.840.1.101.3.4.2.4  | 0x30, 0x2D, 0x30, 0x0D, |
    |            |                         | 0x06, 0x09, 0x60, 0x86, |
    |            |                         | 0x48, 0x01, 0x65, 0x03, |
    |            |                         | 0x04, 0x02, 0x04, 0x05, |
    |            |                         | 0x00, 0x04, 0x1C        |
    +------------+-------------------------+-------------------------+
    | SHA3-256   | 2.16.840.1.101.3.4.2.8  | 0x30, 0x31, 0x30, 0x0D, |
    |            |                         | 0x06, 0x09, 0x60, 0x86, |
    |            |                         | 0x48, 0x01, 0x65, 0x03, |
    |            |                         | 0x04, 0x02, 0x08, 0x05, |
    |            |                         | 0x00, 0x04, 0x20        |
    +------------+-------------------------+-------------------------+
    | SHA3-512   | 2.16.840.1.101.3.4.2.10 | 0x30, 0x51, 0x30, 0x0D, |
    |            |                         | 0x06, 0x09, 0x60, 0x86, |
    |            |                         | 0x48, 0x01, 0x65, 0x03, |
    |            |                         | 0x04, 0x02, 0x0a, 0x05, |
    |            |                         | 0x00, 0x04, 0x40        |
    +------------+-------------------------+-------------------------+

           Table 24: OpenPGP Hash Algorithm Identifiers for RSA
           Signatures' Use of EMSA-PKCS1-v1_5 Padding Registry

   Implementations MUST implement SHA2-256.  Implementations SHOULD
   implement SHA2-384 and SHA2-512.  Implementations MAY implement other
   algorithms.  Implementations SHOULD NOT create messages that require
   the use of SHA-1, with the exception of computing version 4 key
   fingerprints for purposes of the MDC in version 1 Symmetrically
   Encrypted and Integrity Protected Data packets.  Implementations MUST
   NOT generate signatures with MD5, SHA-1, or RIPEMD-160.
   Implementations MUST NOT use MD5, SHA-1, or RIPEMD-160 as a hash
   function in an ECDH KDF.  Implementations MUST NOT generate packets
   using MD5, SHA-1, or RIPEMD-160 as a hash function in an S2K KDF.
   Implementations MUST NOT decrypt a secret using MD5, SHA-1, or
   RIPEMD-160 as a hash function in an S2K KDF in a version 6 (or later)
   packet.  Implementations MUST NOT validate any recent signature that
   depends on MD5, SHA-1, or RIPEMD-160.  Implementations SHOULD NOT
   validate any old signature that depends on MD5, SHA-1, or RIPEMD-160
   unless the signature's creation date predates known weakness of the
   algorithm used, and the implementation is confident that the message
   has been in the secure custody of the user the whole time.

9.6.  AEAD Algorithms

    +=========+==================+==============+====================+
    |      ID | Name             | Nonce Length | Authentication Tag |
    |         |                  | (Octets)     | Length (Octets)    |
    +=========+==================+==============+====================+
    |       0 | Reserved         |              |                    |
    +---------+------------------+--------------+--------------------+
    |       1 | EAX [EAX]        | 16           | 16                 |
    +---------+------------------+--------------+--------------------+
    |       2 | OCB [RFC7253]    | 15           | 16                 |
    +---------+------------------+--------------+--------------------+
    |       3 | GCM [SP800-38D]  | 12           | 16                 |
    +---------+------------------+--------------+--------------------+
    | 100-110 | Private or       |              |                    |
    |         | Experimental Use |              |                    |
    +---------+------------------+--------------+--------------------+

                Table 25: OpenPGP AEAD Algorithms Registry

   Implementations MUST implement OCB.  Implementations MAY implement
   EAX, GCM, and other algorithms.

10.  Packet Sequence Composition

   OpenPGP packets are assembled into sequences in order to create
   messages and to transfer keys.  Not all possible packet sequences are
   meaningful and correct.  This section describes the rules for how
   packets should be placed into sequences.

   There are three distinct sequences of packets:

   *  Transferable Public Keys (Section 10.1) and their close
      counterpart, Transferable Secret Keys (Section 10.2)

   *  OpenPGP Messages (Section 10.3)

   *  Detached Signatures (Section 10.4)

   Each sequence has an explicit grammar of what packet types (Table 3)
   can appear in what place.  The presence of an unknown critical
   packet, or a known but unexpected packet, is a critical error,
   invalidating the entire sequence (see Section 4.3).  On the other
   hand, unknown non-critical packets can appear anywhere within any
   sequence.  This provides a structured way to introduce new packets
   into OpenPGP, while making sure that certain packets will be handled
   strictly.

   An implementation may "recognize" a packet but not implement it.  The
   purpose of Packet Criticality is to allow the producer to tell the
   consumer whether it would prefer a new, unknown packet to generate an
   error or be ignored.

   Note that previous versions of this document did not have a concept
   of Packet Criticality and did not give clear guidance on what to do
   when unknown packets are encountered.  Therefore, implementations of
   the previous versions may reject unknown non-critical packets or
   accept unknown critical packets.

   When generating a sequence of OpenPGP packets according to one of the
   three grammars, an implementation MUST NOT inject a critical packet
   of a type that does not adhere to the grammar.

   When consuming a sequence of OpenPGP packets, if an implementation
   encounters a critical packet of an inappropriate type according to
   the relevant grammar, the implementation MUST reject the sequence
   with an error.

10.1.  Transferable Public Keys

   OpenPGP users may transfer public keys.  This section describes the
   structure of public keys in transit to ensure interoperability.  An
   OpenPGP Transferable Public Key is also known as an OpenPGP
   certificate, in order to distinguish it from both its constituent
   Public Key packets (Sections 5.5.1.1 and 5.5.1.2) and the underlying
   cryptographic key material.

10.1.1.  OpenPGP Version 6 Certificate Structure

   The format of an OpenPGP version 6 certificate is as follows.
   Entries in square brackets are optional and ellipses indicate
   repetition.

   Primary Key
      [Revocation Signature...]
       Direct Key Signature...
      [User ID or User Attribute
              [Certification Revocation Signature...]
              [Certification Signature...]]...
      [Subkey [Subkey Revocation Signature...]
              Subkey Binding Signature...]...
      [Padding]

   In addition to these rules, a Marker packet (Section 5.8) can appear
   anywhere in the sequence.

   Note that a version 6 key uses a self-signed Direct Key signature to
   store algorithm preferences.

   Every subkey for a version 6 primary key MUST be a version 6 subkey.
   Every subkey MUST have at least one Subkey Binding signature.  Every
   Subkey Binding signature MUST be a self-signature (that is, made by
   the version 6 primary key).  Like all other signatures, every self-
   signature made by a version 6 key MUST be a version 6 signature.

10.1.2.  OpenPGP Version 6 Revocation Certificate

   When a primary version 6 Public Key is revoked, it is sometimes
   distributed with only the Revocation Signature:

   Primary Key
       Revocation Signature

   In this case, the Direct Key signature is no longer necessary, since
   the primary key itself has been marked as unusable.

10.1.3.  OpenPGP Version 4 Certificate Structure

   The format of an OpenPGP version 4 key is as follows.

   Primary Key
      [Revocation Signature]
      [Direct Key Signature...]
      [User ID or User Attribute [Signature...]]...
      [Subkey [Subkey Revocation Signature...]
              Subkey Binding Signature...]...

   In addition to these rules, a Marker packet (Section 5.8) can appear
   anywhere in the sequence.

   A subkey always has at least one Subkey Binding signature after it
   that is issued using the primary key to tie the two keys together.
   These binding signatures may be in either version 3 or version 4
   format, but they SHOULD be in version 4 format.  Subkeys that can
   issue signatures MUST have a version 4 binding signature due to the
   REQUIRED embedded Primary Key Binding signature.

   Every subkey for a version 4 primary key MUST be a version 4 subkey.

   When a primary version 4 Public Key is revoked, the Revocation
   Signature is sometimes distributed by itself, without the primary key
   packet it applies to.  This is referred to as a "revocation
   certificate".  Instead, a version 6 revocation certificate MUST
   include the primary key packet, as described in Section 10.1.2.

10.1.4.  OpenPGP Version 3 Key Structure

   The format of an OpenPGP version 3 key is as follows.

   RSA Public Key
      [Revocation Signature]
       User ID [Signature...]
      [User ID [Signature...]]...

   In addition to these rules, a Marker packet (Section 5.8) can appear
   anywhere in the sequence.

   Each signature certifies the RSA public key and the preceding User
   ID.  The RSA public key can have many User IDs, and each User ID can
   have many signatures.  Version 3 keys are deprecated.
   Implementations MUST NOT generate new version 3 keys but MAY continue
   to use existing ones.

   Version 3 keys MUST NOT have subkeys.

10.1.5.  Common Requirements

   The Public Key packet occurs first.

   The primary key MUST be an algorithm capable of making signatures
   (that is, not an encryption-only algorithm).  This is because the
   primary key needs to be able to create self-signatures (see
   Section 5.2.3.10).  The subkeys may be keys of any type.  For
   example, there may be a single-key RSA key, an Ed25519 primary key
   with an RSA encryption subkey, an Ed25519 primary key with an X25519
   subkey, etc.

   Each of the following User ID packets provides the identity of the
   owner of this public key.  If there are multiple User ID packets,
   this corresponds to multiple means of identifying the same unique
   individual user; for example, a user may have more than one email
   address and construct a User ID for each one.  A Transferable Public
   Key SHOULD include at least one User ID packet unless storage
   requirements prohibit this.

   Immediately following each User ID packet, there are zero or more
   Signature packets.  Each Signature packet is calculated on the
   immediately preceding User ID packet and the initial Public Key
   packet.  The signature serves to certify the corresponding public key
   and User ID.  In effect, the signer is testifying to the belief that
   this public key belongs to the user identified by this User ID.

   Within the same section as the User ID packets, there are zero or
   more User Attribute packets.  Like the User ID packets, a User
   Attribute packet is followed by zero or more Signature packets
   calculated on the immediately preceding User Attribute packet and the
   initial Public Key packet.

   User Attribute packets and User ID packets may be freely intermixed
   in this section, as long as the signatures that follow them are
   maintained on the proper User Attribute or User ID packet.

   After the sequence of User ID packets and Attribute packets and their
   associated signatures, zero or more Subkey packets follow, each with
   their own signatures.  In general, subkeys are provided in cases
   where the top-level public key is a certification-only key.  However,
   any version 4 or version 6 key may have subkeys, and the subkeys may
   be encryption keys, signing keys, authentication keys, etc.  It is
   good practice to use separate subkeys for every operation (i.e.,
   signature-only, encryption-only, authentication-only keys, etc.).

   Each Subkey packet MUST be followed by one Signature packet, which
   should be a Subkey Binding signature issued by the top-level key.
   For subkeys that can issue signatures, the Subkey Binding signature
   MUST contain an Embedded Signature subpacket with a Primary Key
   Binding signature (Type ID 0x19) issued by the subkey on the top-
   level key.

   Subkey and Key packets may each be followed by a Revocation Signature
   packet to indicate that the key is revoked.  Revocation Signatures
   are only accepted if they are issued by the key itself or by a key
   that is authorized to issue revocations via a Revocation Key
   subpacket in a self-signature by the top-level key.

   The optional trailing Padding packet is a mechanism to defend against
   traffic analysis (see Section 13.11).  For maximum interoperability,
   if the Public Key packet is a version 4 key, the optional Padding
   packet SHOULD NOT be present unless the recipient has indicated that
   they are capable of ignoring it successfully.  An implementation that
   is capable of receiving a Transferable Public Key with a version 6
   Public Key primary key MUST be able to accept (and ignore) the
   trailing optional Padding packet.

   Transferable Public Key packet sequences may be concatenated to allow
   transferring multiple public keys in one operation (see Section 3.6).

10.2.  Transferable Secret Keys

   OpenPGP users may transfer secret keys.  The format of a Transferable
   Secret Key is the same as a Transferable Public Key except that
   Secret Key and Secret Subkey packets can be used in addition to the
   Public Key and Public Subkey packets.  If a single Secret Key or
   Secret Subkey packet is included in a packet sequence, it is a
   Transferable Secret Key and should be handled and marked as such (see
   Section 6.2.1).  An implementation SHOULD include self-signatures on
   any User IDs and subkeys, as this allows for a complete public key to
   be automatically extracted from the Transferable Secret Key. An
   implementation MAY choose to omit the self-signatures, especially if
   a Transferable Public Key accompanies the Transferable Secret Key.

10.3.  OpenPGP Messages

   An OpenPGP Message is a packet or sequence of packets that adheres to
   the following grammatical rules (a comma (,) represents sequential
   composition, and a vertical bar (|) separates alternatives):

   OpenPGP Message:  Encrypted Message | Signed Message | Compressed
      Message | Literal Message.

   Compressed Message:  Compressed Data Packet.

   Literal Message:  Literal Data Packet.

   ESK:  Public Key Encrypted Session Key Packet | Symmetric Key
      Encrypted Session Key Packet.

   ESK Sequence:  ESK | ESK Sequence, ESK.

   Encrypted Data:  Symmetrically Encrypted Data Packet | Symmetrically
      Encrypted and Integrity Protected Data Packet.

   Encrypted Message:  Encrypted Data | ESK Sequence, Encrypted Data.

   One-Pass Signed Message:  One-Pass Signature Packet, OpenPGP Message,
      Corresponding Signature Packet.

   Signed Message:  Signature Packet, OpenPGP Message | One-Pass Signed
      Message.

   Optionally Padded Message:  OpenPGP Message | OpenPGP Message,
      Padding Packet.

   In addition to these rules, a Marker packet (Section 5.8) can appear
   anywhere in the sequence.

10.3.1.  Unwrapping Encrypted and Compressed Messages

   In addition to the above grammar, certain messages can be "unwrapped"
   to yield new messages.  In particular:

   *  Decrypting a version 2 Symmetrically Encrypted and Integrity
      Protected Data packet MUST yield a valid Optionally Padded
      Message.

   *  Decrypting a version 1 Symmetrically Encrypted and Integrity
      Protected Data packet or -- for historic data -- a Symmetrically
      Encrypted Data packet MUST yield a valid OpenPGP Message.

   *  Decompressing a Compressed Data packet MUST also yield a valid
      OpenPGP Message.

   When any unwrapping is performed, the resulting stream of octets is
   parsed into a series of OpenPGP packets like any other stream of
   octets.  The packet boundaries found in the series of octets are
   expected to align with the length of the unwrapped octet stream.  An
   implementation MUST NOT interpret octets beyond the boundaries of the
   unwrapped octet stream as part of any OpenPGP packet.  If an
   implementation encounters a packet whose header length indicates that
   it would extend beyond the boundaries of the unwrapped octet stream,
   the implementation MUST reject that packet as malformed and unusable.

10.3.2.  Additional Constraints on Packet Sequences

   Note that some subtle combinations that are formally acceptable by
   this grammar are nonetheless unacceptable.

10.3.2.1.  Packet Versions in Encrypted Messages

   As noted above, an Encrypted Message is a sequence of zero or more
   PKESK packets (Section 5.1) and SKESK packets (Section 5.3), followed
   by an SEIPD (Section 5.13) payload.  In some historic data, the
   payload may be a deprecated SED packet (Section 5.7) instead of
   SEIPD, though implementations MUST NOT generate SED packets (see
   Section 13.7).  The versions of the preceding ESK packets within an
   Encrypted Message MUST align with the version of the payload SEIPD
   packet, as described in this section.

   v3 PKESK and v4 SKESK packets both contain the Symmetric Cipher
   Algorithm ID and the session key for the subsequent SEIPD packet in
   their cleartext.  Since a v1 SEIPD does not contain a symmetric
   algorithm ID, all ESK packets preceding a v1 SEIPD payload MUST be
   either v3 PKESK or v4 SKESK.

   On the other hand, the cleartext of the v6 ESK packets (either PKESK
   or SKESK) do not contain a Symmetric Cipher Algorithm ID, so they
   cannot be used in combination with a v1 SEIPD payload.  The payload
   following any v6 PKESK or v6 SKESK packet MUST be a v2 SEIPD.

   Additionally, to avoid potentially conflicting cipher algorithm IDs,
   and for simplicity, implementations MUST NOT precede a v2 SEIPD
   payload with either v3 PKESK or v4 SKESK packets.

   The versions of packets found in an Encrypted Message are summarized
   in the following table.  An implementation MUST only generate an
   Encrypted Message using packet versions that match a row with "Yes"
   in the "Generate?" column.  Other rows are provided for the purpose
   of historic interoperability.  A conforming implementation MUST only
   generate an Encrypted Message using packets whose versions correspond
   to a single row.

   +==============+=====================+==================+===========+
   | Version of   | Version of          | Version of       | Generate? |
   | Encrypted    | Preceding Symmetric | Preceding        |           |
   | Data Payload | Key ESK (If Any)    | Public Key       |           |
   |              |                     | ESK (If Any)     |           |
   +==============+=====================+==================+===========+
   | SED (Section | -                   | v2 PKESK         | No        |
   | 5.7)         |                     | [RFC2440]        |           |
   +--------------+---------------------+------------------+-----------+
   | SED (Section | v4 SKESK            | v3 PKESK         | No        |
   | 5.7)         | (Section 5.3.1)     | (Section         |           |
   |              |                     | 5.1.1)           |           |
   +--------------+---------------------+------------------+-----------+
   | v1 SEIPD     | v4 SKESK            | v3 PKESK         | Yes       |
   | (Section     | (Section 5.3.1)     | (Section         |           |
   | 5.13.1)      |                     | 5.1.1)           |           |
   +--------------+---------------------+------------------+-----------+
   | v2 SEIPD     | v6 SKESK            | v6 PKESK         | Yes       |
   | (Section     | (Section 5.3.2)     | (Section         |           |
   | 5.13.2)      |                     | 5.1.2)           |           |
   +--------------+---------------------+------------------+-----------+

        Table 26: OpenPGP Encrypted Message Packet Versions Registry

   An implementation processing an Encrypted Message MUST discard any
   preceding ESK packet with a version that does not align with the
   version of the payload.

10.3.2.2.  Packet Versions in Signatures

   OpenPGP Key packets and Signature packets are also versioned.  The
   version of a Signature typically matches the version of the signing
   key.  When a version 6 key produces a Signature packet, it MUST
   produce a version 6 Signature packet, regardless of the Signature
   packet type.  When a message is signed or verified using the one-pass
   construction, the version of the One-Pass Signature packet
   (Section 5.4) should also be aligned to the other versions.

   Some legacy implementations have produced unaligned signature
   versions for older key material, which are also described in the
   table below for the purpose of historic interoperability.  A
   conforming implementation MUST only generate Signature packets with
   version numbers matching rows with "Yes" in the "Generate?" column.

     +=====================+================+============+===========+
     | Signing Key Version | Signature      | OPS Packet | Generate? |
     |                     | Packet Version | Version    |           |
     +=====================+================+============+===========+
     | 3 (Section 5.5.2.1) | 3 (Section     | 3 (Section | No        |
     |                     | 5.2.2)         | 5.4)       |           |
     +---------------------+----------------+------------+-----------+
     | 4 (Section 5.5.2.2) | 3 (Section     | 3 (Section | No        |
     |                     | 5.2.2)         | 5.4)       |           |
     +---------------------+----------------+------------+-----------+
     | 4 (Section 5.5.2.2) | 4 (Section     | 3 (Section | Yes       |
     |                     | 5.2.3)         | 5.4)       |           |
     +---------------------+----------------+------------+-----------+
     | 6 (Section 5.5.2.3) | 6 (Section     | 6 (Section | Yes       |
     |                     | 5.2.3)         | 5.4)       |           |
     +---------------------+----------------+------------+-----------+

           Table 27: OpenPGP Key and Signature Versions Registry

   Note, however, that a version mismatch between these packets does not
   invalidate the packet sequence as a whole; it merely invalidates the
   signature, as a signature with an unknown version SHOULD be discarded
   (see Section 5.2.5).

10.4.  Detached Signatures

   Some OpenPGP applications use so-called "detached signatures".  For
   example, a program bundle may contain a file, and with it a second
   file that is a detached signature of the first file.  These detached
   signatures are simply one or more Signature packets stored separately
   from the data for which they are a signature.

   In addition, a Marker packet (Section 5.8) and a Padding packet
   (Section 5.14) can appear anywhere in the sequence.

11.  Elliptic Curve Cryptography

   This section describes algorithms and parameters used with Elliptic
   Curve Cryptography (ECC) keys.  A thorough introduction to ECC can be
   found in [KOBLITZ].  Refer to [FIPS186], Appendix B.4, for the
   methods to generate a uniformly distributed ECC private key.

   None of the ECC methods described in this document are allowed with
   deprecated version 3 keys.

11.1.  ECC Curves

   This document references three named prime field curves defined in
   [FIPS186] as "Curve P-256", "Curve P-384", and "Curve P-521" and
   three named prime field curves defined in [RFC5639] as
   "brainpoolP256r1", "brainpoolP384r1", and "brainpoolP512r1".  All six
   curves can be used with ECDSA and ECDH public key algorithms.  They
   are referenced using a sequence of octets, referred to as the curve
   OID.  Section 9.2 describes in detail how this sequence of octets is
   formed.

   Separate algorithms are also defined for the use of X25519 and X448
   [RFC7748] and Ed25519 and Ed448 [RFC8032].  Additionally, legacy OIDs
   are defined for "Curve25519Legacy" (for encryption using the ECDH
   algorithm) and "Ed25519Legacy" (for signing using the EdDSALegacy
   algorithm).

11.2.  EC Point Wire Formats

   A point on an elliptic curve will always be represented on the wire
   as an MPI.  Each curve uses a specific point format for the data
   within the MPI itself.  Each format uses a designated prefix octet to
   ensure that the high octet has at least 1 bit set to make the MPI a
   constant size.

           +=================+================+================+
           |            Name | Wire Format    | Reference      |
           +=================+================+================+
           |            SEC1 | 0x04 || x || y | Section 11.2.1 |
           +-----------------+----------------+----------------+
           | Prefixed native | 0x40 || native | Section 11.2.2 |
           +-----------------+----------------+----------------+

                Table 28: OpenPGP Elliptic Curve Point Wire
                              Formats Registry

11.2.1.  SEC1 EC Point Wire Format

   For a SEC1-encoded (uncompressed) point, the content of the MPI is:

   B = 04 || x || y

   where x and y are coordinates of the point P = (x, y), and each is
   encoded in the big-endian format and zero-padded to the adjusted
   underlying field size.  The adjusted underlying field size is the
   underlying field size rounded up to the nearest 8-bit boundary, as
   noted in the "fsize" column in Section 9.2.  This encoding is
   compatible with the definition given in [SEC1].

11.2.2.  Prefixed Native EC Point Wire Format

   For a custom compressed point, the content of the MPI is:

   B = 40 || p

   where p is the public key of the point encoded using the rules
   defined for the specified curve.  This format is used for ECDH keys
   based on curves expressed in Montgomery form and for points when
   using EdDSA.

11.2.3.  Notes on EC Point Wire Formats

   Given the above definitions, the exact size of the MPI payload for an
   encoded point is 515 bits for both NIST P-256 and brainpoolP256r1,
   771 for both NIST P-384 and brainpoolP384r1, 1059 for NIST P-521,
   1027 for brainpoolP512r1, and 263 for both Curve25519Legacy and
   Ed25519Legacy.  For example, the length of an EdDSALegacy public key
   for the curve Ed25519Legacy is 263 bits: 7 bits to represent the 0x40
   prefix octet and 32 octets for the native value of the public key.

   Even though the zero point (also called the "point at infinity") may
   occur as a result of arithmetic operations on points of an elliptic
   curve, it SHALL NOT appear in data structures defined in this
   document.

   Each particular curve uses a designated wire format for the point
   found in its public key or ECDH data structure.  An implementation
   MUST NOT use a different wire format for a point other than the wire
   format associated with the curve.

11.3.  EC Scalar Wire Formats

   Some non-curve values in elliptic curve cryptography (for example,
   secret keys and signature components) are not points on a curve, but
   they are also encoded on the wire in OpenPGP as an MPI.

   Because of different patterns of deployment, some curves treat these
   values as opaque bit strings with the high bit set, while others are
   treated as actual integers, encoded in the standard OpenPGP big-
   endian form.  The choice of encoding is specific to the public key
   algorithm in use.

   +==========+===========================================+===========+
   | Type     | Description                               | Reference |
   +==========+===========================================+===========+
   | integer  | An integer encoded in big-endian format   | Section   |
   |          | as a standard OpenPGP MPI                 | 3.2       |
   +----------+-------------------------------------------+-----------+
   | octet    | An octet string of fixed length that may  | Section   |
   | string   | be shorter on the wire due to leading     | 11.3.1    |
   |          | zeros being stripped by the MPI encoding  |           |
   |          | and may need to be zero-padded before use |           |
   +----------+-------------------------------------------+-----------+
   | prefixed | An octet string of fixed length N,        | Section   |
   | N octets | prefixed with octet 0x40 to ensure no     | 11.3.2    |
   |          | leading zero octet                        |           |
   +----------+-------------------------------------------+-----------+

        Table 29: OpenPGP Elliptic Curve Scalar Encodings Registry

11.3.1.  EC Octet String Wire Format

   Some opaque strings of octets are represented on the wire as an MPI
   by simply stripping the leading zeros and counting the remaining
   bits.  These strings are of known, fixed length.  They are
   represented in this document as MPI(N octets of X), where N is the
   expected length in octets of the octet string.

   For example, a 5-octet opaque string (MPI(5 octets of X)) where X has
   the value 00 02 EE 19 00 would be represented on the wire as an MPI
   like so: 00 1A 02 EE 19 00.

   To encode X to the wire format, set the MPI's 2-octet bit counter to
   the value of the highest set bit (bit 26, or 0x001A), and do not
   transfer the leading all-zero octet to the wire.

   To reverse the process, an implementation can take the following
   steps, if it knows that X has an expected length of, for example, 5
   octets:

   *  Ensure that the MPI's 2-octet bit count is less than or equal to
      40 (5 octets of 8 bits)

   *  Allocate 5 octets, setting all to zero initially

   *  Copy the MPI data octets (without the two count octets) into the
      lower octets of the allocated space

11.3.2.  EC Prefixed Octet String Wire Format

   Another way to ensure that a fixed-length bytes string is encoded
   simply to the wire while remaining in MPI format is to prefix the
   byte string with a dedicated non-zero octet.  This specification uses
   0x40 as the prefix octet.  This is represented in this specification
   as MPI(prefixed N octets of X), where N is the known byte string
   length.

   For example, a 5-octet opaque string using MPI(prefixed 5 octets of
   X) where X has the value 00 02 EE 19 00 would be written to the wire
   form as: 00 2F 40 00 02 EE 19 00.

   To encode the string, prefix it with the octet 0x40 (whose 7th bit is
   set), and then set the MPI's 2-octet bit counter to 47 (0x002F -- 7
   bits for the prefix octet and 40 bits for the string).

   To decode the string from the wire, an implementation that knows that
   the variable is formed in this way can:

   *  ensure that the first three octets of the MPI (the 2-bit count
      octets plus the prefix octet) are 00 2F 40, and

   *  use the remainder of the MPI directly off the wire.

   Note that this is a similar approach to that used in the EC point
   encodings found in Section 11.2.2.

11.4.  Key Derivation Function

   A key derivation function (KDF) is necessary to implement EC
   encryption.  The Concatenation Key Derivation Function (Approved
   Alternative 1) [SP800-56A] with the KDF hash function that is
   SHA2-256 [FIPS180] or stronger is REQUIRED.

   For convenience, the synopsis of the encoding method is given below
   with significant simplifications attributable to the restricted
   choice of hash functions in this document.  However, [SP800-56A] is
   the normative source of the definition.

   //   Implements KDF( X, oBits, Param );
   //   Input: point X = (x,y)
   //   oBits - the desired size of output
   //   hBits - the size of output of hash function Hash
   //   Param - octets representing the parameters
   //   Assumes that oBits <= hBits
   // Convert the point X to the octet string:
   //   ZB' = 04 || x || y
   // and extract the x portion from ZB'
   ZB = x;
   MB = Hash ( 00 || 00 || 00 || 01 || ZB || Param );
   return oBits leftmost bits of MB.

   Note that ZB in the KDF description above is the compact
   representation of X as defined in Section 4.2 of [RFC6090].

11.5.  ECDH Algorithm

   This section describes the One-Pass Diffie-Hellman method, which is a
   combination of the ECC Diffie-Hellman method that establishes a
   shared secret and the key derivation method that processes the shared
   secret into a derived key.  Additionally, this section describes a
   key wrapping method that uses the derived key to protect a session
   key used to encrypt a message.

   The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [SP800-56A] MUST
   be implemented with the following restrictions: the ECC Cofactor
   Diffie-Hellman (CDH) primitive employed by this method is modified to
   always assume the cofactor is 1, the KDF specified in Section 11.4 is
   used, and the KDF parameters specified below are used.

   The KDF parameters are encoded as a concatenation of the following 5
   variable-length and fixed-length fields, which are compatible with
   the definition of the OtherInfo bit string [SP800-56A]:

   *  A variable-length field containing a curve OID, which is formatted
      as follows:

      -  A 1-octet size of the following field.

      -  The octets representing a curve OID, as defined in Section 9.2.

   *  A 1-octet public key algorithm ID, as defined in Section 9.1.

   *  A variable-length field containing KDF parameters, which are
      identical to the corresponding field in the ECDH public key and
      formatted as follows:

      -  A 1-octet size of the following fields; values 0 and 0xFF are
         reserved for future extensions.

      -  A 1-octet value 0x01, reserved for future extensions.

      -  A 1-octet hash function ID used with the KDF.

      -  A 1-octet algorithm ID for the symmetric algorithm that is used
         to wrap the symmetric key for message encryption; see
         Section 11.5 for details.

   *  20 octets representing the UTF-8 encoding of the string "Anonymous
      Sender" padded at the end with spaces (0x20) to 20 octets, which
      is the octet sequence 41 6E 6F 6E 79 6D 6F 75 73 20 53 65 6E 64 65
      72 20 20 20 20.

   *  A variable-length field containing the fingerprint of the
      recipient encryption subkey identifying the key material that is
      needed for decryption.  For version 4 keys, this field is 20
      octets.  For version 6 keys, this field is 32 octets.

   The size in octets of the KDF parameters sequence, as defined above,
   for encrypting to a version 4 key is 54 for curve NIST P-256; 51 for
   curves NIST P-384 and NIST P-521; 55 for curves brainpoolP256r1,
   brainpoolP384r1, and brainpoolP512r1; or 56 for Curve25519Legacy.
   For encrypting to a version 6 key, the size of the sequence is 66 for
   curve NIST P-256; 63 for curves NIST P-384 and NIST P-521; or 67 for
   curves brainpoolP256r1, brainpoolP384r1, and brainpoolP512r1.

   The key wrapping method is described in [RFC3394].  The KDF produces
   a symmetric key that is used as a KEK as specified in [RFC3394].
   Refer to Section 11.5.1 for the details regarding the choice of the
   KEK algorithm, which SHOULD be one of the three AES algorithms.  Key
   wrapping and unwrapping is performed with the default initial value
   of [RFC3394].

   To produce the input to the key wrapping method, first concatenate
   the following values:

   *  The 1-octet algorithm identifier, if it was passed (in the case of
      a v3 PKESK packet).

   *  The session key.

   *  A 2-octet checksum of the session key, equal to the sum of the
      session key octets, modulo 65536.

   Then, the above values are padded to an 8-octet granularity using the
   method described in [RFC8018].

   For example, in a version 3 Public Key Encrypted Session Key packet,
   an AES-256 session key is encoded as follows, forming a 40-octet
   sequence:

   09 k0 k1 ... k31 s0 s1 05 05 05 05 05

   The octets k0 to k31 above denote the session key, and the octets s0
   and s1 denote the checksum of the session key octets.  This encoding
   allows the sender to obfuscate the size of the symmetric encryption
   key used to encrypt the data.  For example, assuming that an AES
   algorithm is used for the session key, the sender MAY use 21, 13, and
   5 octets of padding for AES-128, AES-192, and AES-256, respectively,
   to provide the same number of octets, 40 total, as an input to the
   key wrapping method.

   In a version 6 Public Key Encrypted Session Key packet, the symmetric
   algorithm is not included, as described in Section 5.1.  For example,
   an AES-256 session key would be composed as follows:

   k0 k1 ... k31 s0 s1 06 06 06 06 06 06

   The octets k0 to k31 above again denote the session key, and the
   octets s0 and s1 denote the checksum.  In this case, assuming that an
   AES algorithm is used for the session key, the sender MAY use 22, 14,
   and 6 octets of padding for AES-128, AES-192, and AES-256,
   respectively, to provide the same number of octets, 40 total, as an
   input to the key wrapping method.

   The output of the method consists of two fields.  The first field is
   the MPI containing the ephemeral key used to establish the shared
   secret.  The second field is composed of the following two subfields:

   *  One octet encoding the size in octets of the result of the key
      wrapping method; the value 255 is reserved for future extensions.

   *  Up to 254 octets representing the result of the key wrapping
      method, applied to the 8-octet padded session key, as described
      above.

   Note that for session key sizes 128, 192, and 256 bits, the size of
   the result of the key wrapping method is, respectively, 32, 40, and
   48 octets, unless size obfuscation is used.

   For convenience, the synopsis of the encoding method is given below;
   however, this section, [SP800-56A], and [RFC3394] are the normative
   sources of the definition.

   *  Obtain the authenticated recipient public key R

   *  Generate an ephemeral, single-use key pair {v, V=vG}

   *  Compute the shared point S = vR

   *  m = symm_alg_ID || session key || checksum || pkcs5_padding

   *  curve_OID_len = (octet)len(curve_OID)

   *  Param = curve_OID_len || curve_OID || public_key_alg_ID || 03 ||
      01 || KDF_hash_ID || KEK_alg_ID for AESKeyWrap || 41 6E 6F 6E 79
      6D 6F 75 73 20 53 65 6E 64 65 72 20 20 20 20 ||
      recipient_fingerprint

   *  Z_len = the key size for the KEK_alg_ID used with AESKeyWrap

   *  Compute Z = KDF( S, Z_len, Param )

   *  Compute C = AESKeyWrap( Z, m ) (per [RFC3394])

   *  Wipe the memory that contained S, v, and Z to avoid leaking
      ephemeral secrets

   *  VB = convert point V to the octet string

   *  Output (MPI(VB) || len(C) || C)

   The decryption is the inverse of the method given.  Note that the
   recipient with key pair (r,R) obtains the shared secret by
   calculating:

   S = rV = rvG

11.5.1.  ECDH Parameters

   ECDH keys have a hash algorithm parameter for key derivation and a
   symmetric algorithm for key encapsulation.

   For version 6 keys, the following algorithms MUST be used depending
   on the curve.  An implementation MUST NOT generate a version 6 ECDH
   key over any listed curve that uses different KDF or KEK parameters.
   An implementation MUST NOT encrypt any message to a version 6 ECDH
   key over a listed curve that announces a different KDF or KEK
   parameter.

   For version 4 keys, the following algorithms SHOULD be used depending
   on the curve.  An implementation SHOULD only use an AES algorithm as
   a KEK algorithm.

        +==================+================+=====================+
        | Curve            | Hash Algorithm | Symmetric Algorithm |
        +==================+================+=====================+
        | NIST P-256       | SHA2-256       | AES-128             |
        +------------------+----------------+---------------------+
        | NIST P-384       | SHA2-384       | AES-192             |
        +------------------+----------------+---------------------+
        | NIST P-521       | SHA2-512       | AES-256             |
        +------------------+----------------+---------------------+
        | brainpoolP256r1  | SHA2-256       | AES-128             |
        +------------------+----------------+---------------------+
        | brainpoolP384r1  | SHA2-384       | AES-192             |
        +------------------+----------------+---------------------+
        | brainpoolP512r1  | SHA2-512       | AES-256             |
        +------------------+----------------+---------------------+
        | Curve25519Legacy | SHA2-256       | AES-128             |
        +------------------+----------------+---------------------+

           Table 30: OpenPGP ECDH KDF and KEK Parameters Registry

12.  Notes on Algorithms

12.1.  PKCS#1 Encoding in OpenPGP

   This specification makes use of the PKCS#1 functions EME-PKCS1-v1_5
   and EMSA-PKCS1-v1_5.  However, the calling conventions of these
   functions have changed in the past.  To avoid potential confusion and
   interoperability problems, we are including local copies in this
   document, adapted from those in PKCS#1 v2.1 [RFC8017].  [RFC8017]
   should be treated as the ultimate authority on PKCS#1 for OpenPGP.
   Nonetheless, we believe that there is value in having a self-
   contained document that avoids problems in the future with needed
   changes in the conventions.

12.1.1.  EME-PKCS1-v1_5-ENCODE

   Input:

   k =  key modulus length in octets.

   M =  message to be encoded; an octet string of length mLen, where
      mLen <= k - 11.

   Output:

   EM =  encoded message; an octet string of length k.

   Error: "message too long".

   1.  Length checking: If mLen > k - 11, output "message too long" and
       stop.

   2.  Generate an octet string PS of length k - mLen - 3 consisting of
       pseudorandomly generated non-zero octets.  The length of PS will
       be at least 8 octets.

   3.  Concatenate PS, the message M, and other padding to form an
       encoded message EM of length k octets as

      EM = 0x00 || 0x02 || PS || 0x00 || M.

   4.  Output EM.

12.1.2.  EME-PKCS1-v1_5-DECODE

   Input:

   EM =  encoded message; an octet string.

   Output:

   M =  decoded message; an octet string.

   Error: "decryption error".

   To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
   into an octet string PS consisting of non-zero octets and a message M
   as follows

     EM = 0x00 || 0x02 || PS || 0x00 || M.

   If the first octet of EM does not have hexadecimal value 0x00, the
   second octet of EM does not have hexadecimal value 0x02, there is no
   octet with hexadecimal value 0x00 to separate PS from M, or the
   length of PS is less than 8 octets, output "decryption error" and
   stop.  See also Section 13.5 regarding differences in reporting
   between a decryption error and a padding error.

12.1.3.  EMSA-PKCS1-v1_5

   This encoding method is deterministic and only has an encoding
   operation.

   Input:

   Hash =  hash function to be used.

   M =  message to be encoded.

   emLen =  intended length of the encoded message in octets, at least
      tLen + 11, where tLen is the octet length of the DER encoding T of
      a certain value computed during the encoding operation.

   Output:

   EM =  encoded message; an octet string of length emLen.

   Errors: "message too long"; "intended encoded message length too
   short".

   Steps:

   1.  Apply the hash function to the message M to produce hash value H:

       H = Hash(M).

       If the hash function outputs "message too long," output "message
       too long" and stop.

   2.  Let T be the Full Hash Prefix from Table 24 for the given hash
       function, concatenated with the hash digest H (representing an
       ASN.1 DER value for the hash function used and the hash digest).
       Let tLen be the length in octets of T.

   3.  If emLen < tLen + 11, output "intended encoded message length too
       short" and stop.

   4.  Generate an octet string PS consisting of emLen - tLen - 3 octets
       with hexadecimal value 0xFF.  The length of PS will be at least 8
       octets.

   5.  Concatenate PS, the hash prefix T, and other padding to form the
       encoded message EM as

      EM = 0x00 || 0x01 || PS || 0x00 || T.

   6.  Output EM.

12.2.  Symmetric Algorithm Preferences

   The symmetric algorithm preference is an ordered list of algorithms
   that the keyholder accepts.  Since it is found on a self-signature,
   it is possible that a keyholder may have multiple, different
   preferences.  For example, Alice may have AES-128 only specified for
   "alice@work.com" but Camellia-256, Twofish, and AES-128 specified for
   "alice@home.org".  Note that it is also possible for preferences to
   be in a subkey's binding signature.

   Since AES-128 is the algorithm that MUST be implemented, if it is not
   explicitly in the list, it is tacitly at the end.  However, it is
   good form to place it there explicitly.  Note also that if an
   implementation does not implement the preference, then it is
   implicitly an AES-128-only implementation.  Furthermore, note that
   implementations conforming to the previous version of this
   specification [RFC4880] have TripleDES as the only algorithm that
   MUST be implemented.

   An implementation MUST NOT use a symmetric algorithm that is not in
   the recipient's preference list.  When encrypting to more than one
   recipient, the implementation finds a suitable algorithm by taking
   the intersection of the preferences of the recipients.  Note that
   since the AES-128 algorithm MUST be implemented, the intersection is
   guaranteed to be non-empty.

   If an implementation can decrypt a message that a keyholder doesn't
   have in their preferences, the implementation SHOULD decrypt the
   message anyway, but it MUST warn the keyholder.  For example, suppose
   that Alice (above) has an implementation that implements all
   algorithms in this specification.  Nonetheless, she prefers subsets
   for work or home.  If she is sent a message encrypted with IDEA,
   which is not in her preferences, the implementation warns her that
   someone sent an IDEA-encrypted message, but it would ideally decrypt
   it anyway.

12.2.1.  Plaintext

   Algorithm 0, "plaintext", may only be used to denote secret keys that
   are stored in the clear.  An implementation MUST NOT use algorithm 0
   as the indicated symmetric cipher for an encrypted data packet
   (Sections 5.7 or 5.13); it can use a Literal Data packet
   (Section 5.9) to encode unencrypted literal data.

12.3.  Other Algorithm Preferences

   Other algorithm preferences work similarly to the symmetric algorithm
   preference in that they specify which algorithms the keyholder
   accepts.  There are two interesting cases in which further comments
   are needed: the compression preferences and the hash preferences.

12.3.1.  Compression Preferences

   Like the algorithm preferences, an implementation MUST NOT use an
   algorithm that is not in the preference vector.  If Uncompressed (0)
   is not explicitly in the list, it is tacitly at the end.  That is,
   uncompressed messages may always be sent.

   Note that earlier implementations may assume that the absence of
   compression preferences means that [ZIP(1), Uncompressed(0)] are
   preferred, and default to ZIP compression.  Therefore, an
   implementation that prefers uncompressed data SHOULD explicitly state
   this in the Preferred Compression Algorithms.

12.3.1.1.  Uncompressed

   Algorithm 0, "uncompressed", may only be used to denote a preference
   for uncompressed data.  An implementation MUST NOT use algorithm 0 as
   the indicated compression algorithm in a Compressed Data packet
   (Section 5.6); it can use a Literal Data packet (Section 5.9) to
   encode uncompressed literal data.

12.3.2.  Hash Algorithm Preferences

   Typically, the signer chooses which hash algorithm to use, rather
   than the verifier, because a signer rarely knows who is going to be
   verifying the signature.  This preference allows a protocol based
   upon digital signatures ease in negotiation.

   Thus, if Alice is authenticating herself to Bob with a signature, it
   makes sense for her to use a hash algorithm that Bob's implementation
   uses.  This preference allows Bob to state which algorithms Alice may
   use in his key.

   Since SHA2-256 is the hash algorithm that MUST be implemented, if it
   is not explicitly in the list, it is tacitly at the end.  However, it
   is good form to place it there explicitly.

12.4.  RSA

   The PKCS1-v1_5 padding scheme, used by the RSA algorithms defined in
   this document, is no longer recommended, and its use is deprecated by
   [SP800-131A].  Therefore, an implementation SHOULD NOT generate RSA
   keys.

   There are algorithm types for RSA Sign-Only and RSA Encrypt-Only
   keys.  These types are deprecated in favor of the Key Flags signature
   subpacket.  An implementation MUST NOT create such a key, but it MAY
   interpret it.

   An implementation MUST NOT generate RSA keys of a size less than 3072
   bits.  An implementation SHOULD NOT encrypt, sign, or verify using
   RSA keys of a size less than 3072 bits.  An implementation MUST NOT
   encrypt, sign, or verify using RSA keys of a size less than 2048
   bits.  An implementation that decrypts a message using an RSA secret
   key of a size less than 3072 bits SHOULD generate a deprecation
   warning that the key is too weak for modern use.

12.5.  DSA

   DSA is no longer recommended.  It has also been deprecated in
   [FIPS186].  Therefore, an implementation MUST NOT generate DSA keys.

   An implementation MUST NOT sign or verify using DSA keys.

12.6.  Elgamal

   The PKCS1-v1_5 padding scheme, used by the Elgamal algorithm defined
   in this document, is no longer recommended, and its use is deprecated
   by [SP800-131A].  Therefore, an implementation MUST NOT generate
   Elgamal keys.

   An implementation MUST NOT encrypt using Elgamal keys.  An
   implementation that decrypts a message using an Elgamal secret key
   SHOULD generate a deprecation warning that the key is too weak for
   modern use.

12.7.  EdDSA

   Although the EdDSA algorithm allows arbitrary data as input, its use
   with OpenPGP requires that a digest of the message be used as input
   (pre-hashed).  See Section 5.2.4 for details.  Truncation of the
   resulting digest is never applied; the resulting digest value is used
   verbatim as input to the EdDSA algorithm.

   For clarity: while [RFC8032] describes different variants of EdDSA,
   OpenPGP uses the "pure" variant (PureEdDSA).  The hashing that
   happens with OpenPGP is done as part of the standard OpenPGP
   signature process, and that hash itself is fed as the input message
   to the PureEdDSA algorithm.

   As specified in [RFC8032], Ed448 also expects a "context string".  In
   OpenPGP, Ed448 is used with the empty string as a context string.

12.8.  Reserved Algorithm IDs

   A number of algorithm IDs have been reserved for algorithms that
   would be useful to use in an OpenPGP implementation, yet there are
   issues that prevent an implementer from actually implementing the
   algorithm.  These are marked as reserved in Section 9.1.

   The reserved public key algorithm X9.42 (21) does not have the
   necessary parameters, parameter order, or semantics defined.  The
   same is currently true for reserved public key algorithms AEDH (23)
   and AEDSA (24).

   Previous versions of the OpenPGP specification permitted Elgamal
   [ELGAMAL] signatures with a public key algorithm ID of 20.  These are
   no longer permitted.  An implementation MUST NOT generate such keys.
   An implementation MUST NOT generate Elgamal signatures; see
   [BLEICHENBACHER].

12.9.  CFB Mode

   The Cipher Feedback (CFB) mode used in this document is defined in
   Section 6.3 of [SP800-38A].

   The CFB segment size s is equal to the block size of the cipher
   (i.e., n-bit CFB mode, where n is the block size used).

12.10.  Private or Experimental Parameters

   S2K Specifiers, Signature Subpacket Type IDs, User Attribute
   Subpacket Type IDs, image format IDs, and the various algorithm IDs
   described in Section 9 all reserve the range 100 to 110 for Private
   and Experimental Use. Packet Type IDs reserve the range 60 to 63 for
   Private and Experimental Use. These are intentionally managed by the
   Private Use and Experimental Use policies, as described in [RFC8126].

   However, implementations need to be careful with these and promote
   them to full IANA-managed parameters when they grow beyond the
   original, limited system.

12.11.  Meta Considerations for Expansion

   If OpenPGP is extended in a way that is not backward compatible,
   meaning that old implementations will not gracefully handle their
   absence of a new feature, the extension proposal can be declared in
   the keyholder's self-signature as part of the Features signature
   subpacket.

   We cannot state definitively what extensions will not be forward
   compatible, but typically new algorithms are forward compatible,
   whereas new packets are not.

   If an extension proposal does not update the Features system, it
   SHOULD include an explanation of why this is unnecessary.  If the
   proposal contains neither an extension to the Features system nor an
   explanation of why such an extension is unnecessary, the proposal
   SHOULD be rejected.

13.  Security Considerations

   *  As with any technology involving cryptography, implementers should
      check the current literature to determine if any algorithms used
      here have been found to be vulnerable to an attack.  If so,
      implementers should consider disallowing such algorithms for new
      data and warning the end user, or preventing use, when they are
      trying to consume data protected by such algorithms that are now
      vulnerable.

   *  This specification uses Public Key Cryptography technologies.  It
      is assumed that the private key portion of a public-private key
      pair is controlled and secured by the proper party or parties.

   *  The MD5 and SHA-1 hash algorithms have been found to have
      weaknesses, with collisions found in a number of cases.  MD5 and
      SHA-1 are deprecated for use in OpenPGP (see Section 9.5).

   *  Many security protocol designers think that it is a bad idea to
      use a single key for both privacy (encryption) and integrity
      (signatures).  In fact, this was one of the motivating forces
      behind the version 4 key format with separate signature and
      encryption keys.  Using a single key for encrypting and signing is
      discouraged.

   *  The DSA algorithm will work with any hash, but it is sensitive to
      the quality of the hash algorithm.  Verifiers should be aware that
      even if the signer used a strong hash, an attacker could have
      modified the signature to use a weak one.  Only signatures using
      acceptably strong hash algorithms should be accepted as valid.

   *  As OpenPGP combines many different asymmetric, symmetric, and hash
      algorithms, each with different measures of strength, care should
      be taken to ensure that the weakest element of an OpenPGP Message
      is still sufficiently strong for the purpose at hand.  While
      consensus about the strength of a given algorithm may evolve, NIST
      Special Publication 800-57 [SP800-57] contains recommendations
      (current at the time of this publication) about equivalent
      security levels of different algorithms.

   *  There is a somewhat-related potential security problem in
      signatures.  If an attacker can find a message that hashes to the
      same hash with a different algorithm, a bogus signature structure
      can be constructed that evaluates correctly.

      For example, suppose Alice DSA-signs message M using hash
      algorithm H.  Suppose that Mallet finds a message M' that has the
      same hash value as M with H'.  Mallet can then construct a
      signature block that verifies as Alice's signature of M' with H'.
      However, this would also constitute a weakness in either H or H',
      or both.  Should this ever occur, a revision will have to be made
      to this document to revise the allowed hash algorithms.

   *  If you are building an authentication system, the recipient may
      specify a preferred signing algorithm.  However, the signer would
      be foolish to use a weak algorithm simply because the recipient
      requests it.

   *  Some of the encryption algorithms mentioned in this document have
      been analyzed less than others.  For example, although TWOFISH is
      presently considered reasonably strong, it has been analyzed much
      less than AES.  Other algorithms may have other concerns
      surrounding them.

   *  In late summer 2002, Jallad, Katz, and Schneier published an
      interesting attack on previous versions of the OpenPGP
      specification and some of its implementations [JKS02].  In this
      attack, the attacker modifies a message and sends it to a user who
      then returns the erroneously decrypted message to the attacker.
      The attacker is thus using the user as a decryption oracle and can
      often decrypt the message.  This attack is a particular form of
      ciphertext malleability.  See Section 13.7 for information on how
      to defend against such an attack using more recent versions of
      OpenPGP.

13.1.  SHA-1 Collision Detection

   As described in [SHAMBLES], the SHA-1 digest algorithm is not
   collision resistant.  However, an OpenPGP implementation cannot
   completely discard the SHA-1 algorithm, because it is required for
   implementing version 4 public keys.  In particular, the version 4
   fingerprint derivation uses SHA-1.  So as long as an OpenPGP
   implementation supports version 4 public keys, it will need to
   implement SHA-1 in at least some scenarios.

   To avoid the risk of uncertain breakage from a maliciously introduced
   SHA-1 collision, an OpenPGP implementation MAY attempt to detect when
   a hash input is likely from a known collision attack and then either
   reject the hash input deliberately or modify the hash output.  This
   should convert an uncertain breakage (where it is unclear what the
   effect of a collision will be) to an explicit breakage, which is more
   desirable for a robust implementation.

   [STEVENS2013] describes a method for detecting indicators of well-
   known SHA-1 collision attacks.  Some example C code implementing this
   technique can be found at [SHA1CD].

13.2.  Advantages of Salted Signatures

   Version 6 signatures include a salt that is hashed first, and it's
   size depends on the hashing algorithm.  This makes version 6 OpenPGP
   signatures non-deterministic and protects against a broad class of
   attacks that depend on creating a signature over a predictable
   message.  By selecting a new random salt for each signature made, the
   signed hashes and the signatures are not predictable.

   While the material to be signed could be attacker controlled, hashing
   the salt first means that there is no attacker-controlled hashed
   prefix.  An example of this kind of attack is described in the paper
   "SHA-1 is a Shambles" [SHAMBLES], which leverages a chosen prefix
   collision attack against SHA-1.  This means that an adversary
   carrying out a chosen-message attack will not be able to control the
   hash that is being signed and will need to break second-preimage
   resistance instead of the simpler collision resistance to create two
   messages having the same signature.  The size of the salt is bound to
   the hash function to match the expected collision-resistance level
   and is at least 16 octets.

   In some cases, an attacker may be able to induce a signature to be
   made, even if they do not control the content of the message.  In
   some scenarios, a repeated signature over the exact same message may
   risk leakage of part or all of the signing key; for example, see
   discussion of hardware faults over EdDSA and deterministic ECDSA in
   [PSSLR17].  Choosing a new random salt for each signature ensures
   that no repeated signatures are produced, which mitigates this risk.

13.3.  Elliptic Curve Side Channels

   Side-channel attacks are a concern when a compliant application's use
   of the OpenPGP format can be modeled by a decryption or signing
   oracle, for example, when an application is a network service
   performing decryption to unauthenticated remote users.  ECC scalar
   multiplication operations used in ECDSA and ECDH are vulnerable to
   side-channel attacks.  Countermeasures can often be taken at the
   higher protocol level, such as limiting the number of allowed
   failures or time-blinding the operations associated with each network
   interface.  Mitigations at the scalar multiplication level seek to
   eliminate any measurable distinction between the ECC point addition
   and doubling operations.

13.4.  Risks of a Quick Check Oracle

   In winter 2005, Serge Mister and Robert Zuccherato from Entrust
   released a paper describing a way that the "quick check" in v1 SEIPD
   and SED packets can be used as an oracle to decrypt two octets of
   every cipher block [MZ05].  This check was intended for early
   detection of session key decryption errors, particularly to detect a
   wrong passphrase, since v4 SKESK packets do not include an integrity
   check.

   There is a danger when using the quick check if timing or error
   information about the check can be exposed to an attacker,
   particularly via an automated service that allows rapidly repeated
   queries.

   Disabling the quick check prevents the attack.

   For very large encrypted data whose session key is protected by a
   passphrase using a v4 SKESK, the quick check may be convenient to the
   user by informing them early that they typed the wrong passphrase.
   But the implementation should use the quick check with care.  The
   recommended approach for secure and early detection of decryption
   failure is to encrypt data using v2 SEIPD.  If the session key is
   public key encrypted, the quick check is not useful as the public key
   encryption of the session key should guarantee that it is the right
   session key.

   The quick check oracle attack is a particular type of attack that
   exploits ciphertext malleability.  For information about other
   similar attacks, see Section 13.7.

13.5.  Avoiding Leaks from PKCS#1 Errors

   The PKCS#1 padding (used in RSA-encrypted and ElGamal-encrypted
   PKESK) has been found to be vulnerable to attacks in which a system
   that allows distinguishing padding errors from other decryption
   errors can act as a decryption and/or signing oracle that can leak
   the session key or allow signing arbitrary data, respectively
   [BLEICHENBACHER-PKCS1].  The number of queries required to carry out
   an attack can range from thousands to millions, depending on how
   strict and careful an implementation is in processing the padding.

   To make the attack more difficult, an implementation SHOULD implement
   strict, robust, and constant time padding checks.

   To prevent the attack, in settings where the attacker does not have
   access to timing information concerning message decryption, the
   simplest solution is to report a single error code for all variants
   of PKESK processing errors as well as SEIPD integrity errors (this
   also includes session key parsing errors, such as on an invalid
   cipher algorithm for v3 PKESK, or a session key size mismatch for v6
   PKESK).  If the attacker may have access to timing information, then
   a constant time solution is also needed.  This requires careful
   design, especially for v3 PKESK, where session key size and cipher
   information is typically not known in advance, as it is part of the
   PKESK encrypted payload.

13.6.  Fingerprint Usability

   This specification uses fingerprints in several places on the wire
   (e.g., Sections 5.2.3.23, 5.2.3.35, and 5.2.3.36) and in processing
   (e.g., in ECDH KDF Section 11.5).  An implementation may also use the
   fingerprint internally, for example, as an index to a keystore.

   Additionally, some OpenPGP users have historically used manual
   fingerprint comparison to verify the public key of a peer.  For a
   version 4 fingerprint, this has typically been done with the
   fingerprint represented as 40 hexadecimal digits, often broken into
   groups of four digits with whitespace between each group.

   When a human is actively involved, the result of such a verification
   is dubious.  There is little evidence that most humans are good at
   precise comparison of high-entropy data, particularly when that data
   is represented in compact textual form like a hexadecimal
   [USENIX-STUDY].

   The version 6 fingerprint makes the challenge for a human verifier
   even worse.  At 256 bits (compared to version 4's 160-bit
   fingerprint), a version 6 fingerprint is even harder for a human to
   successfully compare.

   An OpenPGP implementation should prioritize mechanical fingerprint
   transfer and comparison where possible and SHOULD NOT promote manual
   transfer or comparison of full fingerprints by a human unless there
   is no other way to achieve the desired result.

   While this subsection acknowledges existing practice for human-
   representable version 4 fingerprints, this document does not attempt
   to standardize any specific human-readable form of version 6
   fingerprint for this discouraged use case.

   NOTE: the topic of interoperable human-in-the-loop key verification
   needs more work, which will be done in a separate document.

13.7.  Avoiding Ciphertext Malleability

   If ciphertext can be modified by an attacker but still subsequently
   decrypted to some new plaintext, it is considered "malleable".  A
   number of attacks can arise in any cryptosystem that uses malleable
   encryption, so [RFC4880] and later versions of OpenPGP offer
   mechanisms to defend against it.  However, OpenPGP data may have been
   created before these defense mechanisms were available.  Because
   OpenPGP implementations deal with historic stored data, they may
   encounter malleable ciphertexts.

   When an OpenPGP implementation discovers that it is decrypting data
   that appears to be malleable, it MUST generate a clear error message
   that indicates the integrity of the message is suspect, it SHOULD NOT
   attempt to parse nor release decrypted data to the user, and it
   SHOULD halt with an error.  Parsing or releasing decrypted data
   before having confirmed its integrity can leak the decrypted data
   [EFAIL] [MRLG15].

   In the case of AEAD encrypted data, if the authentication tag fails
   to verify, the implementation MUST NOT attempt to parse nor release
   decrypted data to the user, and it MUST halt with an error.

   An implementation that encounters malleable ciphertext MAY choose to
   release cleartext to the user if it is not encrypted using AEAD, it
   is known to be dealing with historic archived legacy data, and the
   user is aware of the risks.

   In the case of AEAD encrypted messages, if the message is truncated,
   i.e., the final zero-octet chunk and possibly (part of) some chunks
   before it are missing, the implementation MAY choose to release
   cleartext from the fully authenticated chunks before it to the user
   if it is operating in a streaming fashion, but it MUST indicate a
   clear error message as soon as the truncation is detected.

   Any of the following OpenPGP data elements indicate that malleable
   ciphertext is present:

   *  All Symmetrically Encrypted Data packets (Section 5.7).

   *  Within any encrypted container, any Compressed Data packet
      (Section 5.6) where there is a decompression failure.

   *  Any version 1 Symmetrically Encrypted and Integrity Protected Data
      packet (Section 5.13.1) where the internal Modification Detection
      Code does not validate.

   *  Any version 2 Symmetrically Encrypted and Integrity Protected Data
      packet (Section 5.13.2) where the authentication tag of any chunk
      fails or where there is no final zero-octet chunk.

   *  Any Secret-Key packet with encrypted secret key material
      (Section 3.7.2.1) where there is an integrity failure, based on
      the value of the secret key protection octet:

      -  Value 253 (AEAD): where the AEAD authentication tag is invalid.

      -  Value 254 (CFB): where the SHA1 checksum is mismatched.

      -  Value 255 (MalleableCFB) or raw cipher algorithm: where the
         trailing 2-octet checksum does not match.

   To avoid these circumstances, an implementation that generates
   OpenPGP encrypted data SHOULD select the encrypted container format
   with the most robust protections that can be handled by the intended
   recipients.  In particular:

   *  The SED packet is deprecated and MUST NOT be generated.

   *  When encrypting to one or more public keys:

      -  If all recipient keys indicate support for a version 2
         Symmetrically Encrypted and Integrity Protected Data packet in
         their Features signature subpacket (Section 5.2.3.32), if all
         recipient keys are version 6 keys without a Features signature
         subpacket, or the implementation can otherwise infer that all
         recipients support v2 SEIPD packets, the implementation SHOULD
         encrypt using a v2 SEIPD packet.

      -  If one of the recipients does not support v2 SEIPD packets,
         then the message generator MAY use a v1 SEIPD packet instead.

   *  Passphrase-protected secret key material in a version 6 Secret Key
      or version 6 Secret Subkey packet SHOULD be protected with AEAD
      encryption (S2K usage octet 253) unless it will be transferred to
      an implementation that is known to not support AEAD.  An
      implementation should be aware that, in scenarios where an
      attacker has write access to encrypted private keys, CFB-encrypted
      keys (S2K usage octet 254 or 255) are vulnerable to corruption
      attacks that can cause leakage of secret data when the secret key
      is used [KOPENPGP] [KR02].

   Implementers should implement AEAD (v2 SEIPD and S2K usage octet 253)
   promptly and encourage its spread.

   Users are RECOMMENDED to migrate to AEAD.

13.8.  Secure Use of the v2 SEIPD Session-Key-Reuse Feature

   The salted key derivation of v2 SEIPD packets (Section 5.13.2) allows
   the recipient of an encrypted message to reply to the sender and all
   other recipients without needing their public keys but by using the
   same v6 PKESK packets it received and a different random salt value.
   This ensures a secure mechanism on the cryptographic level that
   enables the use of message encryption in cases where a sender does
   not have a copy of an encryption-capable certificate for one or more
   participants in the conversation and thus can enhance the overall
   security of an application.  However, care must be taken when using
   this mechanism not to create security vulnerabilities, such as the
   following:

   *  Replying to only a subset of the original recipients and the
      original sender by use of the session-key-reuse feature would mean
      that the remaining recipients (including the sender) of the
      original message could read the encrypted reply message, too.

   *  Adding a further recipient to the reply that is encrypted using
      the session-key-reuse feature gives that further recipient also
      cryptographic access to the original message that is being replied
      to (and potentially to a longer history of previous messages).

   *  A modification of the list of recipients addressed in the above
      points also needs to be safeguarded when a message is initially
      composed as a reply with session-key reuse but then is stored
      (e.g., as a draft) and later reopened for further editing and to
      be finally sent.

   *  There is the potential threat that an attacker with network or
      mailbox access, who is at the same time a recipient of the
      original message, silently removes themselves from the message
      before the victim's client receives it.  The victim's client that
      then uses the mechanism for replying with session-key reuse would
      unknowingly compose an encrypted message that could be read by the
      attacker.  Implementations are encouraged to use the Intended
      Recipient Fingerprint subpacket (Section 5.2.3.36) when composing
      messages and checking the consistency of the set of recipients of
      a message before replying to it with session-key reuse.

   *  When using the session-key-reuse feature in any higher-layer
      protocol, care should be taken to ensure that there is no other
      potentially interfering practice of session-key reuse established
      in that protocol.  Such interfering session-key reuse could, for
      instance, be given -- if an initial message is already composed --
      by reusing the session key of an existing encrypted file that may
      have been shared among a group of users already.  Using the
      session-key-reuse feature to compose an encrypted reply to such a
      message would unknowingly give this whole group of users
      cryptographic access to the encrypted message.

   *  Generally, the use of the session-key-reuse feature should be
      under the control of the user.  Specifically, care should be taken
      so that this feature is not silently used when the user assumes
      that proper public key encryption is used.  This can be the case,
      for instance, when the public key of one of the recipients of the
      reply is known but has expired.  Special care should be taken to
      ensure that users do not get caught in continued use of the
      session-key reuse unknowingly but instead receive the chance to
      switch to proper fresh public key encryption as soon as possible.

   *  Whenever possible, a client should prefer a fresh public key
      encryption over the session-key reuse.

   Even though this is not necessarily a security aspect, note that
   initially composing an encrypted reply using the session-key-reuse
   feature on one client and then storing it (e.g., as a draft) and
   later reopening the stored unfinished reply with another client that
   does not support the session-key-reuse feature may lead to
   interoperability problems.

   Avoiding the pitfalls described above requires context-specific
   expertise.  An implementation should only make use of the session-
   key-reuse feature in any particular application layer when it can
   follow reasonable documentation about how to deploy the feature
   safely in the specific application.  At the time of this writing,
   there is no known documentation about safe reuse of OpenPGP session
   keys for any specific context.  An implementer that intends to make
   use of this feature should publish their own proposed guidance for
   others to review.

13.9.  Escrowed Revocation Signatures

   A keyholder, Alice, may wish to designate a third party to be able to
   revoke her own key.

   The preferred way for Alice to do this is to produce a specific
   Revocation Signature (Signature Type ID 0x20, 0x28, or 0x30) and
   distribute it securely to a preferred revoker who can hold it in
   escrow.  The preferred revoker can then publish the escrowed
   Revocation Signature at whatever time is deemed appropriate rather
   than generating the Revocation Signature themselves.

   There are multiple advantages of using an escrowed Revocation
   Signature over the deprecated Revocation Key subpacket
   (Section 5.2.3.23):

   *  The keyholder can constrain what types of revocation the preferred
      revoker can issue, by only escrowing those specific signatures.

   *  There is no public/visible linkage between the keyholder and the
      preferred revoker.

   *  Third parties can verify the revocation without needing to find
      the key of the preferred revoker.

   *  The preferred revoker doesn't even need to have a public OpenPGP
      Key if some other secure transport is possible between them and
      the keyholder.

   *  Implementation support for enforcing a revocation from an
      authorized Revocation Key subpacket is uneven and unreliable.

   *  If the fingerprint mechanism suffers a cryptanalytic flaw, the
      escrowed Revocation Signature is not affected.

   A Revocation Signature may also be split up into shares and
   distributed among multiple parties, requiring some subset of those
   parties to collaborate before the escrowed Revocation Signature is
   recreated.

13.10.  Random Number Generation and Seeding

   OpenPGP requires a cryptographically secure pseudorandom number
   generator (CSPRNG).  In most cases, the operating system provides an
   appropriate facility such as a getrandom() syscall on Linux or BSD,
   which should be used absent other (for example, performance)
   concerns.  It is RECOMMENDED to use an existing CSPRNG implementation
   as opposed to crafting a new one.  Many adequate cryptographic
   libraries are already available under favorable license terms.
   Should those prove unsatisfactory, [RFC4086] provides guidance on the
   generation of random values.

   OpenPGP uses random data with three different levels of visibility:

   *  In publicly visible fields such as nonces, IVs, public padding
      material, or salts.

   *  In shared-secret values, such as session keys for encrypted data
      or padding material within an encrypted packet.

   *  In entirely private data, such as asymmetric key generation.

   With a properly functioning CSPRNG, this range of visibility does not
   present a security problem, as it is not feasible to determine the
   CSPRNG state from its output.  However, with a broken CSPRNG, it may
   be possible for an attacker to use visible output to determine the
   CSPRNG internal state and thereby predict less-visible data like
   keying material, as documented in [CHECKOWAY].

   An implementation can provide extra security against this form of
   attack by using separate CSPRNGs to generate random data with
   different levels of visibility.

13.11.  Traffic Analysis

   When sending OpenPGP data through the network, the size of the data
   may leak information to an attacker.  There are circumstances where
   such a leak could be unacceptable from a security perspective.

   For example, if possible cleartext messages for a given protocol are
   known to be either yes (3 octets) or no (2 octets) and the messages
   are sent within a Symmetrically Encrypted and Integrity Protected
   Data packet, the length of the encrypted message will reveal the
   contents of the cleartext.

   In another example, sending an OpenPGP Transferable Public Key over
   an encrypted network connection might reveal the length of the
   certificate.  Since the length of an OpenPGP certificate varies based
   on the content, an external observer interested in metadata (e.g.,
   which individual is trying to contact another individual) may be able
   to guess the identity of the certificate sent, if its length is
   unique.

   In both cases, an implementation can adjust the size of the compound
   structure by including a Padding packet (see Section 5.14).

13.12.  Surreptitious Forwarding

   When an attacker obtains a signature for some text, e.g., by
   receiving a signed message, they may be able to use that signature
   maliciously by sending a message purporting to come from the original
   sender, with the same body and signature, to a different recipient.
   To prevent this, an implementation SHOULD implement the Intended
   Recipient Fingerprint subpacket (Section 5.2.3.36).

13.13.  Hashed vs. Unhashed Subpackets

   Each OpenPGP signature can have subpackets in two different sections.
   The first set of subpackets (the "hashed section") is covered by the
   signature itself.  The second set has no cryptographic protections
   and is used for advisory material only, including locally stored
   annotations about the signature.

   For example, consider an implementation working with a specific
   signature that happens to know that the signature was made by a
   certain key, even though the signature contains no Issuer Fingerprint
   subpacket (Section 5.2.3.35) in the hashed section.  That
   implementation MAY synthesize an Issuer Fingerprint subpacket and
   store it in the unhashed section so that it will be able to recall
   which key issued the signature in the future.

   Some subpackets are only useful when they are in the hashed section,
   and an implementation SHOULD ignore them when they are found with
   unknown provenance in the unhashed section.  For example, a Preferred
   AEAD Ciphersuites subpacket (Section 5.2.3.15) in a Direct Key self-
   signature indicates the preferences of the keyholder when encrypting
   v2 SEIPD data to the key.  An implementation that observes such a
   subpacket found in the unhashed section would open itself to an
   attack where the recipient's certificate is tampered with to
   encourage the use of a specific cipher or mode of operation.

13.14.  Malicious Compressed Data

   It is possible to form a compression quine that produces itself upon
   decompression, leading to infinite regress in any implementation
   willing to parse arbitrary numbers of layers of compression.  This
   could cause resource exhaustion, which itself could lead to
   termination by the operating system.  If the operating system creates
   a "crash report", that report could contain confidential information.

   An OpenPGP implementation SHOULD limit the number of layers of
   compression it is willing to decompress in a single message.

14.  Implementation Considerations

   This section is a collection of comments to help an implementer who
   has a particular interest in backward compatibility.  Often the
   differences are small, but small differences are frequently more
   vexing than large differences.  Thus, this is a non-comprehensive
   list of potential problems and gotchas for a developer who is trying
   to achieve backward compatibility.

   *  There are many possible ways for two keys to have the same key
      material but different fingerprints (and thus different Key IDs).
      For example, since a version 4 fingerprint is constructed by
      hashing the key creation time along with other things, two version
      4 keys created at different times yet with the same key material
      will have different fingerprints.

   *  OpenPGP does not put limits on the size of public keys.  However,
      larger keys are not necessarily better keys.  Larger keys take
      more computation time to use, and this can quickly become
      impractical.  Different OpenPGP implementations may also use
      different upper bounds for public key sizes, so care should be
      taken when choosing sizes to maintain interoperability.

   *  ASCII Armor is an optional feature of OpenPGP.  The OpenPGP
      Working Group strives for a minimal set of mandatory-to-implement
      features, and since there could be useful implementations that
      only use binary object formats, this is not a "MUST" feature for
      an implementation.  For example, an implementation that is using
      OpenPGP as a mechanism for file signatures may find ASCII Armor
      unnecessary.  OpenPGP permits an implementation to declare what
      features it does and does not support, but ASCII Armor is not one
      of these.  Since most implementations allow binary and armored
      objects to be used indiscriminately, an implementation that does
      not implement ASCII Armor may find itself with compatibility
      issues with general-purpose implementations.  Moreover,
      implementations of OpenPGP-MIME [RFC3156] already have a
      requirement for ASCII Armor, so those implementations will
      necessarily have support.

   *  What this document calls the "Legacy packet format"
      (Section 4.2.2) is what older documents called the "old packet
      format".  It is the packet format used by implementations
      predating [RFC2440].  The current "OpenPGP packet format"
      (Section 4.2.1) was called the "new packet format" by older RFCs.
      This is the format introduced in [RFC2440] and maintained through
      [RFC4880] to this document.

14.1.  Constrained Legacy Fingerprint Storage for Version 6 Keys

   Some OpenPGP implementations have fixed length constraints for key
   fingerprint storage that will not fit all 32 octets of a version 6
   fingerprint.  For example, [OPENPGPCARD] reserves 20 octets for each
   stored fingerprint.

   An OpenPGP implementation MUST NOT attempt to map any part of a
   version 6 fingerprint to such a constrained field unless the relevant
   specification for the constrained environment has explicit guidance
   for storing a version 6 fingerprint that distinguishes it from a
   version 4 fingerprint.  An implementation interacting with such a
   constrained field SHOULD directly calculate the version 6 fingerprint
   from public key material and associated metadata instead of relying
   on the constrained field.

15.  IANA Considerations

   This document obsoletes [RFC4880].  IANA has updated all registration
   information that references [RFC4880] to reference this RFC instead.

15.1.  Renamed Protocol Group

   IANA bundles a set of registries associated with a particular
   protocol into a "protocol group".  IANA has updated the name of the
   "Pretty Good Privacy (PGP)" protocol group (i.e., the group of
   registries described at <https://www.iana.org/assignments/pgp-
   parameters>) to "OpenPGP".  IANA has arranged a permanent redirect
   from the existing URL to the new URL for the registries in this
   protocol group.  All further updates specified below are for
   registries within this same OpenPGP protocol group.

15.2.  Renamed and Updated Registries

   IANA has renamed the "PGP String-to-Key (S2K)" registry to "OpenPGP
   String-to-Key (S2K) Types" and updated its contents as shown in
   Table 1.

   IANA has renamed the "PGP Packet Types/Tags" registry to "OpenPGP
   Packet Types" and updated its contents as shown in Table 3.

   IANA has renamed the "Signature Subpacket Types" registry to "OpenPGP
   Signature Subpacket Types" and updated its contents as shown in
   Table 5.

   IANA has renamed the "Key Server Preference Extensions" registry to
   "OpenPGP Key Server Preference Flags" and updated its contents as
   shown in Table 8.

   IANA has renamed the "Key Flags Extensions" registry to "OpenPGP Key
   Flags" and updated its contents as shown in Table 9.

   IANA has renamed the "Reason for Revocation Extensions" registry to
   "OpenPGP Reason for Revocation (Revocation Octet)" and updated its
   contents as shown in Table 10.

   IANA has renamed the "Implementation Features" registry to "OpenPGP
   Features Flags" and updated its contents as shown in Table 11.

   IANA has renamed the "PGP User Attribute Types" registry to "OpenPGP
   User Attribute Subpacket Types" and updated its contents as shown in
   Table 13.

   IANA has renamed the "Image Format Subpacket Types" registry to
   "OpenPGP Image Attribute Encoding Format" and updated its contents as
   shown in Table 15.

   IANA has renamed the "Public Key Algorithms" registry to "OpenPGP
   Public Key Algorithms" and updated its contents as shown in Table 18.

   IANA has renamed the "Symmetric Key Algorithms" registry to "OpenPGP
   Symmetric Key Algorithms" and updated its contents as shown in
   Table 21.

   IANA has renamed the "Compression Algorithms" registry to "OpenPGP
   Compression Algorithms" and updated its contents as shown in
   Table 22.

   IANA has renamed the "Hash Algorithms" registry to "OpenPGP Hash
   Algorithms" and updated its contents as shown in Table 23.

15.3.  Removed Registry

   IANA has marked the empty "New Packet Versions" registry as OBSOLETE.

   A tombstone note has been added to the OpenPGP protocol group with
   the following content:

   |  Those wishing to use the removed "New Packet Versions" registry
   |  should instead register new versions of the relevant packets in
   |  the "OpenPGP Key and Signature Versions", "OpenPGP Key IDs and
   |  Fingerprints", and "OpenPGP Encrypted Message Packet Versions"
   |  registries.

15.4.  Added Registries

   IANA has added the following registries in the OpenPGP protocol
   group.  Note that the initial contents of each registry is shown in
   the corresponding table.

   *  "OpenPGP Secret Key Encryption (S2K Usage Octet)" (Table 2).

   *  "OpenPGP Signature Types" (Table 4).

   *  "OpenPGP Signature Notation Data Subpacket Notation Flags"
      (Table 6).

   *  "OpenPGP Signature Notation Data Subpacket Types" (Table 7).

   *  "OpenPGP Key IDs and Fingerprints" (Table 12).

   *  "OpenPGP Image Attribute Versions" (Table 14).

   *  "OpenPGP Armor Header Lines" (Table 16).

   *  "OpenPGP Armor Header Keys" (Table 17).

   *  "OpenPGP ECC Curve OIDs and Usage" (Table 19).

   *  "OpenPGP ECC Curve-Specific Wire Formats" (Table 20).

   *  "OpenPGP Hash Algorithm Identifiers for RSA Signatures' Use of
      EMSA-PKCS1-v1_5 Padding" (Table 24).

   *  "OpenPGP AEAD Algorithms" (Table 25).

   *  "OpenPGP Encrypted Message Packet Versions" (Table 26).

   *  "OpenPGP Key and Signature Versions" (Table 27).

   *  "OpenPGP Elliptic Curve Point Wire Formats" (Table 28).

   *  "OpenPGP Elliptic Curve Scalar Encodings" (Table 29).

   *  "OpenPGP ECDH KDF and KEK Parameters" (Table 30).

15.5.  Registration Policies

   All registries within the OpenPGP protocol group, with the exception
   of the registries listed in Section 15.5.1, use the Specification
   Required registration policy; see Section 4.6 of [RFC8126].  This
   policy means that review and approval by a designated expert is
   required and that the IDs and their meanings must be documented in a
   permanent and readily available public specification, in sufficient
   detail, so that interoperability between independent implementations
   is possible.

15.5.1.  Registries That Use RFC Required

   The following registries use the RFC Required registration policy, as
   described in Section 4.7 of [RFC8126]:

   *  "OpenPGP Packet Types" (Table 3).

   *  "OpenPGP Key IDs and Fingerprints" (Table 12).

   *  "OpenPGP Encrypted Message Packet Versions" (Table 26).

   *  "OpenPGP Key and Signature Versions" (Table 27).

15.6.  Designated Experts

   The designated experts will determine whether the new registrations
   retain the security properties that are expected by the base
   implementation and whether these new registrations do not cause
   interoperability issues with existing implementations, other than not
   producing or consuming the IDs associated with these new
   registrations.  Registration proposals that fail to meet these
   criteria could instead be proposed as new work items for the OpenPGP
   Working Group or its successor.

   The subsections below describe specific guidance for classes of
   registry updates that a designated expert will consider.

   The designated experts should also consider Section 12.11 when
   reviewing proposed additions to the OpenPGP protocol group.

15.6.1.  Key and Signature Versions

   When defining a new version of OpenPGP Keys or Signatures, the
   "OpenPGP Key and Signature Versions" registry (Table 27) should be
   updated.  When a new version of OpenPGP Key is defined, the "OpenPGP
   Key IDs and Fingerprints" registry (Table 12) should also be updated.

15.6.2.  Encryption Versions

   When defining a new version of the Symmetrically Encrypted and
   Integrity Protected Data packet (Section 5.13), Public Key Encrypted
   Session Key packet (Section 5.1), and/or Symmetric Key Encrypted
   Session Key packet (Section 5.3), the "OpenPGP Encrypted Message
   Packet Versions" registry (Table 26) should be updated.  When the
   SEIPD is updated, consider also adding a corresponding flag to the
   "OpenPGP Features Flags" registry (Table 11).

15.6.3.  Algorithms

   Section 9 lists the cryptographic and compression algorithms that
   OpenPGP uses.  Adding new algorithms is usually simple; in some
   cases, allocating an ID and pointing to a reference is only needed.
   But some algorithm registries require some subtle additional details
   when a new algorithm is introduced.

15.6.3.1.  Elliptic Curve Algorithms

   To register a new elliptic curve for use with OpenPGP, its OID needs
   to be registered in the "OpenPGP ECC Curve OIDs and Usage" registry
   (Table 19), its wire format needs to be documented in the "OpenPGP
   ECC Curve-Specific Wire Formats" registry (Table 20), and if used for
   ECDH, its KDF and KEK parameters must be populated in the "OpenPGP
   ECDH KDF and KEK Parameters" registry (Table 30).  If the wire
   format(s) used is not already defined in the "OpenPGP Elliptic Curve
   Point Wire Formats" (Table 28) or "OpenPGP Elliptic Curve Scalar
   Encodings" (Table 29) registries, they should be defined there as
   well.

15.6.3.2.  Symmetric Key Algorithms

   When registering a new symmetric cipher with a block size of 64 or
   128 bits and a key size that is a multiple of 64 bits, no new
   considerations are needed.

   If the new cipher has a different block size, there needs to be
   additional documentation describing how to use the cipher in CFB
   mode.

   If the new cipher has an unusual key size, then padding needs to be
   considered for X25519 and X448 key wrapping, which currently needs no
   padding.

15.6.3.3.  Hash Algorithms

   When registering a new hash algorithm in the "OpenPGP Hash
   Algorithms" registry (Table 23), if the algorithm is also to be used
   with RSA signing schemes, it must also have an entry in the "OpenPGP
   Hash Algorithm Identifiers for RSA Signatures' Use of EMSA-PKCS1-v1_5
   Padding" registry (Table 24).

16.  References

16.1.  Normative References

   [AES]      NIST, "Advanced Encryption Standard (AES)", Updated May
              2023, FIPS PUB 197, DOI 10.6028/NIST.FIPS.197-upd1,
              November 2001, <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.197-upd1.pdf>.

   [BLOWFISH] Schneier, B., "Description of a New Variable-Length Key,
              64-Bit Block Cipher (Blowfish)", Fast Software Encryption,
              Cambridge Security Workshop Proceedings, pp. 191-204,
              December 1993,
              <https://www.schneier.com/academic/archives/1994/09/
              description_of_a_new.html>.

   [BZ2]      bzip2, "bzip2 and libbzip2", 2010,
              <https://sourceware.org/bzip2/>.

   [EAX]      Bellare, M., Rogaway, P., and D. Wagner, "A Conventional
              Authenticated-Encryption Mode", April 2003,
              <https://seclab.cs.ucdavis.edu/papers/eax.pdf>.

   [ELGAMAL]  Elgamal, T., "A Public Key Cryptosystem and a Signature
              Scheme Based on Discrete Logarithms", IEEE Transactions on
              Information Theory, Vol. 31, Issue 4, pp. 469-472,
              DOI 10.1109/TIT.1985.1057074, July 1985,
              <https://doi.org/10.1109/TIT.1985.1057074>.

   [FIPS180]  NIST, "Secure Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <https://nvlpubs.nist.gov/nistpubs/fips/
              nist.fips.180-4.pdf>.

   [FIPS186]  NIST, "Digital Signature Standard (DSS)", FIPS PUB 186-5,
              DOI 10.6028/NIST.FIPS.186-5, February 2023,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-5.pdf>.

   [FIPS202]  NIST, "SHA-3 Standard: Permutation-Based Hash and
              Extendable-Output Functions", FIPS PUB 202,
              DOI 10.6028/NIST.FIPS.202, August 2015,
              <https://nvlpubs.nist.gov/nistpubs/fips/
              nist.fips.202.pdf>.

   [IDEA]     Lai, X. and J. L. Massey, "A Proposal for a New Block
              Encryption Standard", Advances in Cryptology - EUROCRYPT
              '90, Vol. 473, pp. 389-404, DOI 10.1007/3-540-46877-3_35,
              January 1991, <https://link.springer.com/
              chapter/10.1007/3-540-46877-3_35>.

   [ISO10646] ISO, "Information technology - Universal coded character
              set (UCS)", ISO/IEC 10646:2020, December 2020,
              <https://www.iso.org/standard/76835.html>.

   [JFIF]     ITU-T, "Information technology - Digital compression and
              coding of continuous-tone still images: JPEG File
              Interchange Format (JFIF)", Recommendation ITU-T T.871,
              May 2011, <https://www.itu.int/rec/T-REC-T.871-201105-I>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <https://www.rfc-editor.org/info/rfc1321>.

   [RFC1950]  Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
              Specification version 3.3", RFC 1950,
              DOI 10.17487/RFC1950, May 1996,
              <https://www.rfc-editor.org/info/rfc1950>.

   [RFC1951]  Deutsch, P., "DEFLATE Compressed Data Format Specification
              version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
              <https://www.rfc-editor.org/info/rfc1951>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2144]  Adams, C., "The CAST-128 Encryption Algorithm", RFC 2144,
              DOI 10.17487/RFC2144, May 1997,
              <https://www.rfc-editor.org/info/rfc2144>.

   [RFC2822]  Resnick, P., Ed., "Internet Message Format", RFC 2822,
              DOI 10.17487/RFC2822, April 2001,
              <https://www.rfc-editor.org/info/rfc2822>.

   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
              "MIME Security with OpenPGP", RFC 3156,
              DOI 10.17487/RFC3156, August 2001,
              <https://www.rfc-editor.org/info/rfc3156>.

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, DOI 10.17487/RFC3394,
              September 2002, <https://www.rfc-editor.org/info/rfc3394>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC3713]  Matsui, M., Nakajima, J., and S. Moriai, "A Description of
              the Camellia Encryption Algorithm", RFC 3713,
              DOI 10.17487/RFC3713, April 2004,
              <https://www.rfc-editor.org/info/rfc3713>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC7253]  Krovetz, T. and P. Rogaway, "The OCB Authenticated-
              Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May
              2014, <https://www.rfc-editor.org/info/rfc7253>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8018]  Moriarty, K., Ed., Kaliski, B., and A. Rusch, "PKCS #5:
              Password-Based Cryptography Specification Version 2.1",
              RFC 8018, DOI 10.17487/RFC8018, January 2017,
              <https://www.rfc-editor.org/info/rfc8018>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9106]  Biryukov, A., Dinu, D., Khovratovich, D., and S.
              Josefsson, "Argon2 Memory-Hard Function for Password
              Hashing and Proof-of-Work Applications", RFC 9106,
              DOI 10.17487/RFC9106, September 2021,
              <https://www.rfc-editor.org/info/rfc9106>.

   [RIPEMD-160]
              ISO, "Information technology - Security techniques - Hash-
              functions - Part 3: Dedicated hash-functions", ISO/
              IEC 10118-3:1998, May 1998.

   [SP800-38A]
              NIST, "Recommendation for Block Cipher Modes of Operation:
              Methods and Techniques", NIST Special Publication 800-38A,
              DOI 10.6028/NIST.SP.800-38A, December 2001,
              <https://nvlpubs.nist.gov/nistpubs/legacy/sp/
              nistspecialpublication800-38a.pdf>.

   [SP800-38D]
              NIST, "Recommendation for Block Cipher Modes of Operation:
              Galois/Counter Mode (GCM) and GMAC", NIST Special
              Publication 800-38D, DOI 10.6028/NIST.SP.800-38D, November
              2007, <https://nvlpubs.nist.gov/nistpubs/legacy/sp/
              nistspecialpublication800-38d.pdf>.

   [SP800-56A]
              NIST, "Recommendation for Pair-Wise Key Establishment
              Schemes Using Discrete Logarithm Cryptography", NIST
              Special Publication 800-56A Revision 3,
              DOI 10.6028/NIST.SP.800-56Ar, April 2018,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              nist.sp.800-56Ar3.pdf>.

   [SP800-67] NIST, "Recommendation for the Triple Data Encryption
              Algorithm (TDEA) Block Cipher", NIST Special
              Publication 800-67 Revision 2,
              DOI 10.6028/NIST.SP.800-67r2, November 2017,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-67r2.pdf>.

   [TWOFISH]  Schneier, B., Kelsey, J., Whiting, D., Wagner, D., Hall,
              C., and N. Ferguson, "Twofish: A 128-Bit Block Cipher",
              June 1998, <https://www.schneier.com/wp-
              content/uploads/2016/02/paper-twofish-paper.pdf>.

16.2.  Informative References

   [BLEICHENBACHER]
              Bleichenbacher, D., "Generating ElGamal Signatures Without
              Knowing the Secret Key", EUROCRYPT'96: International
              Conference on the Theory and Applications of Cryptographic
              Techniques Proceedings, Vol. 1070, pp. 10-18, May 1996.

   [BLEICHENBACHER-PKCS1]
              Bleichenbacher, D., "Chosen Ciphertext Attacks Against
              Protocols Based on the RSA Encryption Standard PKCS #1",
              CRYPTO '98: International Cryptology Conference
              Proceedings, Vol. 1462, pp. 1-12, August 1998,
              <http://archiv.infsec.ethz.ch/education/fs08/secsem/
              Bleichenbacher98.pdf>.

   [C99]      ISO, "Information technology - Programming languages: C",
              ISO/IEC 9899:2018, June 2018,
              <https://www.iso.org/standard/74528.html>.

   [CHECKOWAY]
              Checkoway, S., Maskiewicz, J., Garman, C., Fried, J.,
              Cohney, S., Green, M., Heninger, N., Weinmann, RP.,
              Rescorla, E., and H. Shacham, "A Systematic Analysis of
              the Juniper Dual EC Incident", Proceedings of the 2016 ACM
              SIGSAC Conference on Computer and Communications Security,
              DOI 10.1145/2976749.2978395, October 2016,
              <https://doi.org/10.1145/2976749.2978395>.

   [EFAIL]    Poddebniak, D., Dresen, C., Müller, J., Ising, F.,
              Schinzel, S., Friedberger, S., Somorovsky, J., and J.
              Schwenk, "Efail: Breaking S/MIME and OpenPGP Email
              Encryption using Exfiltration Channels", Proceedings of
              the 27th USENIX Security Symposium, August 2018,
              <https://www.usenix.org/system/files/conference/
              usenixsecurity18/sec18-poddebniak.pdf>.

   [Errata-2199]
              RFC Errata, Erratum ID 2199, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2199>.

   [Errata-2200]
              RFC Errata, Erratum ID 2200, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2200>.

   [Errata-2206]
              RFC Errata, Erratum ID 2206, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2206>.

   [Errata-2208]
              RFC Errata, Erratum ID 2208, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2208>.

   [Errata-2214]
              RFC Errata, Erratum ID 2214, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2214>.

   [Errata-2216]
              RFC Errata, Erratum ID 2216, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2216>.

   [Errata-2219]
              RFC Errata, Erratum ID 2219, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2219>.

   [Errata-2222]
              RFC Errata, Erratum ID 2222, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2222>.

   [Errata-2226]
              RFC Errata, Erratum ID 2226, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2226>.

   [Errata-2234]
              RFC Errata, Erratum ID 2234, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2234>.

   [Errata-2235]
              RFC Errata, Erratum ID 2235, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2235>.

   [Errata-2236]
              RFC Errata, Erratum ID 2236, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2236>.

   [Errata-2238]
              RFC Errata, Erratum ID 2238, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2238>.

   [Errata-2240]
              RFC Errata, Erratum ID 2240, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2240>.

   [Errata-2242]
              RFC Errata, Erratum ID 2242, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2242>.

   [Errata-2243]
              RFC Errata, Erratum ID 2243, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2243>.

   [Errata-2270]
              RFC Errata, Erratum ID 2270, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2270>.

   [Errata-2271]
              RFC Errata, Erratum ID 2271, RFC 4880,
              <https://www.rfc-editor.org/errata/eid2271>.

   [Errata-3298]
              RFC Errata, Erratum ID 3298, RFC 4880,
              <https://www.rfc-editor.org/errata/eid3298>.

   [Errata-5491]
              RFC Errata, Erratum ID 5491, RFC 4880,
              <https://www.rfc-editor.org/errata/eid5491>.

   [Errata-7545]
              RFC Errata, Erratum ID 7545, RFC 4880,
              <https://www.rfc-editor.org/errata/eid7545>.

   [Errata-7889]
              RFC Errata, Erratum ID 7889, RFC 4880,
              <https://www.rfc-editor.org/errata/eid7889>.

   [HASTAD]   Hastad, J., "Solving Simultaneous Modular Equations of Low
              Degree", DOI 10.1137/0217019, April 1988,
              <https://doi.org/10.1137/0217019>.

   [JKS02]    Jallad, K., Katz, J., and B. Schneier, "Implementation of
              Chosen-Ciphertext Attacks against PGP and GnuPG",
              DOI 0.1007/3-540-45811-5_7, September 2002,
              <https://www.schneier.com/academic/archives/2002/01/
              implementation_of_ch.html>.

   [KOBLITZ]  Koblitz, N., "A course in number theory and cryptography",
              Chapter VI: Elliptic Curves, DOI 10.2307/3618498, 1997,
              <https://doi.org/10.2307/3618498>.

   [KOPENPGP] Bruseghini, L., Paterson, K. G., and D. Huigens, "Victory
              by KO: Attacking OpenPGP Using Key Overwriting",
              Proceedings of the ACM SIGSAC Conference on Computer and
              Communications Security, pp. 411-423,
              DOI 10.1145/3548606.3559363, November 2022,
              <https://dl.acm.org/doi/10.1145/3548606.3559363>.

   [KR02]     Klíma, V. and T. Rosa, "Attack on Private Signature Keys
              of the OpenPGP Format, PGP(TM) Programs and Other
              Applications Compatible with OpenPGP", Cryptology ePrint
              Archive, Paper 2002/076, March 2001,
              <https://eprint.iacr.org/2002/076>.

   [MRLG15]   Maury, F., Reinhard, JR., Levillain, O., and H. Gilbert,
              "Format Oracles on OpenPGP", Topics in Cryptology -- CT-
              RSA 2015, Vol. 9048, pp. 220-236,
              DOI 10.1007/978-3-319-16715-2_12, January 2015,
              <https://doi.org/10.1007/978-3-319-16715-2_12>.

   [MZ05]     Mister, S. and R. Zuccherato, "An Attack on CFB Mode
              Encryption As Used By OpenPGP", Cryptology ePrint Archive,
              Paper 2005/033, February 2005,
              <http://eprint.iacr.org/2005/033>.

   [OPENPGPCARD]
              Pietig, A., "Functional Specification of the OpenPGP
              application on ISO Smart Card Operating Systems", Version
              3.4.1, March 2020, <https://gnupg.org/ftp/specs/OpenPGP-
              smart-card-application-3.4.1.pdf>.

   [PAX]      The Open Group, "The Open Group Base Specifications", 'pax
              - portable archive interchange', Issue 7, 2018 Edition,
              IEEE Std 1003.1-2017, 2018,
              <https://pubs.opengroup.org/onlinepubs/9699919799/
              utilities/pax.html>.

   [PSSLR17]  Poddebniak, D., Somorovsky, J., Schinzel, S., Lochter, M.,
              and P. Rösler, "Attacking Deterministic Signature Schemes
              using Fault Attacks", Cryptology ePrint Archive, Paper
              2017/1014, October 2017,
              <https://eprint.iacr.org/2017/1014>.

   [REGEX]    regex, "Henry Spencer's regular expression libraries",
              <https://garyhouston.github.io/regex/>.

   [RFC1991]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
              Exchange Formats", RFC 1991, DOI 10.17487/RFC1991, August
              1996, <https://www.rfc-editor.org/info/rfc1991>.

   [RFC2440]  Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
              "OpenPGP Message Format", RFC 2440, DOI 10.17487/RFC2440,
              November 1998, <https://www.rfc-editor.org/info/rfc2440>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <https://www.rfc-editor.org/info/rfc2978>.

   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <https://www.rfc-editor.org/info/rfc4880>.

   [RFC5581]  Shaw, D., "The Camellia Cipher in OpenPGP", RFC 5581,
              DOI 10.17487/RFC5581, June 2009,
              <https://www.rfc-editor.org/info/rfc5581>.

   [RFC5639]  Lochter, M. and J. Merkle, "Elliptic Curve Cryptography
              (ECC) Brainpool Standard Curves and Curve Generation",
              RFC 5639, DOI 10.17487/RFC5639, March 2010,
              <https://www.rfc-editor.org/info/rfc5639>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <https://www.rfc-editor.org/info/rfc6090>.

   [RFC6637]  Jivsov, A., "Elliptic Curve Cryptography (ECC) in
              OpenPGP", RFC 6637, DOI 10.17487/RFC6637, June 2012,
              <https://www.rfc-editor.org/info/rfc6637>.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", May 2009,
              <https://www.secg.org/sec1-v2.pdf>.

   [SHA1CD]   "sha1collisiondetection", commit b4a7b0b, December 2020,
              <https://github.com/cr-marcstevens/
              sha1collisiondetection>.

   [SHAMBLES] Leurent, G. and T. Peyrin, "Sha-1 is a shambles: first
              chosen-prefix collision on sha-1 and application to the
              PGP web of trust", August 2020,
              <https://dl.acm.org/doi/abs/10.5555/3489212.3489316/>.

   [SP800-131A]
              NIST, "Transitioning the Use of Cryptographic Algorithms
              and Key Lengths", NIST Special Publication 800-131A,
              Revision 2, DOI 10.6028/NIST.SP.800-131Ar2, March 2019,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-131Ar2.pdf>.

   [SP800-57] NIST, "Recommendation for Key Management: Part 1 -
              General", NIST Special Publication 800-57 Part 1, Revision
              5, DOI 10.6028/NIST.SP.800-57pt1r5, May 2020,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-57pt1r5.pdf>.

   [STEVENS2013]
              Stevens, M., "Counter-cryptanalysis", Cryptology ePrint
              Archive, Paper 2013/358, June 2013,
              <https://eprint.iacr.org/2013/358>.

   [UNIFIED-DIFF]
              Free Software Foundation, "Comparing and Merging Files",
              'Detailed Description of Unified Format', Section 2.2.2.2,
              January 2021,
              <https://www.gnu.org/software/diffutils/manual/html_node/
              Detailed-Unified.html>.

   [USENIX-STUDY]
              Dechand, S., Schürmann, D., Busse, K., Acar, Y., Fahl, S.,
              and M. Smith, "An Empirical Study of Textual Key-
              Fingerprint Representations", ISBN 978-1-931971-32-4,
              August 2016,
              <https://www.usenix.org/system/files/conference/
              usenixsecurity16/sec16_paper_dechand.pdf>.

Appendix A.  Test Vectors

   To help with the implementation of this specification, a set of non-
   normative examples follow.

A.1.  Sample Version 4 Ed25519Legacy Key

   The secret key used for this example is:

   D: 1a8b1ff05ded48e18bf50166c664ab023ea70003d78d9e41f5758a91d850f8d2

   Note that this is the raw secret key used as input to the EdDSA
   signing operation.  The key was created on 2014-08-19 14:28:27 and
   thus the fingerprint of the OpenPGP Key is:

      C959 BDBA FA32 A2F8 9A15  3B67 8CFD E121 9796 5A9A

   The algorithm-specific input parameters without the MPI length
   headers are:

   oid: 2b06010401da470f01

   q: 403f098994bdd916ed4053197934e4a87c80733a1280d62f8010992e43ee3b2406

   The entire Public Key packet is thus:

      98 33 04 53 f3 5f 0b 16  09 2b 06 01 04 01 da 47
      0f 01 01 07 40 3f 09 89  94 bd d9 16 ed 40 53 19
      79 34 e4 a8 7c 80 73 3a  12 80 d6 2f 80 10 99 2e
      43 ee 3b 24 06

   The same packet represented in ASCII-armored form is:

   -----BEGIN PGP PUBLIC KEY BLOCK-----

   xjMEU/NfCxYJKwYBBAHaRw8BAQdAPwmJlL3ZFu1AUxl5NOSofIBzOhKA1i+AEJku
   Q+47JAY=
   -----END PGP PUBLIC KEY BLOCK-----

A.2.  Sample Version 4 Ed25519Legacy Signature

   The signature is created using the sample key over the input data
   "OpenPGP" on 2015-09-16 12:24:53 UTC and thus the input to the hash
   function is:

   m: 4f70656e504750040016080006050255f95f9504ff0000000c

   Using the SHA2-256 hash algorithm yields the digest:

   d: f6220a3f757814f4c2176ffbb68b00249cd4ccdc059c4b34ad871f30b1740280

   which is fed into the EdDSA signature function and yields the
   following signature:

   r: 56f90cca98e2102637bd983fdb16c131dfd27ed82bf4dde5606e0d756aed3366

   s: d09c4fa11527f038e0f57f2201d82f2ea2c9033265fa6ceb489e854bae61b404

   The entire Signature packet is thus:

      88 5e 04 00 16 08 00 06  05 02 55 f9 5f 95 00 0a
      09 10 8c fd e1 21 97 96  5a 9a f6 22 00 ff 56 f9
      0c ca 98 e2 10 26 37 bd  98 3f db 16 c1 31 df d2
      7e d8 2b f4 dd e5 60 6e  0d 75 6a ed 33 66 01 00
      d0 9c 4f a1 15 27 f0 38  e0 f5 7f 22 01 d8 2f 2e
      a2 c9 03 32 65 fa 6c eb  48 9e 85 4b ae 61 b4 04

   The same packet represented in ASCII-armored form is:

   -----BEGIN PGP SIGNATURE-----

   iF4EABYIAAYFAlX5X5UACgkQjP3hIZeWWpr2IgD/VvkMypjiECY3vZg/2xbBMd/S
   ftgr9N3lYG4NdWrtM2YBANCcT6EVJ/A44PV/IgHYLy6iyQMyZfps60iehUuuYbQE
   -----END PGP SIGNATURE-----

A.3.  Sample Version 6 Certificate (Transferable Public Key)

   Here is a Transferable Public Key consisting of:

   *  A version 6 Ed25519 Public Key packet

   *  A version 6 Direct Key self-signature

   *  A version 6 X25519 Public Subkey packet

   *  A version 6 Subkey Binding signature

   The primary key has the following fingerprint:

   CB186C4F0609A697E4D52DFA6C722B0C1F1E27C18A56708F6525EC27BAD9ACC9

   The subkey has the following fingerprint:

   12C83F1E706F6308FE151A417743A1F033790E93E9978488D1DB378DA9930885

   -----BEGIN PGP PUBLIC KEY BLOCK-----

   xioGY4d/4xsAAAAg+U2nu0jWCmHlZ3BqZYfQMxmZu52JGggkLq2EVD34laPCsQYf
   GwoAAABCBYJjh3/jAwsJBwUVCg4IDAIWAAKbAwIeCSIhBssYbE8GCaaX5NUt+mxy
   KwwfHifBilZwj2Ul7Ce62azJBScJAgcCAAAAAK0oIBA+LX0ifsDm185Ecds2v8lw
   gyU2kCcUmKfvBXbAf6rhRYWzuQOwEn7E/aLwIwRaLsdry0+VcallHhSu4RN6HWaE
   QsiPlR4zxP/TP7mhfVEe7XWPxtnMUMtf15OyA51YBM4qBmOHf+MZAAAAIIaTJINn
   +eUBXbki+PSAld2nhJh/LVmFsS+60WyvXkQ1wpsGGBsKAAAALAWCY4d/4wKbDCIh
   BssYbE8GCaaX5NUt+mxyKwwfHifBilZwj2Ul7Ce62azJAAAAAAQBIKbpGG2dWTX8
   j+VjFM21J0hqWlEg+bdiojWnKfA5AQpWUWtnNwDEM0g12vYxoWM8Y81W+bHBw805
   I8kWVkXU6vFOi+HWvv/ira7ofJu16NnoUkhclkUrk0mXubZvyl4GBg==
   -----END PGP PUBLIC KEY BLOCK-----

   The corresponding Transferable Secret Key can be found in
   Appendix A.4.

A.3.1.  Hashed Data Stream for Signature Verification

   The Direct Key self-signature in the certificate in Appendix A.3 is
   made over the following sequence of data:

   0x0000  10 3e 2d 7d 22 7e c0 e6
   0x0008  d7 ce 44 71 db 36 bf c9
   0x0010  70 83 25 36 90 27 14 98
   0x0018  a7 ef 05 76 c0 7f aa e1
   0x0020  9b 00 00 00 2a 06 63 87
   0x0028  7f e3 1b 00 00 00 20 f9
   0x0030  4d a7 bb 48 d6 0a 61 e5
   0x0038  67 70 6a 65 87 d0 33 19
   0x0040  99 bb 9d 89 1a 08 24 2e
   0x0048  ad 84 54 3d f8 95 a3 06
   0x0050  1f 1b 0a 00 00 00 42 05
   0x0058  82 63 87 7f e3 03 0b 09
   0x0060  07 05 15 0a 0e 08 0c 02
   0x0068  16 00 02 9b 03 02 1e 09
   0x0070  22 21 06 cb 18 6c 4f 06
   0x0078  09 a6 97 e4 d5 2d fa 6c
   0x0080  72 2b 0c 1f 1e 27 c1 8a
   0x0088  56 70 8f 65 25 ec 27 ba
   0x0090  d9 ac c9 05 27 09 02 07
   0x0098  02 06 ff 00 00 00 4a

   The same data, broken out by octet and semantics, is:

   0x0000  10 3e 2d 7d 22 7e c0 e6  salt
   0x0008  d7 ce 44 71 db 36 bf c9
   0x0010  70 83 25 36 90 27 14 98
   0x0018  a7 ef 05 76 c0 7f aa e1
           [ pubkey begins ]
   0x0020  9b                       key packet
   0x0021     00 00 00 2a           pubkey length
   0x0025                 06        pubkey version
   0x0026                    63 87  creation time
   0x0028  7f e3                      (2022-11-30T16:08:03Z)
   0x002a        1b                 key algo: Ed25519
   0x002b           00 00 00 20     key length
   0x002f                       f9  Ed25519 public key
   0x0030  4d a7 bb 48 d6 0a 61 e5
   0x0038  67 70 6a 65 87 d0 33 19
   0x0040  99 bb 9d 89 1a 08 24 2e
   0x0048  ad 84 54 3d f8 95 a3
            [ trailer begins ]
   0x004f                       06  sig version 6
   0x0050  1f                       sig type: Direct Key signature
   0x0051     1b                    sig algo: Ed25519
   0x0052        0a                 hash ago: SHA2-512
   0x0053           00 00 00 42     hashed subpackets length
   0x0057                       05  subpkt length
   0x0058  82                       critical subpkt: Sig Creation Time
   0x0059     63 87 7f e3           Signature Creation Time
   0x005d                 03        subpkt length
   0x005e                    0b     subpkt type: Pref. v1 SEIPD Ciphers
   0x005f                       09  Ciphers: [AES256 AES128]
   0x0060  07
   0x0061     05                    subpkt length
   0x0062        15                 subpkt type: Pref. Hash Algorithms
   0x0063           0a 0e           Hashes: [SHA2-512 SHA3-512
   0x0065                 08 0c              SHA2-256 SHA3-256]
   0x0067                       02  subpkt length
   0x0068  16                       subpkt type: Pref. Compression
   0x0069     00                    Compression: [none]
   0x006a        02                 subpkt length
   0x006b           9b              critical subpkt: Key Flags
   0x006c              03           Key Flags: {certify, sign}
   0x006d                 02        subpkt length
   0x006e                    1e     subpkt type: Features
   0x006f                       09  Features: {v1SEIPD, v2SEIPD}
   0x0070  22                       subpkt length
   0x0071     21                    subpkt type: Issuer Fingerprint
   0x0072        06                 Fingerprint version 6
   0x0073           cb 18 6c 4f 06  Fingerprint
   0x0078  09 a6 97 e4 d5 2d fa 6c
   0x0080  72 2b 0c 1f 1e 27 c1 8a
   0x0088  56 70 8f 65 25 ec 27 ba
   0x0090  d9 ac c9
   0x0093           05              subpkt length
   0x0094              27           subpkt type: Pref. AEAD Ciphersuites
   0x0095                 09 02 07  Ciphersuites:
   0x0098  02                         [ AES256-OCB, AES128-OCB ]
   0x0099     06                    sig version 6
   0x009a        ff                 sentinel octet
   0x009b           00 00 00 4a     trailer length

   The Subkey Binding signature in Appendix A.3 is made over the
   following sequence of data:

   0x0000  a6 e9 18 6d 9d 59 35 fc
   0x0008  8f e5 63 14 cd b5 27 48
   0x0010  6a 5a 51 20 f9 b7 62 a2
   0x0018  35 a7 29 f0 39 01 0a 56
   0x0020  9b 00 00 00 2a 06 63 87
   0x0028  7f e3 1b 00 00 00 20 f9
   0x0030  4d a7 bb 48 d6 0a 61 e5
   0x0038  67 70 6a 65 87 d0 33 19
   0x0040  99 bb 9d 89 1a 08 24 2e
   0x0048  ad 84 54 3d f8 95 a3 9b
   0x0050  00 00 00 2a 06 63 87 7f
   0x0058  e3 19 00 00 00 20 86 93
   0x0060  24 83 67 f9 e5 01 5d b9
   0x0068  22 f8 f4 80 95 dd a7 84
   0x0070  98 7f 2d 59 85 b1 2f ba
   0x0078  d1 6c af 5e 44 35 06 18
   0x0080  1b 0a 00 00 00 2c 05 82
   0x0088  63 87 7f e3 02 9b 0c 22
   0x0090  21 06 cb 18 6c 4f 06 09
   0x0098  a6 97 e4 d5 2d fa 6c 72
   0x00a0  2b 0c 1f 1e 27 c1 8a 56
   0x00a8  70 8f 65 25 ec 27 ba d9
   0x00b0  ac c9 06 ff 00 00 00 34

   The same data, broken out by octet and semantics, is:

   0x0000  a6 e9 18 6d 9d 59 35 fc  salt
   0x0008  8f e5 63 14 cd b5 27 48
   0x0010  6a 5a 51 20 f9 b7 62 a2
   0x0018  35 a7 29 f0 39 01 0a 56
         [ primary pubkey begins ]
   0x0020  9b                       key packet
   0x0021     00 00 00 2a           pubkey length
   0x0025                 06        pubkey version
   0x0026                    63 87  creation time
   0x0028  7f e3                      (2022-11-30T16:08:03Z)
   0x002a        1b                 key algo: Ed25519
   0x002b           00 00 00 20     key length
   0x002f                       f9  Ed25519 public key
   0x0030  4d a7 bb 48 d6 0a 61 e5
   0x0038  67 70 6a 65 87 d0 33 19
   0x0040  99 bb 9d 89 1a 08 24 2e
   0x0048  ad 84 54 3d f8 95 a3
         [ subkey pubkey begins ]
   0x004f                       9b  key packet
   0x0050  00 00 00 2a              pubkey length
   0x0054              06           pubkey version
   0x0055                 63 87 7f  creation time (2022-11-30T16:08:03Z)
   0x0058  e3
   0x0059     19                    key algo: X25519
   0x005a        00 00 00 20        key length
   0x005e                    86 93  X25519 public key
   0x0060  24 83 67 f9 e5 01 5d b9
   0x0068  22 f8 f4 80 95 dd a7 84
   0x0070  98 7f 2d 59 85 b1 2f ba
   0x0078  d1 6c af 5e 44 35
          [ trailer begins ]
   0x007e                    06     sig version 6
   0x007f                       18  sig type: Subkey Binding sig
   0x0080  1b                       sig algo Ed25519
   0x0081     0a                    hash algo: SHA2-512
   0x0082        00 00 00 2c        hashed subpackets length
   0x0086                    05     subpkt length
   0x0087                       82  critical subpkt: Sig Creation Time
   0x0088  63 87 7f e3              Signature Creation Time
   0x008c              02           subpkt length
   0x008d                 9b        critical subpkt: Key Flags
   0x008e                    0c     Key Flags: {EncComms, EncStorage}
   0x008f                       22  subpkt length
   0x0090  21                       subpkt type: Issuer Fingerprint
   0x0091     06                    Fingerprint version 6
   0x0092        cb 18 6c 4f 06 09  Fingerprint
   0x0098  a6 97 e4 d5 2d fa 6c 72
   0x00a0  2b 0c 1f 1e 27 c1 8a 56
   0x00a8  70 8f 65 25 ec 27 ba d9
   0x00b0  ac c9
   0x00b2        06                 sig version 6
   0x00b3           ff              sentinel octet
   0x00b4              00 00 00 34  trailer length

A.4.  Sample Version 6 Secret Key (Transferable Secret Key)

   Here is a Transferable Secret Key consisting of:

   *  A version 6 Ed25519 Secret Key packet

   *  A version 6 Direct Key self-signature

   *  A version 6 X25519 Secret Subkey packet

   *  A version 6 Subkey Binding signature

   -----BEGIN PGP PRIVATE KEY BLOCK-----

   xUsGY4d/4xsAAAAg+U2nu0jWCmHlZ3BqZYfQMxmZu52JGggkLq2EVD34laMAGXKB
   exK+cH6NX1hs5hNhIB00TrJmosgv3mg1ditlsLfCsQYfGwoAAABCBYJjh3/jAwsJ
   BwUVCg4IDAIWAAKbAwIeCSIhBssYbE8GCaaX5NUt+mxyKwwfHifBilZwj2Ul7Ce6
   2azJBScJAgcCAAAAAK0oIBA+LX0ifsDm185Ecds2v8lwgyU2kCcUmKfvBXbAf6rh
   RYWzuQOwEn7E/aLwIwRaLsdry0+VcallHhSu4RN6HWaEQsiPlR4zxP/TP7mhfVEe
   7XWPxtnMUMtf15OyA51YBMdLBmOHf+MZAAAAIIaTJINn+eUBXbki+PSAld2nhJh/
   LVmFsS+60WyvXkQ1AE1gCk95TUR3XFeibg/u/tVY6a//1q0NWC1X+yui3O24wpsG
   GBsKAAAALAWCY4d/4wKbDCIhBssYbE8GCaaX5NUt+mxyKwwfHifBilZwj2Ul7Ce6
   2azJAAAAAAQBIKbpGG2dWTX8j+VjFM21J0hqWlEg+bdiojWnKfA5AQpWUWtnNwDE
   M0g12vYxoWM8Y81W+bHBw805I8kWVkXU6vFOi+HWvv/ira7ofJu16NnoUkhclkUr
   k0mXubZvyl4GBg==
   -----END PGP PRIVATE KEY BLOCK-----

   The corresponding Transferable Public Key can be found in
   Appendix A.3.

A.5.  Sample Locked Version 6 Secret Key (Transferable Secret Key)

   Here is the same secret key as in Appendix A.4, but the secret key
   material is locked with a passphrase using AEAD and Argon2.

   The passphrase is the ASCII string:

   correct horse battery staple

   -----BEGIN PGP PRIVATE KEY BLOCK-----

   xYIGY4d/4xsAAAAg+U2nu0jWCmHlZ3BqZYfQMxmZu52JGggkLq2EVD34laP9JgkC
   FARdb9ccngltHraRe25uHuyuAQQVtKipJ0+r5jL4dacGWSAheCWPpITYiyfyIOPS
   3gIDyg8f7strd1OB4+LZsUhcIjOMpVHgmiY/IutJkulneoBYwrEGHxsKAAAAQgWC
   Y4d/4wMLCQcFFQoOCAwCFgACmwMCHgkiIQbLGGxPBgmml+TVLfpscisMHx4nwYpW
   cI9lJewnutmsyQUnCQIHAgAAAACtKCAQPi19In7A5tfORHHbNr/JcIMlNpAnFJin
   7wV2wH+q4UWFs7kDsBJ+xP2i8CMEWi7Ha8tPlXGpZR4UruETeh1mhELIj5UeM8T/
   0z+5oX1RHu11j8bZzFDLX9eTsgOdWATHggZjh3/jGQAAACCGkySDZ/nlAV25Ivj0
   gJXdp4SYfy1ZhbEvutFsr15ENf0mCQIUBA5hhGgp2oaavg6mFUXcFMwBBBUuE8qf
   9Ock+xwusd+GAglBr5LVyr/lup3xxQvHXFSjjA2haXfoN6xUGRdDEHI6+uevKjVR
   v5oAxgu7eJpaXNjCmwYYGwoAAAAsBYJjh3/jApsMIiEGyxhsTwYJppfk1S36bHIr
   DB8eJ8GKVnCPZSXsJ7rZrMkAAAAABAEgpukYbZ1ZNfyP5WMUzbUnSGpaUSD5t2Ki
   Nacp8DkBClZRa2c3AMQzSDXa9jGhYzxjzVb5scHDzTkjyRZWRdTq8U6L4da+/+Kt
   ruh8m7Xo2ehSSFyWRSuTSZe5tm/KXgYG
   -----END PGP PRIVATE KEY BLOCK-----

A.5.1.  Intermediate Data for Locked Primary Key

   The S2K-derived material for the primary key is:

   832bd2662a5c2b251ee3fc82aec349a766ca539015880133002e5a21960b3bcf

   After HKDF, the symmetric key used for AEAD encryption of the primary
   key is:

   9e37cb26787f37e18db172795c4c297550d39ac82511d9af4c8706db6a77fd51

   The additional data for AEAD for the primary key is:

   c50663877fe31b00000020f94da7bb48d60a61e567706a6587d0331999bb9d89
   1a08242ead84543df895a3

A.5.2.  Intermediate Data for Locked Subkey

   The S2K-derived key material for the subkey is:

   f74a6ce873a089ef13a3da9ac059777bb22340d15eaa6c9dc0f8ef09035c67cd

   After HKDF, the symmetric key used for AEAD encryption of the subkey
   is:

   3c60cb63285f62f4c3de49835786f011cf6f4c069f61232cd7013ff5fd31e603

   The additional data for AEAD for the subkey is:

   c70663877fe319000000208693248367f9e5015db922f8f48095dda784987f2d
   5985b12fbad16caf5e4435

A.6.  Sample Cleartext Signed Message

   Here is a signed message that uses the Cleartext Signature Framework
   (Section 7).  It can be verified with the certificate from
   Appendix A.3.

   Note that this message makes use of dash-escaping (Section 7.2) due
   to its contents.

   -----BEGIN PGP SIGNED MESSAGE-----

   What we need from the grocery store:

   - - tofu
   - - vegetables
   - - noodles

   -----BEGIN PGP SIGNATURE-----

   wpgGARsKAAAAKQWCY5ijYyIhBssYbE8GCaaX5NUt+mxyKwwfHifBilZwj2Ul7Ce6
   2azJAAAAAGk2IHZJX1AhiJD39eLuPBgiUU9wUA9VHYblySHkBONKU/usJ9BvuAqo
   /FvLFuGWMbKAdA+epq7V4HOtAPlBWmU8QOd6aud+aSunHQaaEJ+iTFjP2OMW0KBr
   NK2ay45cX1IVAQ==
   -----END PGP SIGNATURE-----

   The Signature packet here is:

   0x0000  c2                       packet type: Signature
   0x0001     98                    packet length
   0x0002        06                 sig version 6
   0x0003           01              sig type: Canonical Text
   0x0004              1b           pubkey algorithm: Ed25519
   0x0005                 0a        hash algorithm used: SHA2-512
   0x0006                    00 00  hashed subpackets length: 41
   0x0008  00 29
   0x000a        05                 subpkt length
   0x000b           82              critical subpkt: Sig Creation Time
   0x000c              63 98 a3 63   (2022-12-13T16:08:03Z)
   0x0010  22                       subpkt length
   0x0011     21                    subpkt type: Issuer Fingerprint
   0x0012        06                 Fingerprint version 6
   0x0013           cb 18 6c 4f 06  Fingerprint
   0x001a  09 a6 97 e4 d5 2d fa 6c
   0x0020  72 2b 0c 1f 1e 27 c1 8a
   0x0028  56 70 8f 65 25 ec 27 ba
   0x0030  d9 ac c9
   0x0033           00 00 00 00     unhashed subpackets length: 0
   0x0037                       69  left 16 bits of signed hash
   0x0038  36
   0x0039     20                    salt length
   0x003a        76 49 5f 50 21 88  salt
   0x0040  90 f7 f5 e2 ee 3c 18 22
   0x0048  51 4f 70 50 0f 55 1d 86
   0x0050  e5 c9 21 e4 04 e3 4a 53
   0x0058  fb ac
   0x005a        27 d0 6f b8 0a a8  Ed25519 signature
   0x0060  fc 5b cb 16 e1 96 31 b2
   0x0068  80 74 0f 9e a6 ae d5 e0
   0x0070  73 ad 00 f9 41 5a 65 3c
   0x0078  40 e7 7a 6a e7 7e 69 2b
   0x0080  a7 1d 06 9a 10 9f a2 4c
   0x0088  58 cf d8 e3 16 d0 a0 6b
   0x0090  34 ad 9a cb 8e 5c 5f 52
   0x0098  15 01

   The signature is made over the following data:

   0x0000  76 49 5f 50 21 88 90 f7
   0x0008  f5 e2 ee 3c 18 22 51 4f
   0x0010  70 50 0f 55 1d 86 e5 c9
   0x0018  21 e4 04 e3 4a 53 fb ac
   0x0020  57 68 61 74 20 77 65 20
   0x0028  6e 65 65 64 20 66 72 6f
   0x0030  6d 20 74 68 65 20 67 72
   0x0038  6f 63 65 72 79 20 73 74
   0x0040  6f 72 65 3a 0d 0a 0d 0a
   0x0048  2d 20 74 6f 66 75 0d 0a
   0x0050  2d 20 76 65 67 65 74 61
   0x0058  62 6c 65 73 0d 0a 2d 20
   0x0060  6e 6f 6f 64 6c 65 73 0d
   0x0068  0a 06 01 1b 0a 00 00 00
   0x0070  29 05 82 63 98 a3 63 22
   0x0078  21 06 cb 18 6c 4f 06 09
   0x0080  a6 97 e4 d5 2d fa 6c 72
   0x0088  2b 0c 1f 1e 27 c1 8a 56
   0x0090  70 8f 65 25 ec 27 ba d9
   0x0098  ac c9 06 ff 00 00 00 31

   The same data, broken out by octet and semantics, is:

   0x0000  76 49 5f 50 21 88 90 f7  salt
   0x0008  f5 e2 ee 3c 18 22 51 4f
   0x0010  70 50 0f 55 1d 86 e5 c9
   0x0018  21 e4 04 e3 4a 53 fb ac
         [ message begins ]
   0x0020  57 68 61 74 20 77 65 20  canonicalized message
   0x0028  6e 65 65 64 20 66 72 6f
   0x0030  6d 20 74 68 65 20 67 72
   0x0038  6f 63 65 72 79 20 73 74
   0x0040  6f 72 65 3a 0d 0a 0d 0a
   0x0048  2d 20 74 6f 66 75 0d 0a
   0x0050  2d 20 76 65 67 65 74 61
   0x0058  62 6c 65 73 0d 0a 2d 20
   0x0060  6e 6f 6f 64 6c 65 73 0d
   0x0068  0a
         [ trailer begins ]
   0x0069     06                    sig version 6
   0x006a        01                 sig type: Canonical Text
   0x006b           1b              pubkey algorithm: Ed25519
   0x006c              0a           hash algorithm: SHA2-512
   0x006d                 00 00 00  hashed subpackets length
   0x0070  29
   0x0071     05                    subpacket length
   0x0072        82                 critical subpkt: Sig Creation Time
   0x0073           63 98 a3 63       (2022-12-13T16:08:03Z)
   0x0077                       22  subpkt length
   0x0078  21                       subpkt type: Issuer Fingerprint
   0x0079     06                    Fingerprint version 6
   0x007a        cb 18 6c 4f 06 09  Fingerprint
   0x0080  a6 97 e4 d5 2d fa 6c 72
   0x0088  2b 0c 1f 1e 27 c1 8a 56
   0x0090  70 8f 65 25 ec 27 ba d9
   0x0098  ac c9
   0x009a        06                 sig version 6
   0x009b           ff              sentinel octet
   0x009c              00 00 00 31  trailer length

   The calculated SHA2-512 hash digest over this data is:

   69365bf44a97af1f0844f1f6ab83fdf6b36f26692efaa621a8aac91c4e29ea07
   e894cabc6e2f20eedfce6c03b89141a2cc7cbe245e6e7a5654addbec5000b89b

A.7.  Sample Inline-Signed Message

   This is the same message and signature as in Appendix A.6 but as an
   inline-signed message.  The hashed data is exactly the same, and all
   intermediate values and annotated hex dumps are also applicable.

   -----BEGIN PGP MESSAGE-----

   xEYGAQobIHZJX1AhiJD39eLuPBgiUU9wUA9VHYblySHkBONKU/usyxhsTwYJppfk
   1S36bHIrDB8eJ8GKVnCPZSXsJ7rZrMkBy0p1AAAAAABXaGF0IHdlIG5lZWQgZnJv
   bSB0aGUgZ3JvY2VyeSBzdG9yZToKCi0gdG9mdQotIHZlZ2V0YWJsZXMKLSBub29k
   bGVzCsKYBgEbCgAAACkFgmOYo2MiIQbLGGxPBgmml+TVLfpscisMHx4nwYpWcI9l
   JewnutmsyQAAAABpNiB2SV9QIYiQ9/Xi7jwYIlFPcFAPVR2G5ckh5ATjSlP7rCfQ
   b7gKqPxbyxbhljGygHQPnqau1eBzrQD5QVplPEDnemrnfmkrpx0GmhCfokxYz9jj
   FtCgazStmsuOXF9SFQE=
   -----END PGP MESSAGE-----

A.8.  Sample X25519-AEAD-OCB Encryption and Decryption

   This example encrypts the cleartext string Hello, world! for the
   sample cert (see Appendix A.3), using AES-128 with AEAD-OCB
   encryption.

A.8.1.  Sample Version 6 Public Key Encrypted Session Key Packet

   This packet contains the following series of octets:

   0x0000  c1 5d 06 21 06 12 c8 3f
   0x0008  1e 70 6f 63 08 fe 15 1a
   0x0010  41 77 43 a1 f0 33 79 0e
   0x0018  93 e9 97 84 88 d1 db 37
   0x0020  8d a9 93 08 85 19 87 cf
   0x0028  18 d5 f1 b5 3f 81 7c ce
   0x0030  5a 00 4c f3 93 cc 89 58
   0x0038  bd dc 06 5f 25 f8 4a f5
   0x0040  09 b1 7d d3 67 64 18 de
   0x0048  a3 55 43 79 56 61 79 01
   0x0050  e0 69 57 fb ca 8a 6a 47
   0x0058  a5 b5 15 3e 8d 3a b7

   The same data, broken out by octet and semantics, is:

   0x0000  c1                       packet type: PKESK
   0x0001     5d                    packet length
   0x0002        06                 v6 PKESK
   0x0003           21              length of fingerprint
   0x0004              06           Key version 6
   0x0005                 12 c8 3f  Key fingerprint
   0x0008  1e 70 6f 63 08 fe 15 1a
   0x0010  41 77 43 a1 f0 33 79 0e
   0x0018  93 e9 97 84 88 d1 db 37
   0x0020  8d a9 93 08 85
   0x0025                 19        algorithm: X25519
   0x0026                    87 cf  Ephemeral key
   0x0028  18 d5 f1 b5 3f 81 7c ce
   0x0030  5a 00 4c f3 93 cc 89 58
   0x0038  bd dc 06 5f 25 f8 4a f5
   0x0040  09 b1 7d d3 67 64
   0x0046                    18     ESK length
   0x0047                       de  ESK
   0x0048  a3 55 43 79 56 61 79 01
   0x0050  e0 69 57 fb ca 8a 6a 47
   0x0058  a5 b5 15 3e 8d 3a b7

A.8.2.  X25519 Encryption/Decryption of the Session Key

   Ephemeral key:

     87 cf 18 d5 f1 b5 3f 81 7c ce 5a 00 4c f3 93 cc
     89 58 bd dc 06 5f 25 f8 4a f5 09 b1 7d d3 67 64

   This ephemeral key is derived from the following ephemeral secret key
   material, which is never placed on the wire:

     af 1e 43 c0 d1 23 ef e8 93 a7 d4 d3 90 f3 a7 61
     e3 fa c3 3d fc 7f 3e da a8 30 c9 01 13 52 c7 79

   Public key from the target certificate (see Appendix A.3):

     86 93 24 83 67 f9 e5 01 5d b9 22 f8 f4 80 95 dd
     a7 84 98 7f 2d 59 85 b1 2f ba d1 6c af 5e 44 35

   The corresponding long-lived X25519 private key material (see
   Appendix A.4):

     4d 60 0a 4f 79 4d 44 77 5c 57 a2 6e 0f ee fe d5
     58 e9 af ff d6 ad 0d 58 2d 57 fb 2b a2 dc ed b8

   Shared point:

     67 e3 0e 69 cd c7 ba b2 a2 68 0d 78 ac a4 6a 2f
     8b 6e 2a e4 4d 39 8b dc 6f 92 c5 ad 4a 49 25 14

   HKDF output:

     f6 6d ad cf f6 45 92 23 9b 25 45 39 b6 4f f6 07

   Decrypted session key:

     dd 70 8f 6f a1 ed 65 11 4d 68 d2 34 3e 7c 2f 1d

A.8.3.  Sample v2 SEIPD Packet

   This packet contains the following series of octets:

   0x0000  d2 69 02 07 02 06 61 64
   0x0008  16 53 5b e0 b0 71 6d 60
   0x0010  e0 52 a5 6c 4c 40 7f 9e
   0x0018  b3 6b 0e fa fe 9a d0 a0
   0x0020  df 9b 03 3c 69 a2 1b a9
   0x0028  eb d2 c0 ec 95 bf 56 9d
   0x0030  25 c9 99 ee 4a 3d e1 70
   0x0038  58 f4 0d fa 8b 4c 68 2b
   0x0040  e3 fb bb d7 b2 7e b0 f5
   0x0048  9b b5 00 5f 80 c7 c6 f4
   0x0050  03 88 c3 0a d4 06 ab 05
   0x0058  13 dc d6 f9 fd 73 76 56
   0x0060  28 6e 11 77 d0 0f 88 8a
   0x0068  db 31 c4

   The same data, broken out by octet and semantics, is:

   0x0000  d2                       packet type: SEIPD
   0x0001     69                    packet length
   0x0002        02                 v2 SEIPD
   0x0003           07              cipher: AES128
   0x0004              02           AEAD mode: OCB
   0x0005                 06        chunk size (2^12 octets)
   0x0006                    61 64  salt
   0x0008  16 53 5b e0 b0 71 6d 60
   0x0010  e0 52 a5 6c 4c 40 7f 9e
   0x0018  b3 6b 0e fa fe 9a d0 a0
   0x0020  df 9b 03 3c 69 a2
   0x0026                    1b a9  chunk #0 encrypted data
   0x0028  eb d2 c0 ec 95 bf 56 9d
   0x0030  25 c9 99 ee 4a 3d e1 70
   0x0038  58 f4 0d fa 8b 4c 68 2b
   0x0040  e3 fb bb d7 b2 7e b0 f5
   0x0048  9b b5 00
   0x004b           5f 80 c7 c6 f4  chunk #0 AEAD tag
   0x0050  03 88 c3 0a d4 06 ab 05
   0x0058  13 dc d6
   0x005b           f9 fd 73 76 56  final AEAD tag (#1)
   S0x0060  28 6e 11 77 d0 0f 88 8a
   0x0068  db 31 c4

A.8.4.  Decryption of Data

   Starting AEAD-OCB decryption of data, using the session key.

   HKDF info:

     d2 02 07 02 06

   HKDF output:

     45 12 f7 14 9d 86 33 41 52 7c 65 67 d5 bf fc 42
     5f af 32 50 21 2f f9

   Message key:

     45 12 f7 14 9d 86 33 41 52 7c 65 67 d5 bf fc 42

   Initialization vector:

     5f af 32 50 21 2f f9

   Chunk #0:

   Nonce:

     5f af 32 50 21 2f f9 00 00 00 00 00 00 00 00

   Additional authenticated data:

     d2 02 07 02 06

   Encrypted data chunk:

     1b a9 eb d2 c0 ec 95 bf 56 9d 25 c9 99 ee 4a 3d
     e1 70 58 f4 0d fa 8b 4c 68 2b e3 fb bb d7 b2 7e
     b0 f5 9b b5 00 5f 80 c7 c6 f4 03 88 c3 0a d4 06
     ab 05 13 dc d6

   Decrypted chunk #0.

   Literal Data packet with the string contents Hello, world!:

     cb 13 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77
     6f 72 6c 64 21

   Padding packet:

     d5 0e c5 a2 93 07 29 91 62 81 47 d7 2c 8f 86 b7

   Authenticating final tag:

   Final nonce:

     5f af 32 50 21 2f f9 00 00 00 00 00 00 00 01

   Final additional authenticated data:

     d2 02 07 02 06 00 00 00 00 00 00 00 25

A.8.5.  Complete X25519-AEAD-OCB Encrypted Packet Sequence

   -----BEGIN PGP MESSAGE-----

   wV0GIQYSyD8ecG9jCP4VGkF3Q6HwM3kOk+mXhIjR2zeNqZMIhRmHzxjV8bU/gXzO
   WgBM85PMiVi93AZfJfhK9QmxfdNnZBjeo1VDeVZheQHgaVf7yopqR6W1FT6NOrfS
   aQIHAgZhZBZTW+CwcW1g4FKlbExAf56zaw76/prQoN+bAzxpohup69LA7JW/Vp0l
   yZnuSj3hcFj0DfqLTGgr4/u717J+sPWbtQBfgMfG9AOIwwrUBqsFE9zW+f1zdlYo
   bhF30A+IitsxxA==
   -----END PGP MESSAGE-----

A.9.  Sample AEAD-EAX Encryption and Decryption

   This example encrypts the cleartext string Hello, world! with the
   passphrase password, using AES-128 with AEAD-EAX encryption.

A.9.1.  Sample Version 6 Symmetric Key Encrypted Session Key Packet

   This packet contains the following series of octets:

   0x0000  c3 40 06 1e 07 01 0b 03
   0x0008  08 a5 ae 57 9d 1f c5 d8
   0x0010  2b ff 69 22 4f 91 99 93
   0x0018  b3 50 6f a3 b5 9a 6a 73
   0x0020  cf f8 c5 ef c5 f4 1c 57
   0x0028  fb 54 e1 c2 26 81 5d 78
   0x0030  28 f5 f9 2c 45 4e b6 5e
   0x0038  be 00 ab 59 86 c6 8e 6e
   0x0040  7c 55

   The same data, broken out by octet and semantics, is:

   0x0000  c3                       packet type: SKESK
   0x0001     40                    packet length
   0x0002        06                 v6 SKESK
   0x0003           1e              length through end of AEAD nonce
   0x0004              07           cipher: AES128
   0x0005                 01        AEAD mode: EAX
   0x0006                    0b     length of S2K
   0x0007                       03  S2K type: iterated+salted
   0x0008  08                       S2K hash: SHA2-256
   0x0009     a5 ae 57 9d 1f c5 d8  S2K salt
   0x0010  2b
   0x0011     ff                    S2K iterations (65011712 octets)
   0x0012        69 22 4f 91 99 93  AEAD nonce
   0x0018  b3 50 6f a3 b5 9a 6a 73
   0x0020  cf f8
   0x0022        c5 ef c5 f4 1c 57  encrypted session key
   0x0028  fb 54 e1 c2 26 81 5d 78
   0x0030  28 f5
   0x0032        f9 2c 45 4e b6 5e  AEAD tag
   0x0038  be 00 ab 59 86 c6 8e 6e
   0x0040  7c 55

A.9.2.  Starting AEAD-EAX Decryption of the Session Key

   The derived key is:

     15 49 67 e5 90 aa 1f 92 3e 1c 0a c6 4c 88 f2 3d

   HKDF info:

     c3 06 07 01

   HKDF output:

     2f ce 33 1f 39 dd 95 5c c4 1e 95 d8 70 c7 21 39

   Authenticated Data:

     c3 06 07 01

   Nonce:

     69 22 4f 91 99 93 b3 50 6f a3 b5 9a 6a 73 cf f8

   Decrypted session key:

     38 81 ba fe 98 54 12 45 9b 86 c3 6f 98 cb 9a 5e

A.9.3.  Sample v2 SEIPD Packet

   This packet contains the following series of octets:

   0x0000  d2 69 02 07 01 06 9f f9
   0x0008  0e 3b 32 19 64 f3 a4 29
   0x0010  13 c8 dc c6 61 93 25 01
   0x0018  52 27 ef b7 ea ea a4 9f
   0x0020  04 c2 e6 74 17 5d 4a 3d
   0x0028  22 6e d6 af cb 9c a9 ac
   0x0030  12 2c 14 70 e1 1c 63 d4
   0x0038  c0 ab 24 1c 6a 93 8a d4
   0x0040  8b f9 9a 5a 99 b9 0b ba
   0x0048  83 25 de 61 04 75 40 25
   0x0050  8a b7 95 9a 95 ad 05 1d
   0x0058  da 96 eb 15 43 1d fe f5
   0x0060  f5 e2 25 5c a7 82 61 54
   0x0068  6e 33 9a

   The same data, broken out by octet and semantics, is:

   0x0000  d2                       packet type: SEIPD
   0x0001     69                    packet length
   0x0002        02                 v2 SEIPD
   0x0003           07              cipher: AES128
   0x0004              01           AEAD mode: EAX
   0x0005                 06        chunk size (2^12 octets)
   0x0005                    9f f9  salt
   0x0008  0e 3b 32 19 64 f3 a4 29
   0x0010  13 c8 dc c6 61 93 25 01
   0x0018  52 27 ef b7 ea ea a4 9f
   0x0020  04 c2 e6 74 17 5d
   0x0026                    4a 3d  chunk #0 encrypted data
   0x0028  22 6e d6 af cb 9c a9 ac
   0x0030  12 2c 14 70 e1 1c 63 d4
   0x0038  c0 ab 24 1c 6a 93 8a d4
   0x0040  8b f9 9a 5a 99 b9 0b ba
   0x0048  83 25 de
   0x004b           61 04 75 40 25  chunk #0 AEAD tag
   0x0050  8a b7 95 9a 95 ad 05 1d
   0x0058  da 96 eb
   0x005b           15 43 1d fe f5  final AEAD tag (#1)
   0x0060  f5 e2 25 5c a7 82 61 54
   0x0068  6e 33 9a

A.9.4.  Decryption of Data

   Starting AEAD-EAX decryption of data, using the session key.

   HKDF info:

     d2 02 07 01 06

   HKDF output:

     b5 04 22 ac 1c 26 be 9d dd 83 1d 5b bb 36 b6 4f
     78 b8 33 f2 e9 4a 60 c0

   Message key:

     b5 04 22 ac 1c 26 be 9d dd 83 1d 5b bb 36 b6 4f

   Initialization vector:

     78 b8 33 f2 e9 4a 60 c0

   Chunk #0:

   Nonce:

     78 b8 33 f2 e9 4a 60 c0 00 00 00 00 00 00 00 00

   Additional authenticated data:

     d2 02 07 01 06

   Decrypted chunk #0.

   Literal Data packet with the string contents Hello, world!:

     cb 13 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77
     6f 72 6c 64 21

   Padding packet:

     d5 0e ae 5b f0 cd 67 05 50 03 55 81 6c b0 c8 ff

   Authenticating final tag:

   Final nonce:

     78 b8 33 f2 e9 4a 60 c0 00 00 00 00 00 00 00 01

   Final additional authenticated data:

     d2 02 07 01 06 00 00 00 00 00 00 00 25

A.9.5.  Complete AEAD-EAX Encrypted Packet Sequence

   -----BEGIN PGP MESSAGE-----

   w0AGHgcBCwMIpa5XnR/F2Cv/aSJPkZmTs1Bvo7WaanPP+MXvxfQcV/tU4cImgV14
   KPX5LEVOtl6+AKtZhsaObnxV0mkCBwEGn/kOOzIZZPOkKRPI3MZhkyUBUifvt+rq
   pJ8EwuZ0F11KPSJu1q/LnKmsEiwUcOEcY9TAqyQcapOK1Iv5mlqZuQu6gyXeYQR1
   QCWKt5Wala0FHdqW6xVDHf719eIlXKeCYVRuM5o=
   -----END PGP MESSAGE-----

A.10.  Sample AEAD-OCB Encryption and Decryption

   This example encrypts the cleartext string Hello, world! with the
   passphrase password, using AES-128 with AEAD-OCB encryption.

A.10.1.  Sample Version 6 Symmetric Key Encrypted Session Key Packet

   This packet contains the following series of octets:

   0x0000  c3 3f 06 1d 07 02 0b 03
   0x0008  08 56 a2 98 d2 f5 e3 64
   0x0010  53 ff cf cc 5c 11 66 4e
   0x0018  db 9d b4 25 90 d7 dc 46
   0x0020  b0 72 41 b6 12 c3 81 2c
   0x0028  ff fb ea 00 f2 34 7b 25
   0x0030  64 11 23 f8 87 ae 60 d4
   0x0038  fd 61 4e 08 37 d8 19 d3
   0x0040  6c

   The same data, broken out by octet and semantics, is:

   0x0000  c3                       packet type: SKESK
   0x0001     3f                    packet length
   0x0002        06                 v6 SKESK
   0x0003           1d              length through end of AEAD nonce
   0x0004              07           cipher: AES128
   0x0005                 02        AEAD mode: OCB
   0x0006                    0b     length of S2K
   0x0007                       03  S2K type: iterated+salted
   0x0008  08                       S2K hash: SHA2-256
   0x0009     56 a2 98 d2 f5 e3 64  S2K salt
   0x0010  53
   0x0011    ff                     S2K iterations (65011712 octets)
   0x0012        cf cc 5c 11 66 4e  AEAD nonce
   0x0018  db 9d b4 25 90 d7 dc 46
   0x0020  b0
   0x0021     72 41 b6 12 c3 81 2c  encrypted session key
   0x0028  ff fb ea 00 f2 34 7b 25
   0x0030  64
   0x0031     11 23 f8 87 ae 60 d4  AEAD tag
   0x0038  fd 61 4e 08 37 d8 19 d3
   0x0040  6c

A.10.2.  Starting AEAD-OCB Decryption of the Session Key

   The derived key is:

     e8 0d e2 43 a3 62 d9 3b 9d c6 07 ed e9 6a 73 56

   HKDF info:

     c3 06 07 02

   HKDF output:

     38 a9 b3 45 b5 68 0b b6 1b b6 5d 73 ee c7 ec d9

   Authenticated Data:

     c3 06 07 02

   Nonce:

     cf cc 5c 11 66 4e db 9d b4 25 90 d7 dc 46 b0

   Decrypted session key:

     28 e7 9a b8 23 97 d3 c6 3d e2 4a c2 17 d7 b7 91

A.10.3.  Sample v2 SEIPD Packet

   This packet contains the following series of octets:

   0x0000  d2 69 02 07 02 06 20 a6
   0x0008  61 f7 31 fc 9a 30 32 b5
   0x0010  62 33 26 02 7e 3a 5d 8d
   0x0018  b5 74 8e be ff 0b 0c 59
   0x0020  10 d0 9e cd d6 41 ff 9f
   0x0028  d3 85 62 75 80 35 bc 49
   0x0030  75 4c e1 bf 3f ff a7 da
   0x0038  d0 a3 b8 10 4f 51 33 cf
   0x0040  42 a4 10 0a 83 ee f4 ca
   0x0048  1b 48 01 a8 84 6b f4 2b
   0x0050  cd a7 c8 ce 9d 65 e2 12
   0x0058  f3 01 cb cd 98 fd ca de
   0x0060  69 4a 87 7a d4 24 73 23
   0x0068  f6 e8 57

   The same data, broken out by octet and semantics, is:

   0x0000  d2                       packet type: SEIPD
   0x0001     69                    packet length
   0x0002        02                 v2 SEIPD
   0x0003           07              cipher: AES128
   0x0004              02           AEAD mode: OCB
   0x0005                 06        chunk size (2^12 octets)
   0x0006                    20 a6  salt
   0x0008  61 f7 31 fc 9a 30 32 b5
   0x0010  62 33 26 02 7e 3a 5d 8d
   0x0018  b5 74 8e be ff 0b 0c 59
   0x0020  10 d0 9e cd d6 41
   0x0026                    ff 9f  chunk #0 encrypted data
   0x0028  d3 85 62 75 80 35 bc 49
   0x0030  75 4c e1 bf 3f ff a7 da
   0x0038  d0 a3 b8 10 4f 51 33 cf
   0x0040  42 a4 10 0a 83 ee f4 ca
   0x0048  1b 48 01
   0x004b           a8 84 6b f4 2b  chunk #0 authentication tag
   0x0050  cd a7 c8 ce 9d 65 e2 12
   0x0058  f3 01 cb
   0x005b           cd 98 fd ca de  final AEAD tag (#1)
   0x0060  69 4a 87 7a d4 24 73 23
   0x0068  f6 e8 57

A.10.4.  Decryption of Data

   Starting AEAD-OCB decryption of data, using the session key.

   HKDF info:

     d2 02 07 02 06

   HKDF output:

     71 66 2a 11 ee 5b 4e 08 14 4e 6d e8 83 a0 09 99
     eb de 12 bb 57 0d cf

   Message key:

     71 66 2a 11 ee 5b 4e 08 14 4e 6d e8 83 a0 09 99

   Initialization vector:

     eb de 12 bb 57 0d cf

   Chunk #0:

   Nonce:

     eb de 12 bb 57 0d cf 00 00 00 00 00 00 00 00

   Additional authenticated data:

     d2 02 07 02 06

   Decrypted chunk #0.

   Literal Data packet with the string contents Hello, world!:

     cb 13 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77
     6f 72 6c 64 21

   Padding packet:

     d5 0e ae 6a a1 64 9b 56 aa 83 5b 26 13 90 2b d2

   Authenticating final tag:

   Final nonce:

     eb de 12 bb 57 0d cf 00 00 00 00 00 00 00 01

   Final additional authenticated data:

     d2 02 07 02 06 00 00 00 00 00 00 00 25

A.10.5.  Complete AEAD-OCB Encrypted Packet Sequence

   -----BEGIN PGP MESSAGE-----

   wz8GHQcCCwMIVqKY0vXjZFP/z8xcEWZO2520JZDX3EawckG2EsOBLP/76gDyNHsl
   ZBEj+IeuYNT9YU4IN9gZ02zSaQIHAgYgpmH3MfyaMDK1YjMmAn46XY21dI6+/wsM
   WRDQns3WQf+f04VidYA1vEl1TOG/P/+n2tCjuBBPUTPPQqQQCoPu9MobSAGohGv0
   K82nyM6dZeIS8wHLzZj9yt5pSod61CRzI/boVw==
   -----END PGP MESSAGE-----

A.11.  Sample AEAD-GCM Encryption and Decryption

   This example encrypts the cleartext string Hello, world! with the
   passphrase password, using AES-128 with AEAD-GCM encryption.

A.11.1.  Sample Version 6 Symmetric Key Encrypted Session Key Packet

   This packet contains the following series of octets:

   0x0000  c3 3c 06 1a 07 03 0b 03
   0x0008  08 e9 d3 97 85 b2 07 00
   0x0010  08 ff b4 2e 7c 48 3e f4
   0x0018  88 44 57 cb 37 26 b9 b3
   0x0020  db 9f f7 76 e5 f4 d9 a4
   0x0028  09 52 e2 44 72 98 85 1a
   0x0030  bf ff 75 26 df 2d d5 54
   0x0038  41 75 79 a7 79 9f

   The same data, broken out by octet and semantics, is:

   0x0000  c3                       packet type: SKESK
   0x0001     3c                    packet length
   0x0002        06                 v6 SKESK
   0x0003           1a              length through end of AEAD nonce
   0x0004              07           cipher: AES128
   0x0005                 03        AEAD mode: GCM
   0x0006                    0b     length of S2K
   0x0007                       03  S2K type: iterated+salted
   0x0008  08                       S2K hash: SHA2-256
   0x0009     e9 d3 97 85 b2 07 00  S2K salt
   0x0010  08
   0x0011     ff                    S2K iterations (65011712 octets)
   0x0012        b4 2e 7c 48 3e f4  AEAD nonce
   0x0018  88 44 57 cb 37 26
   0x001e                    b9 b3  encrypted session key
   0x0020  db 9f f7 76 e5 f4 d9 a4
   0x0028  09 52 e2 44 72 98
   0x002e                     85 1a  AEAD tag
   0x0030  bf ff 75 26 df 2d d5 54
   0x0038  41 75 79 a7 79 9f

A.11.2.  Starting AEAD-GCM Decryption of the Session Key

   The derived key is:

     25 02 81 71 5b ba 78 28 ef 71 ef 64 c4 78 47 53

   HKDF info:

     c3 06 07 03

   HKDF output:

     7a 6f 9a b7 f9 9f 7e f8 db ef 84 1c 65 08 00 f5

   Authenticated Data:

     c3 06 07 03

   Nonce:

     b4 2e 7c 48 3e f4 88 44 57 cb 37 26

   Decrypted session key:

     19 36 fc 85 68 98 02 74 bb 90 0d 83 19 36 0c 77

A.11.3.  Sample v2 SEIPD Packet

   This packet contains the following series of octets, is:

   0x0000  d2 69 02 07 03 06 fc b9
   0x0008  44 90 bc b9 8b bd c9 d1
   0x0010  06 c6 09 02 66 94 0f 72
   0x0018  e8 9e dc 21 b5 59 6b 15
   0x0020  76 b1 01 ed 0f 9f fc 6f
   0x0028  c6 d6 5b bf d2 4d cd 07
   0x0030  90 96 6e 6d 1e 85 a3 00
   0x0038  53 78 4c b1 d8 b6 a0 69
   0x0040  9e f1 21 55 a7 b2 ad 62
   0x0048  58 53 1b 57 65 1f d7 77
   0x0050  79 12 fa 95 e3 5d 9b 40
   0x0058  21 6f 69 a4 c2 48 db 28
   0x0060  ff 43 31 f1 63 29 07 39
   0x0068  9e 6f f9

   The same data, broken out by octet and semantics, is:

   0x0000  d2                       packet type: SEIPD
   0x0001     69                    packet length
   0x0002        02                 v2 SEIPD
   0x0003           07              cipher: AES128
   0x0004              03           AEAD mode: GCM
   0x0005                 06        chunk size (2^12 octets)
   0x0006                    fc b9  salt
   0x0008  44 90 bc b9 8b bd c9 d1
   0x0010  06 c6 09 02 66 94 0f 72
   0x0018  e8 9e dc 21 b5 59 6b 15
   0x0020  76 b1 01 ed 0f 9f
   0x0026                    fc 6f  chunk #0 encrypted data
   0x0028  c6 d6 5b bf d2 4d cd 07
   0x0030  90 96 6e 6d 1e 85 a3 00
   0x0038  53 78 4c b1 d8 b6 a0 69
   0x0040  9e f1 21 55 a7 b2 ad 62
   0x0048  58 53 1b
   0x004b           57 65 1f d7 77  chunk #0 authentication tag
   0x0050  79 12 fa 95 e3 5d 9b 40
   0x0058  21 6f 69
   0x005b           a4 c2 48 db 28  final AEAD tag (#1)
   0x0060  ff 43 31 f1 63 29 07 39
   0x0068  9e 6f f9

A.11.4.  Decryption of Data

   Starting AEAD-GCM decryption of data, using the session key.

   HKDF info:

     d2 02 07 03 06

   HKDF output:

     ea 14 38 80 3c b8 a4 77 40 ce 9b 54 c3 38 77 8d
     4d 2b dc 2b

   Message key:

     ea 14 38 80 3c b8 a4 77 40 ce 9b 54 c3 38 77 8d

   Initialization vector:

     4d 2b dc 2b

   Chunk #0:

   Nonce:

     4d 2b dc 2b 00 00 00 00 00 00 00 00

   Additional authenticated data:

     d2 02 07 03 06

   Decrypted chunk #0.

   Literal Data packet with the string contents Hello, world!:

     cb 13 62 00 00 00 00 00 48 65 6c 6c 6f 2c 20 77
     6f 72 6c 64 21

   Padding packet:

     d5 0e 1c e2 26 9a 9e dd ef 81 03 21 72 b7 ed 7c

   Authenticating final tag:

   Final nonce:

     4d 2b dc 2b 00 00 00 00 00 00 00 01

   Final additional authenticated data:

     d2 02 07 03 06 00 00 00 00 00 00 00 25

A.11.5.  Complete AEAD-GCM Encrypted Packet Sequence

   -----BEGIN PGP MESSAGE-----

   wzwGGgcDCwMI6dOXhbIHAAj/tC58SD70iERXyzcmubPbn/d25fTZpAlS4kRymIUa
   v/91Jt8t1VRBdXmneZ/SaQIHAwb8uUSQvLmLvcnRBsYJAmaUD3LontwhtVlrFXax
   Ae0Pn/xvxtZbv9JNzQeQlm5tHoWjAFN4TLHYtqBpnvEhVaeyrWJYUxtXZR/Xd3kS
   +pXjXZtAIW9ppMJI2yj/QzHxYykHOZ5v+Q==
   -----END PGP MESSAGE-----

A.12.  Sample Messages Encrypted Using Argon2

   These messages are the literal data Hello, world! encrypted using v1
   SEIPD, with Argon2 and the passphrase "password", using different
   session key sizes.  In each example, the choice of symmetric cipher
   is the same in both the v4 SKESK packet and v1 SEIPD packet.  In all
   cases, the Argon2 parameters are t = 1, p = 4, and m = 21.

A.12.1.  V4 SKESK Using Argon2 with AES-128

   -----BEGIN PGP MESSAGE-----
   Comment: Encrypted using AES with 128-bit key
   Comment: Session key: 01FE16BBACFD1E7B78EF3B865187374F

   wycEBwScUvg8J/leUNU1RA7N/zE2AQQVnlL8rSLPP5VlQsunlO+ECxHSPgGYGKY+
   YJz4u6F+DDlDBOr5NRQXt/KJIf4m4mOlKyC/uqLbpnLJZMnTq3o79GxBTdIdOzhH
   XfA3pqV4mTzF
   -----END PGP MESSAGE-----

A.12.2.  V4 SKESK Using Argon2 with AES-192

   -----BEGIN PGP MESSAGE-----
   Comment: Encrypted using AES with 192-bit key
   Comment: Session key: 27006DAE68E509022CE45A14E569E91001C2955...
   Comment: Session key: ...AF8DFE194

   wy8ECAThTKxHFTRZGKli3KNH4UP4AQQVhzLJ2va3FG8/pmpIPd/H/mdoVS5VBLLw
   F9I+AdJ1Sw56PRYiKZjCvHg+2bnq02s33AJJoyBexBI4QKATFRkyez2gldJldRys
   LVg77Mwwfgl2n/d572WciAM=
   -----END PGP MESSAGE-----

A.12.3.  V4 SKESK Using Argon2 with AES-256

   -----BEGIN PGP MESSAGE-----
   Comment: Encrypted using AES with 256-bit key
   Comment: Session key: BBEDA55B9AAE63DAC45D4F49D89DACF4AF37FEF...
   Comment: Session key: ...C13BAB2F1F8E18FB74580D8B0

   wzcECQS4eJUgIG/3mcaILEJFpmJ8AQQVnZ9l7KtagdClm9UaQ/Z6M/5roklSGpGu
   623YmaXezGj80j4B+Ku1sgTdJo87X1Wrup7l0wJypZls21Uwd67m9koF60eefH/K
   95D1usliXOEm8ayQJQmZrjf6K6v9PWwqMQ==
   -----END PGP MESSAGE-----

Appendix B.  Upgrade Guidance (Adapting Implementations from RFCs 4880
             and 6637)

   This subsection offers a concise, non-normative summary of the
   substantial additions to and departures from [RFC4880] and [RFC6637].
   It is intended to help implementers who are augmenting an existing
   implementation from those specifications to comply with this
   specification.  Cryptographic algorithms marked with "MTI" are
   mandatory to implement.

   *  Public Key Signing Algorithms:

      -  Ed25519 (Sections 5.5.5.9 and 5.2.3.4) -- MTI

      -  Ed448 (Sections 5.5.5.10 and 5.2.3.5)

      -  EdDSALegacy with Ed25519Legacy (Sections 5.5.5.5 and 5.2.3.3)

      -  ECDSA with Brainpool curves (Section 9.2)

   *  Public Key Encryption Algorithms:

      -  X25519 (Sections 5.5.5.7 and 5.1.6) -- MTI

      -  X448 (Sections 5.5.5.8 and 5.1.7)

      -  ECDH with Curve25519Legacy (Section 9.2)

      -  ECDH with Brainpool curves (Section 9.2)

   *  AEAD Encryption:

      -  V2 SEIPD (Section 5.13.2)

      -  AEAD modes:

         o  OCB mode (Section 5.13.4) -- MTI

         o  EAX mode (Section 5.13.3)

         o  GCM mode (Section 5.13.5)

      -  V6 PKESK (Section 5.1.2)

      -  V6 SKESK (Section 5.3.2)

      -  Features signature subpacket: add flag for v2 SEIPD
         (Section 5.2.3.32)

      -  Signature Subpacket: Preferred AEAD Ciphersuites
         (Section 5.2.3.15)

      -  Secret key encryption: AEAD "S2K usage octet" (Sections 3.7.2
         and 5.5.3)

   *  Version 6 Keys and Signatures:

      -  Version 6 Public Keys (Section 5.5.2.3)

      -  Version 6 Fingerprint and Key ID (Section 5.5.4.3)

      -  Version 6 Secret Keys (Section 5.5.3)

      -  Version 6 Signatures (Section 5.2.3)

      -  Version 6 One-Pass Signatures (Section 5.4)

   *  Certificate (Transferable Public Key) Structure:

      -  Preferences subpackets in Direct Key signatures
         (Section 5.2.3.10)

      -  Self-verifying revocation certificate (Section 10.1.2)

      -  User ID is explicitly optional (Section 10.1.1)

   *  S2K: Argon2 (Section 3.7.1.4)

   *  Subpacket: Intended Recipient Fingerprint (Section 5.2.3.36)

   *  Digest Algorithms: SHA3-256 and SHA3-512 (Section 9.5)

   *  Packet: Padding (Section 5.14)

   *  Message Structure: Packet Criticality (Section 4.3)

   *  Deprecations:

      -  Public Key Algorithms:

         o  Avoid RSA weak keys (Section 12.4)

         o  Avoid DSA (Section 12.5)

         o  Avoid ElGamal (Sections 12.6 and 5.1.4)

         o  For Version 6 Keys: Avoid EdDSA25519Legacy and
            Curve25519Legacy (Section 9.2)

      -  Digest Algorithms:

         o  Avoid MD5, SHA1, and RIPEMD160 (Section 9.5)

      -  Symmetric Key Algorithms:

         o  Avoid IDEA, TripleDES, and CAST5 (Section 9.3)

      -  S2K Specifier:

         o  Avoid Simple S2K (Section 3.7.1.1)

      -  Secret Key Protections (a.k.a. S2K Usage):

         o  Avoid MalleableCFB (Section 3.7.2.1)

      -  Packet Types:

         o  Avoid Symmetrically Encrypted Data (Sections 5.7 and 13.7)

      -  Literal Data Packet Metadata:

         o  Avoid Filename and Date fields (Section 5.9)

         o  Avoid Special _CONSOLE "filename" (Section 5.9.1)

      -  Packet Versions:

         o  Avoid Version 3 Public Keys (Section 5.5.2.1)

         o  Avoid Version 3 Signatures (Section 5.2)

      -  Signature Types:

         o  Avoid Reserved Signature Type ID 0xFF (Sections 5.2.1.16 and
            5.2.4.1)

      -  Signature Subpackets:

         o  For Version 6 Signatures: Avoid Issuer Key ID
            (Section 5.2.3.12)

         o  Avoid Revocation Key (Section 5.2.3.23)

      -  ASCII Armor:

         o  Ignore; do not emit CRC (Section 6.1)

         o  Do not emit "Version" Armor Header (Section 6.2.2.1)

      -  Cleartext Signature Framework:

         o  Ignore; avoid emitting unnecessary Hash: headers
            (Section 6.2.2.3)

         o  Reject Cleartext Signature Framework signatures with invalid
            Hash: headers (Section 6.2.2.3) or any other Armor Header
            (Section 7.1)

B.1.  Terminology Changes

   Note that some of the words used in previous versions of the OpenPGP
   specification have been improved in this document.

   In previous versions, the following terms were used:

   *  "Radix-64" was used to refer to OpenPGP's ASCII Armor base64
      encoding (Section 6).

   *  "Old packet format" was used to refer to the Legacy packet format
      (Section 4.2.2) predating [RFC2440].

   *  "New packet format" was used to refer to the OpenPGP packet format
      (Section 4.2.1) introduced in [RFC2440].

   *  "Certificate" was used ambiguously to mean multiple things.  In
      this document, it means "Transferable Public Key" exclusively.

   *  "Preferred Symmetric Algorithms" was the old name for the
      "Preferred Symmetric Ciphers for v1 SEIPD" subpacket
      (Section 5.2.3.14).

   *  "Modification Detection Code" or "MDC" was originally described as
      a distinct packet (Packet Type ID 19), and its corresponding flag
      in the Features signature subpacket (Section 5.2.3.32) was known
      as "Modification Detection".  It is now described as an intrinsic
      part of v1 SEIPD (Section 5.13.1), and the same corresponding flag
      is known as "Version 1 Symmetrically Encrypted and Integrity
      Protected Data packet".

   *  "Packet Tag" was used to refer to the Packet Type ID (Section 5)
      or sometimes to the encoded Packet Type ID (Section 4.2).

Appendix C.  Errata Addressed by This Document

   The following verified errata have been incorporated or are otherwise
   resolved by this document:

   *  [Errata-2199] - S2K hash/cipher octet correction

   *  [Errata-2200] - No implicit use of IDEA correction

   *  [Errata-2206] - PKESK acronym expansion

   *  [Errata-2208] - Signature key owner clarification

   *  [Errata-2214] - Signature hashing clarification

   *  [Errata-2216] - Self-signature applies to user ID correction

   *  [Errata-2219] - Session key encryption storage clarification

   *  [Errata-2222] - Simple hash MUST/MAY clarification

   *  [Errata-2226] - Native line endings SHOULD clarification

   *  [Errata-2234] - Radix-64/base64 clarification

   *  [Errata-2235] - ASCII/UTF-8 collation sequence clarification

   *  [Errata-2236] - Packet Composition is a sequence clarification

   *  [Errata-2238] - Subkey packets come after all User ID packets
      clarification

   *  [Errata-2240] - Subkey removal clarification

   *  [Errata-2242] - mL/emLen variable correction

   *  [Errata-2243] - CFB mode initialization vector (IV) clarification

   *  [Errata-2270] - SHA-224 octet sequence correction

   *  [Errata-2271] - Radix-64 correction

   *  [Errata-3298] - Key Revocation signatures correction

   *  [Errata-5491] - C code fix for CRC24_POLY define

   *  [Errata-7545] - Armor Header colon hex fix

   *  [Errata-7889] - Signature/certification correction

Acknowledgements

   Thanks to the OpenPGP Design Team for working on this document and
   preparing it for working group consumption: Stephen Farrell, Daniel
   Kahn Gillmor, Daniel Huigens, Jeffrey Lau, Yutaka Niibe, Justus
   Winter, and Paul Wouters.

   Thanks to Werner Koch for the early work on rfc4880bis and Andrey
   Jivsov for the work on [RFC6637].

   This document also draws on much previous work from a number of other
   authors including Derek Atkins, Charles Breed, Dave Del Torto, Marc
   Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Ben Laurie,
   Raph Levien, Colin Plumb, Will Price, Daphne Shaw, William Stallings,
   Mark Weaver, and Philip R. Zimmermann.

Authors' Addresses

   Paul Wouters (editor)
   Aiven
   Email: paul.wouters@aiven.io


   Daniel Huigens
   Proton AG
   Email: d.huigens@protonmail.com


   Justus Winter
   Sequoia PGP
   Email: justus@sequoia-pgp.org


   Yutaka Niibe
   FSIJ
   Email: gniibe@fsij.org