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authorThomas Voss <mail@thomasvoss.com> 2024-11-27 20:54:24 +0100
committerThomas Voss <mail@thomasvoss.com> 2024-11-27 20:54:24 +0100
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+
+Network Working Group J. Callas
+Request for Comments: 2440 Network Associates
+Category: Standards Track L. Donnerhacke
+ IN-Root-CA Individual Network e.V.
+ H. Finney
+ Network Associates
+ R. Thayer
+ EIS Corporation
+ November 1998
+
+
+ OpenPGP Message Format
+
+Status of this Memo
+
+ This document specifies an Internet standards track protocol for the
+ Internet community, and requests discussion and suggestions for
+ improvements. Please refer to the current edition of the "Internet
+ Official Protocol Standards" (STD 1) for the standardization state
+ and status of this protocol. Distribution of this memo is unlimited.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (1998). All Rights Reserved.
+
+IESG Note
+
+ This document defines many tag values, yet it doesn't describe a
+ mechanism for adding new tags (for new features). Traditionally the
+ Internet Assigned Numbers Authority (IANA) handles the allocation of
+ new values for future expansion and RFCs usually define the procedure
+ to be used by the IANA. However, there are subtle (and not so
+ subtle) interactions that may occur in this protocol between new
+ features and existing features which result in a significant
+ reduction in over all security. Therefore, this document does not
+ define an extension procedure. Instead requests to define new tag
+ values (say for new encryption algorithms for example) should be
+ forwarded to the IESG Security Area Directors for consideration or
+ forwarding to the appropriate IETF Working Group for consideration.
+
+Abstract
+
+ 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,
+
+
+
+Callas, et. al. Standards Track [Page 1]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ however, discuss implementation issues necessary to avoid security
+ flaws.
+
+ Open-PGP software uses a combination of strong public-key and
+ symmetric cryptography to provide security services for electronic
+ communications and data storage. These services include
+ confidentiality, key management, authentication, and digital
+ signatures. This document specifies the message formats used in
+ OpenPGP.
+
+Table of Contents
+
+ Status of this Memo 1
+ IESG Note 1
+ Abstract 1
+ Table of Contents 2
+ 1. Introduction 4
+ 1.1. Terms 5
+ 2. General functions 5
+ 2.1. Confidentiality via Encryption 5
+ 2.2. Authentication via Digital signature 6
+ 2.3. Compression 7
+ 2.4. Conversion to Radix-64 7
+ 2.5. Signature-Only Applications 7
+ 3. Data Element Formats 7
+ 3.1. Scalar numbers 8
+ 3.2. Multi-Precision Integers 8
+ 3.3. Key IDs 8
+ 3.4. Text 8
+ 3.5. Time fields 9
+ 3.6. String-to-key (S2K) specifiers 9
+ 3.6.1. String-to-key (S2k) specifier types 9
+ 3.6.1.1. Simple S2K 9
+ 3.6.1.2. Salted S2K 10
+ 3.6.1.3. Iterated and Salted S2K 10
+ 3.6.2. String-to-key usage 11
+ 3.6.2.1. Secret key encryption 11
+ 3.6.2.2. Symmetric-key message encryption 11
+ 4. Packet Syntax 12
+ 4.1. Overview 12
+ 4.2. Packet Headers 12
+ 4.2.1. Old-Format Packet Lengths 13
+ 4.2.2. New-Format Packet Lengths 13
+ 4.2.2.1. One-Octet Lengths 14
+ 4.2.2.2. Two-Octet Lengths 14
+ 4.2.2.3. Five-Octet Lengths 14
+ 4.2.2.4. Partial Body Lengths 14
+ 4.2.3. Packet Length Examples 14
+
+
+
+Callas, et. al. Standards Track [Page 2]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 4.3. Packet Tags 15
+ 5. Packet Types 16
+ 5.1. Public-Key Encrypted Session Key Packets (Tag 1) 16
+ 5.2. Signature Packet (Tag 2) 17
+ 5.2.1. Signature Types 17
+ 5.2.2. Version 3 Signature Packet Format 19
+ 5.2.3. Version 4 Signature Packet Format 21
+ 5.2.3.1. Signature Subpacket Specification 22
+ 5.2.3.2. Signature Subpacket Types 24
+ 5.2.3.3. Signature creation time 25
+ 5.2.3.4. Issuer 25
+ 5.2.3.5. Key expiration time 25
+ 5.2.3.6. Preferred symmetric algorithms 25
+ 5.2.3.7. Preferred hash algorithms 25
+ 5.2.3.8. Preferred compression algorithms 26
+ 5.2.3.9. Signature expiration time 26
+ 5.2.3.10.Exportable Certification 26
+ 5.2.3.11.Revocable 27
+ 5.2.3.12.Trust signature 27
+ 5.2.3.13.Regular expression 27
+ 5.2.3.14.Revocation key 27
+ 5.2.3.15.Notation Data 28
+ 5.2.3.16.Key server preferences 28
+ 5.2.3.17.Preferred key server 29
+ 5.2.3.18.Primary user id 29
+ 5.2.3.19.Policy URL 29
+ 5.2.3.20.Key Flags 29
+ 5.2.3.21.Signer's User ID 30
+ 5.2.3.22.Reason for Revocation 30
+ 5.2.4. Computing Signatures 31
+ 5.2.4.1. Subpacket Hints 32
+ 5.3. Symmetric-Key Encrypted Session-Key Packets (Tag 3) 32
+ 5.4. One-Pass Signature Packets (Tag 4) 33
+ 5.5. Key Material Packet 34
+ 5.5.1. Key Packet Variants 34
+ 5.5.1.1. Public Key Packet (Tag 6) 34
+ 5.5.1.2. Public Subkey Packet (Tag 14) 34
+ 5.5.1.3. Secret Key Packet (Tag 5) 35
+ 5.5.1.4. Secret Subkey Packet (Tag 7) 35
+ 5.5.2. Public Key Packet Formats 35
+ 5.5.3. Secret Key Packet Formats 37
+ 5.6. Compressed Data Packet (Tag 8) 38
+ 5.7. Symmetrically Encrypted Data Packet (Tag 9) 39
+ 5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) 39
+ 5.9. Literal Data Packet (Tag 11) 40
+ 5.10. Trust Packet (Tag 12) 40
+ 5.11. User ID Packet (Tag 13) 41
+ 6. Radix-64 Conversions 41
+
+
+
+Callas, et. al. Standards Track [Page 3]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 6.1. An Implementation of the CRC-24 in "C" 42
+ 6.2. Forming ASCII Armor 42
+ 6.3. Encoding Binary in Radix-64 44
+ 6.4. Decoding Radix-64 46
+ 6.5. Examples of Radix-64 46
+ 6.6. Example of an ASCII Armored Message 47
+ 7. Cleartext signature framework 47
+ 7.1. Dash-Escaped Text 47
+ 8. Regular Expressions 48
+ 9. Constants 49
+ 9.1. Public Key Algorithms 49
+ 9.2. Symmetric Key Algorithms 49
+ 9.3. Compression Algorithms 50
+ 9.4. Hash Algorithms 50
+ 10. Packet Composition 50
+ 10.1. Transferable Public Keys 50
+ 10.2. OpenPGP Messages 52
+ 10.3. Detached Signatures 52
+ 11. Enhanced Key Formats 52
+ 11.1. Key Structures 52
+ 11.2. Key IDs and Fingerprints 53
+ 12. Notes on Algorithms 54
+ 12.1. Symmetric Algorithm Preferences 54
+ 12.2. Other Algorithm Preferences 55
+ 12.2.1. Compression Preferences 56
+ 12.2.2. Hash Algorithm Preferences 56
+ 12.3. Plaintext 56
+ 12.4. RSA 56
+ 12.5. Elgamal 57
+ 12.6. DSA 58
+ 12.7. Reserved Algorithm Numbers 58
+ 12.8. OpenPGP CFB mode 58
+ 13. Security Considerations 59
+ 14. Implementation Nits 60
+ 15. Authors and Working Group Chair 62
+ 16. References 63
+ 17. Full Copyright Statement 65
+
+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 builds on the foundation provided in
+ RFC 1991 "PGP Message Exchange Formats."
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 4]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+1.1. Terms
+
+ * OpenPGP - This is a definition for security software that uses
+ PGP 5.x as a basis.
+
+ * PGP - Pretty Good Privacy. PGP is a family of software systems
+ developed by Philip R. Zimmermann from which OpenPGP is based.
+
+ * PGP 2.6.x - This version of PGP has many variants, hence the term
+ PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic
+ transforms. An informational RFC, RFC 1991, was written
+ describing this version of PGP.
+
+ * PGP 5.x - This version of PGP is formerly known as "PGP 3" in the
+ community and also in the predecessor of this document, RFC 1991.
+ It has new formats and corrects a number of problems in the PGP
+ 2.6.x design. It is referred to here as PGP 5.x because that
+ software was the first release of the "PGP 3" code base.
+
+ "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
+ Network Associates, Inc. and are used with permission.
+
+ This document uses the terms "MUST", "SHOULD", and "MAY" as defined
+ in RFC 2119, along with the negated forms of those terms.
+
+2. General functions
+
+ OpenPGP provides data integrity services for messages and data files
+ by using these core technologies:
+
+ - digital signatures
+
+ - encryption
+
+ - compression
+
+ - radix-64 conversion
+
+ In addition, OpenPGP provides key management and certificate
+ services, but many of these are beyond the scope of this document.
+
+2.1. Confidentiality via Encryption
+
+ OpenPGP uses two encryption methods to provide confidentiality:
+ symmetric-key encryption and public key encryption. With public-key
+ encryption, the object is encrypted using a symmetric encryption
+ algorithm. Each symmetric key is used only once. A new "session key"
+ is generated as a random number for each message. Since it is used
+
+
+
+Callas, et. al. Standards Track [Page 5]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 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 generates a random number to be used as a
+ session key for this message only.
+
+ 3. The session key is encrypted using each recipient's public key.
+ These "encrypted session keys" start the message.
+
+ 4. The sending OpenPGP encrypts the message using the session key,
+ which forms the remainder of the message. Note that the message
+ is also usually compressed.
+
+ 5. The receiving OpenPGP decrypts the session key using the
+ recipient's private key.
+
+ 6. The receiving OpenPGP decrypts the message using the session key.
+ If the message was compressed, it will be decompressed.
+
+ With symmetric-key encryption, an object may be encrypted with a
+ symmetric key derived from a passphrase (or other shared secret), or
+ a two-stage mechanism similar to the public-key method described
+ above in which a session key is itself encrypted with a symmetric
+ algorithm keyed 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 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 hash code or message digest algorithm,
+ and a public-key signature algorithm. The sequence is as follows:
+
+ 1. The sender creates a message.
+
+ 2. The sending software generates a hash code of the message.
+
+ 3. The sending software generates a signature from the hash code
+ using the sender's private key.
+
+ 4. The binary signature is attached to the message.
+
+
+
+Callas, et. al. Standards Track [Page 6]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 5. The receiving software keeps a copy of the message signature.
+
+ 6. The receiving software generates a new hash code 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
+
+ OpenPGP implementations MAY compress the message after applying the
+ signature but before encryption.
+
+2.4. Conversion to Radix-64
+
+ OpenPGP's underlying native representation for encrypted messages,
+ signature certificates, and keys is a stream of arbitrary octets.
+ Some systems only permit the use of blocks consisting of seven-bit,
+ printable text. For transporting OpenPGP's native raw binary octets
+ through channels that are not safe to raw binary data, a printable
+ encoding of these binary octets is needed. OpenPGP provides the
+ service of converting the raw 8-bit binary octet stream to a stream
+ of printable ASCII characters, called Radix-64 encoding or ASCII
+ Armor.
+
+ Implementations SHOULD provide Radix-64 conversions.
+
+ Note that many applications, particularly messaging applications,
+ will want more advanced features as described in the OpenPGP-MIME
+ document, RFC 2015. An application that implements OpenPGP for
+ messaging SHOULD implement OpenPGP-MIME.
+
+2.5. Signature-Only Applications
+
+ OpenPGP is designed for applications that use both encryption and
+ signatures, but there are a number of problems that are solved by a
+ signature-only implementation. Although this specification requires
+ both encryption and signatures, it is reasonable for there to be
+ subset implementations that are non-comformant only in that they omit
+ encryption.
+
+3. Data Element Formats
+
+ This section describes the data elements used by OpenPGP.
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 7]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+3.1. Scalar numbers
+
+ Scalar numbers are unsigned, and are always stored in big-endian
+ format. Using n[k] to refer to the kth octet being interpreted, the
+ value of a two-octet scalar is ((n[0] << 8) + n[1]). The value of a
+ four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
+ n[3]).
+
+3.2. Multi-Precision Integers
+
+ Multi-Precision Integers (also called MPIs) are unsigned integers
+ used to hold large integers such as the ones used in cryptographic
+ calculations.
+
+ An MPI consists of two pieces: a two-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:
+
+ (all numbers are in hexadecimal)
+
+ 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 of 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].
+
+3.3. Key IDs
+
+ A Key ID is an eight-octet scalar that identifies a key.
+ Implementations SHOULD NOT assume that Key IDs are unique. The
+ section, "Enhanced Key Formats" below describes how Key IDs are
+ formed.
+
+3.4. Text
+
+ The default character set for text is the UTF-8 [RFC2279] encoding of
+ Unicode [ISO10646].
+
+
+
+
+Callas, et. al. Standards Track [Page 8]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+3.5. Time fields
+
+ A time field is an unsigned four-octet number containing the number
+ of seconds elapsed since midnight, 1 January 1970 UTC.
+
+3.6. String-to-key (S2K) specifiers
+
+ String-to-key (S2K) specifiers are used to convert passphrase strings
+ into symmetric-key encryption/decryption keys. They are used in two
+ places, currently: to encrypt the secret part of private keys in the
+ private keyring, and to convert passphrases to encryption keys for
+ symmetrically encrypted messages.
+
+3.6.1. String-to-key (S2k) specifier types
+
+ There are three types of S2K specifiers currently supported, as
+ follows:
+
+3.6.1.1. Simple S2K
+
+ This directly hashes the string to produce the key data. See below
+ for how this hashing is done.
+
+ 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 will depend on the cipher used) and the size of the hash
+ algorithm's output. If the hash size is greater than or equal to 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 to say, the first instance has no preloading, the second gets
+ preloaded with 1 octet of zero, the third is preloaded with two
+ 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 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, with any excess octets
+ on the right discarded.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 9]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+3.6.1.2. Salted S2K
+
+ This 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.
+
+3.6.1.3. Iterated and Salted S2K
+
+ This includes both a salt and an octet count. The salt is combined
+ with the passphrase and the resulting value is hashed repeatedly.
+ 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 one-octet, coded value
+
+ The count is coded into a one-octet number using the following
+ formula:
+
+ #define EXPBIAS 6
+ count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
+
+ The above formula is in C, where "Int32" is a type for a 32-bit
+ integer, and the variable "c" is the coded count, Octet 10.
+
+ Iterated-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 as with the other S2K
+ algorithms, depending on how many octets of key data are needed.
+ Then the salt, followed by the passphrase data is repeatedly hashed
+ until the number of octets specified by the octet count has been
+ hashed. The one exception is that if the octet count is less than
+ the size of the salt plus passphrase, the full salt plus passphrase
+ will be hashed even though that is greater than the octet count.
+
+
+
+Callas, et. al. Standards Track [Page 10]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ After the hashing is done the data is unloaded from the hash
+ context(s) as with the other S2K algorithms.
+
+3.6.2. String-to-key usage
+
+ Implementations SHOULD use salted or iterated-and-salted S2K
+ specifiers, as simple S2K specifiers are more vulnerable to
+ dictionary attacks.
+
+3.6.2.1. Secret key encryption
+
+ An S2K specifier can be stored in the secret keyring to specify how
+ to convert the passphrase to a key that unlocks the secret data.
+ Older versions of PGP just stored a cipher algorithm octet preceding
+ the secret data or a zero to indicate that the secret data was
+ unencrypted. The MD5 hash function was always used to convert the
+ passphrase to a key for the specified cipher algorithm.
+
+ For compatibility, when an S2K specifier is used, the special value
+ 255 is stored in the position where the hash algorithm octet would
+ have been in the old data structure. This is then followed
+ immediately by a one-octet algorithm identifier, and then by the S2K
+ specifier as encoded above.
+
+ Therefore, preceding the secret data there will be one of these
+ possibilities:
+
+ 0: secret data is unencrypted (no pass phrase)
+ 255: followed by algorithm octet and S2K specifier
+ Cipher alg: use Simple S2K algorithm using MD5 hash
+
+ This last possibility, the cipher algorithm number with an implicit
+ use of MD5 and IDEA, is provided for backward compatibility; it MAY
+ be understood, but SHOULD NOT be generated, and is deprecated.
+
+ These are followed by an 8-octet Initial Vector for the decryption of
+ the secret values, if they are encrypted, and then the secret key
+ values themselves.
+
+3.6.2.2. Symmetric-key message encryption
+
+ OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) 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 symmetric-key ESKs and public-key ESKs. This allows a message
+ to be decrypted either with a passphrase or a public key.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 11]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ PGP 2.X always used IDEA with Simple string-to-key conversion when
+ encrypting a message with a symmetric algorithm. This is deprecated,
+ but MAY be used 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
+ traditionally called packets. A packet is a chunk of data that has a
+ tag specifying its meaning. An OpenPGP message, keyring, certificate,
+ 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.
+
+4.2. Packet Headers
+
+ The first octet of the packet header is called the "Packet Tag." It
+ determines the format of the header and denotes the packet contents.
+ The remainder of the packet header is the length of the packet.
+
+ Note that the most significant bit is the left-most bit, called bit
+ 7. A mask for this bit is 0x80 in hexadecimal.
+
+ +---------------+
+ PTag |7 6 5 4 3 2 1 0|
+ +---------------+
+ Bit 7 -- Always one
+ Bit 6 -- New packet format if set
+
+ PGP 2.6.x only uses old format packets. Thus, software that
+ interoperates with those versions of PGP must only use old format
+ packets. If interoperability is not an issue, either format may be
+ used. Note that old format packets have four bits of content tags,
+ and new format packets have six; some features cannot be used and
+ still be backward-compatible.
+
+ Old format packets contain:
+
+ Bits 5-2 -- content tag
+ Bits 1-0 - length-type
+
+
+
+
+
+Callas, et. al. Standards Track [Page 12]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ New format packets contain:
+
+ Bits 5-0 -- content tag
+
+4.2.1. Old-Format Packet Lengths
+
+ The meaning of the length-type in old-format packets is:
+
+ 0 - The packet has a one-octet length. The header is 2 octets long.
+
+ 1 - The packet has a two-octet length. The header is 3 octets long.
+
+ 2 - The packet has a four-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, this means that the packet
+ extends until the end of the file. In general, an implementation
+ SHOULD NOT use indeterminate length packets except where the end
+ of the data will be clear from the context, and even then it is
+ better to use a definite length, or a new-format header. The
+ new-format headers described below have a mechanism for precisely
+ encoding data of indeterminate length.
+
+4.2.2. New-Format Packet Lengths
+
+ New format packets have four possible ways of encoding length:
+
+ 1. A one-octet Body Length header encodes packet lengths of up to
+ 191 octets.
+
+ 2. A two-octet Body Length header encodes packet lengths of 192 to
+ 8383 octets.
+
+ 3. A five-octet Body Length header encodes packet lengths of up to
+ 4,294,967,295 (0xFFFFFFFF) octets in length. (This actually
+ encodes a four-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.
+
+
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 13]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+4.2.2.1. One-Octet Lengths
+
+ A one-octet Body Length header encodes a length of from 0 to 191
+ octets. This type of length header is recognized because the one
+ octet value is less than 192. The body length is equal to:
+
+ bodyLen = 1st_octet;
+
+4.2.2.2. Two-Octet Lengths
+
+ A two-octet Body Length header encodes a length of from 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
+
+4.2.2.3. Five-Octet Lengths
+
+ A five-octet Body Length header consists of a single octet holding
+ the value 255, followed by a four-octet scalar. The body length is
+ equal to:
+
+ bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
+ (4th_octet << 8) | 5th_octet
+
+4.2.2.4. Partial Body Lengths
+
+ A Partial Body Length header is one 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 one
+ 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 (of one of the three types --
+ one octet, two-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.
+
+4.2.3. Packet Length Examples
+
+ These examples show ways that new-format packets might encode the
+ packet lengths.
+
+
+
+
+Callas, et. al. Standards Track [Page 14]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ A packet with length 100 may have its length encoded in one octet:
+ 0x64. This is followed by 100 octets of data.
+
+ A packet with length 1723 may have its length coded in two octets:
+ 0xC5, 0xFB. This header is followed by the 1723 octets of data.
+
+ A packet 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 two octets of data; 0xE0, next one
+ octet of data; 0xF0, next 65536 octets of data; 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. Note also that the last Body Length header can
+ be a zero-length header.
+
+ An implementation MAY use Partial Body Lengths for data packets, be
+ they 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.
+
+ 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 Tags
+
+ The packet tag denotes what type of packet the body holds. Note that
+ old format headers can only have tags less than 16, whereas new
+ format headers can have tags as great as 63. The defined tags (in
+ decimal) are:
+
+ 0 -- Reserved - a packet tag must not have this value
+ 1 -- Public-Key Encrypted Session Key Packet
+ 2 -- Signature Packet
+ 3 -- Symmetric-Key Encrypted Session Key Packet
+ 4 -- One-Pass Signature Packet
+ 5 -- Secret Key Packet
+ 6 -- Public Key Packet
+ 7 -- Secret Subkey Packet
+ 8 -- Compressed Data Packet
+ 9 -- Symmetrically Encrypted Data Packet
+ 10 -- Marker Packet
+ 11 -- Literal Data Packet
+ 12 -- Trust Packet
+
+
+
+
+Callas, et. al. Standards Track [Page 15]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 13 -- User ID Packet
+ 14 -- Public Subkey Packet
+ 60 to 63 -- Private or Experimental Values
+
+5. Packet Types
+
+5.1. Public-Key Encrypted Session Key Packets (Tag 1)
+
+ A Public-Key Encrypted Session Key packet holds the session key used
+ to encrypt a message. Zero or more Encrypted Session Key packets
+ (either Public-Key or Symmetric-Key) may precede a Symmetrically
+ Encrypted Data 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
+ Symmetrically Encrypted Data Packet 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 consists of:
+
+ - A one-octet number giving the version number of the packet type.
+ The currently defined value for packet version is 3. An
+ implementation should accept, but not generate a version of 2,
+ which is equivalent to V3 in all other respects.
+
+ - An eight-octet number that gives the key ID of the public key
+ that the session key is encrypted to.
+
+ - A one-octet number giving the public key algorithm used.
+
+ - A string of octets that is the encrypted session key. This string
+ takes up the remainder of the packet, and its contents are
+ dependent on the public key algorithm used.
+
+ Algorithm Specific Fields for RSA encryption
+
+ - multiprecision integer (MPI) of RSA encrypted value m**e mod n.
+
+ 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.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 16]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ The value "m" in the above formulas is derived from the session key
+ as follows. First the session key is prefixed with a one-octet
+ algorithm identifier that specifies the symmetric encryption
+ algorithm used to encrypt the following Symmetrically Encrypted Data
+ Packet. Then a two-octet checksum is appended which is equal to the
+ sum of the preceding session key octets, not including the algorithm
+ identifier, modulo 65536. This value is then padded as described in
+ PKCS-1 block type 02 [RFC2313] to form the "m" value used in the
+ formulas above.
+
+ Note that when an implementation forms several PKESKs with one
+ session key, forming a message that can be decrypted by several keys,
+ the implementation MUST make new PKCS-1 padding for each key.
+
+ An implementation MAY accept or use a Key ID of zero as a "wild card"
+ or "speculative" Key ID. 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 (Tag 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.
+
+ Two versions of signature packets are defined. Version 3 provides
+ basic signature information, while version 4 provides an expandable
+ format with subpackets that can specify more information about the
+ signature. PGP 2.6.x only accepts version 3 signatures.
+
+ Implementations MUST accept V3 signatures. Implementations SHOULD
+ generate V4 signatures. Implementations MAY generate a V3 signature
+ that can be verified by PGP 2.6.x.
+
+ Note that if an implementation is creating an encrypted and signed
+ message that is encrypted to a V3 key, it is reasonable to create a
+ V3 signature.
+
+5.2.1. Signature Types
+
+ There are a number of possible meanings for a signature, which are
+ specified in a signature type octet in any given signature. These
+ meanings are:
+
+ 0x00: Signature of a binary document.
+ Typically, this means the signer owns it, created it, or
+ certifies that it has not been modified.
+
+
+
+
+Callas, et. al. Standards Track [Page 17]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 0x01: Signature of a canonical text document.
+ Typically, 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> and trailing blanks removed.
+
+ 0x02: Standalone signature.
+ This signature is a signature of only its own subpacket
+ contents. It is calculated identically to a signature over a
+ zero-length binary document. Note that it doesn't make sense to
+ have a V3 standalone signature.
+
+ 0x10: Generic certification 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. Note that all PGP "key signatures" are this type of
+ certification.
+
+ 0x11: Persona certification 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.
+
+ 0x12: Casual certification of a User ID and Public Key packet.
+ The issuer of this certification has done some casual
+ verification of the claim of identity.
+
+ 0x13: Positive certification of a User ID and Public Key packet.
+ The issuer of this certification has done substantial
+ verification of the claim of identity.
+
+ Please note that the vagueness of these certification claims is
+ not a flaw, but a feature of the system. Because PGP places
+ final authority for validity upon the receiver of a
+ certification, it may be that one authority's casual
+ certification might be more rigorous than some other
+ authority's positive certification. These classifications allow
+ a certification authority to issue fine-grained claims.
+
+ 0x18: Subkey Binding Signature
+ This signature is a statement by the top-level signing key
+ indicates that it owns the subkey. This signature is calculated
+ directly on the subkey itself, not on any User ID or other
+ packets.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 18]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 0x1F: Signature directly on a key
+ 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 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.
+
+ 0x20: Key revocation signature
+ The 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 an authorized revocation key,
+ should be considered valid revocation signatures.
+
+ 0x28: Subkey revocation signature
+ The signature is calculated directly on 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 an authorized revocation key, should be
+ considered valid revocation signatures.
+
+ 0x30: Certification revocation signature
+ This signature revokes an earlier user ID certification
+ signature (signature class 0x10 through 0x13). It should be
+ issued by the same key that issued the revoked signature or an
+ authorized revocation key The signature should have a later
+ creation date than the signature it revokes.
+
+ 0x40: Timestamp signature.
+ This signature is only meaningful for the timestamp contained
+ in it.
+
+5.2.2. Version 3 Signature Packet Format
+
+ The body of a version 3 Signature Packet contains:
+
+ - One-octet version number (3).
+
+ - One-octet length of following hashed material. MUST be 5.
+
+ - One-octet signature type.
+
+ - Four-octet creation time.
+
+ - Eight-octet key ID of signer.
+
+ - One-octet public key algorithm.
+
+
+
+Callas, et. al. Standards Track [Page 19]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ - One-octet hash algorithm.
+
+ - Two-octet field holding left 16 bits of signed hash value.
+
+ - One or more multi-precision integers comprising the signature.
+ This portion is algorithm specific, as described below.
+
+ The data being signed is hashed, and then the signature type and
+ creation time from the signature packet are hashed (5 additional
+ octets). 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 quick test to reject
+ some invalid signatures.
+
+ Algorithm Specific Fields for RSA signatures:
+
+ - multiprecision integer (MPI) of RSA signature value m**d.
+
+ 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 signature than for RSA signatures.
+
+ With RSA signatures, the hash value is encoded as described in PKCS-1
+ section 10.1.2, "Data encoding", producing an ASN.1 value of type
+ DigestInfo, and then padded using PKCS-1 block type 01 [RFC2313].
+ This requires inserting the hash value as an octet string into an
+ ASN.1 structure. The object identifier for the type of hash being
+ used is included in the structure. The hexadecimal representations
+ for the currently defined hash algorithms are:
+
+ - MD2: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02
+
+ - MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
+
+ - RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
+
+ - SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 20]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ The ASN.1 OIDs are:
+
+ - MD2: 1.2.840.113549.2.2
+
+ - MD5: 1.2.840.113549.2.5
+
+ - RIPEMD-160: 1.3.36.3.2.1
+
+ - SHA-1: 1.3.14.3.2.26
+
+ The full hash prefixes for these are:
+
+ MD2: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
+ 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02, 0x05, 0x00,
+ 0x04, 0x10
+
+ MD5: 0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
+ 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
+ 0x04, 0x10
+
+ RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
+ 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
+
+ SHA-1: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
+ 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
+
+ DSA signatures MUST use hashes with a size of 160 bits, to match q,
+ the size of the group generated by the DSA key's generator value.
+ The hash function result is treated as a 160 bit number and used
+ directly in the DSA signature algorithm.
+
+5.2.3. Version 4 Signature Packet Format
+
+ The body of a version 4 Signature Packet contains:
+
+ - One-octet version number (4).
+
+ - One-octet signature type.
+
+ - One-octet public key algorithm.
+
+ - One-octet hash algorithm.
+
+ - Two-octet scalar octet count for following hashed subpacket
+ data. Note that this is the length in octets of all of the hashed
+ subpackets; a pointer incremented by this number will skip over
+ the hashed subpackets.
+
+
+
+
+Callas, et. al. Standards Track [Page 21]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ - Hashed subpacket data. (zero or more subpackets)
+
+ - Two-octet scalar octet count for following unhashed subpacket
+ data. Note that this is the length in octets of all of the
+ unhashed subpackets; a pointer incremented by this number will
+ skip over the unhashed subpackets.
+
+ - Unhashed subpacket data. (zero or more subpackets)
+
+ - Two-octet field holding left 16 bits of signed hash value.
+
+ - One or more multi-precision integers comprising the signature.
+ This portion is algorithm specific, as described above.
+
+ The data being signed is hashed, and then the signature data from the
+ version number through the hashed subpacket data (inclusive) is
+ hashed. The resulting hash value is what is signed. The left 16 bits
+ of the hash are included in the signature packet to provide a quick
+ test to reject some invalid signatures.
+
+ There are two fields consisting of signature subpackets. The first
+ field is hashed with the rest of the signature data, while the second
+ is unhashed. The second set of subpackets is not cryptographically
+ protected by the signature and should include only advisory
+ information.
+
+ The algorithms for converting the hash function result to a signature
+ are described in a section below.
+
+5.2.3.1. Signature Subpacket Specification
+
+ The subpacket fields consist of zero or more signature subpackets.
+ Each set of subpackets is preceded by a two-octet scalar count of the
+ length of the set of 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 (1 octet)
+
+ and is followed by the subpacket specific data.
+
+ The length includes the type octet but not this length. Its format is
+ similar to the "new" format packet header lengths, but cannot have
+ partial body lengths. That is:
+
+
+
+
+Callas, et. al. Standards Track [Page 22]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 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 = [four-octet scalar starting at 2nd_octet]
+
+ The value of the subpacket type octet may be:
+
+ 2 = signature creation time
+ 3 = signature expiration time
+ 4 = exportable certification
+ 5 = trust signature
+ 6 = regular expression
+ 7 = revocable
+ 9 = key expiration time
+ 10 = placeholder for backward compatibility
+ 11 = preferred symmetric algorithms
+ 12 = revocation key
+ 16 = issuer key ID
+ 20 = notation data
+ 21 = preferred hash algorithms
+ 22 = preferred compression algorithms
+ 23 = key server preferences
+ 24 = preferred key server
+ 25 = primary user id
+ 26 = policy URL
+ 27 = key flags
+ 28 = signer's user id
+ 29 = reason for revocation
+ 100 to 110 = internal or user-defined
+
+ An implementation SHOULD ignore any subpacket of a type that it does
+ not recognize.
+
+ Bit 7 of the subpacket type 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 software, the
+ evaluator SHOULD consider the signature to be in error.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 23]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 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 than be ignored.
+
+ Implementations SHOULD implement "preferences".
+
+5.2.3.2. Signature Subpacket Types
+
+ A number of subpackets are currently defined. 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 user id
+ 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 self-signature is a binding signature made by the key the signature
+ refers to. There are three types of self-signatures, the
+ certification signatures (types 0x10-0x13), the direct-key signature
+ (type 0x1f), and the subkey binding signature (type 0x18). 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 in fact has a self-
+ signature. Subpackets that appear in a certification self-signature
+ apply to the username, 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.
+
+ Implementing software should interpret a self-signature's preference
+ subpackets as narrowly as possible. For example, suppose a key has
+ two usernames, Alice and Bob. Suppose that Alice prefers the
+ symmetric algorithm CAST5, and Bob prefers IDEA or Triple-DES. If the
+ software locates this key via Alice's name, then the preferred
+ algorithm is CAST5, if software locates the key via Bob's name, then
+ the preferred algorithm is IDEA. If the key is located by key id,
+ then algorithm of the default user id of the key provides the default
+ symmetric algorithm.
+
+ A subpacket may be found either in the hashed or 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
+ part of the signature proper.
+
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 24]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+5.2.3.3. Signature creation time
+
+ (4 octet time field)
+
+ The time the signature was made.
+
+ MUST be present in the hashed area.
+
+5.2.3.4. Issuer
+
+ (8 octet key ID)
+
+ The OpenPGP key ID of the key issuing the signature.
+
+5.2.3.5. 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. If this is not present
+ or has a value of zero, the key never expires. This is found only on
+ a self-signature.
+
+5.2.3.6. Preferred symmetric algorithms
+
+ (sequence of one-octet values)
+
+ Symmetric algorithm numbers that indicate which algorithms the key
+ holder prefers to use. The subpacket body is an ordered list of
+ octets with the most preferred listed first. It is assumed that only
+ algorithms listed are supported by the recipient's software.
+ Algorithm numbers in section 9. This is only found on a self-
+ signature.
+
+5.2.3.7. Preferred hash algorithms
+
+ (array of one-octet values)
+
+ Message digest algorithm numbers that indicate which algorithms the
+ key holder prefers to receive. Like the preferred symmetric
+ algorithms, the list is ordered. Algorithm numbers are in section 6.
+ This is only found on a self-signature.
+
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 25]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+5.2.3.8. Preferred compression algorithms
+
+ (array of one-octet values)
+
+ Compression algorithm numbers that indicate which algorithms the key
+ holder prefers to use. Like the preferred symmetric algorithms, the
+ list is ordered. Algorithm numbers are in section 6. If this
+ subpacket is not included, ZIP is preferred. A zero denotes that
+ uncompressed data is preferred; the key holder's software might have
+ no compression software in that implementation. This is only found on
+ a self-signature.
+
+5.2.3.9. 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.
+
+5.2.3.10. Exportable Certification
+
+ (1 octet of exportability, 0 for not, 1 for exportable)
+
+ This subpacket denotes whether a certification signature is
+ "exportable", to be used by other users 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,
+ 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.
+
+
+
+
+Callas, et. al. Standards Track [Page 26]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+5.2.3.11. Revocable
+
+ (1 octet of revocability, 0 for not, 1 for revocable)
+
+ Signature's revocability status. Packet body contains a boolean flag
+ indicating whether the signature is revocable. Signatures that are
+ not revocable have any later revocation signatures ignored. They
+ represent a commitment by the signer that he cannot revoke his
+ signature for the life of his key. If this packet is not present,
+ the signature is revocable.
+
+5.2.3.12. Trust signature
+
+ (1 octet "level" (depth), 1 octet of trust amount)
+
+ 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, i.e. that
+ it 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.
+
+5.2.3.13. Regular expression
+
+ (null-terminated 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 the Henry Spencer's
+ "almost public domain" regular expression package. A description of
+ the syntax is found in a section below.
+
+5.2.3.14. Revocation key
+
+ (1 octet of class, 1 octet of algid, 20 octets of fingerprint)
+
+ Authorizes the specified key to issue revocation signatures for this
+ key. Class octet must have bit 0x80 set. If the bit 0x40 is set,
+ then this means that the revocation information is sensitive. Other
+ bits are for future expansion to other kinds of authorizations. This
+
+
+
+Callas, et. al. Standards Track [Page 27]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ is found on a self-signature.
+
+ 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: 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.
+
+5.2.3.15. 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 four octets of
+ flags.
+
+ All undefined flags MUST be zero. Defined flags are:
+
+ First octet: 0x80 = human-readable. This note is text, a note
+ from one person to another, and has no
+ meaning to software.
+ Other octets: none.
+
+5.2.3.16. Key server preferences
+
+ (N octets of flags)
+
+ This is a list of flags that indicate preferences that the key holder
+ has about how the key is handled on a key server. All undefined flags
+ MUST be zero.
+
+ First octet: 0x80 = No-modify
+ the key holder requests that this key only be modified or updated
+ by the key holder or an administrator of the key server.
+
+ This is found only on a self-signature.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 28]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+5.2.3.17. Preferred key server
+
+ (String)
+
+ This is a URL of a key server that the key holder 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 URL, the
+ key server can actually be a copy of the key retrieved by ftp, http,
+ finger, etc.
+
+5.2.3.18. 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, etc. 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.
+
+5.2.3.19. Policy URL
+
+ (String)
+
+ This subpacket contains a URL of a document that describes the policy
+ that the signature was issued under.
+
+5.2.3.20. Key Flags
+
+ (Octet string)
+
+ 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. This is so 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:
+
+ First octet:
+
+ 0x01 - This key may be used to certify 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.
+
+
+
+
+Callas, et. al. Standards Track [Page 29]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 0x10 - The private component of this key may have been split by a
+ secret-sharing mechanism.
+
+ 0x80 - The private component of this key may be in the possession
+ of more than one person.
+
+ 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. Note however, that it is a thorny issue to
+ determine what is "communications" and what is "storage." 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
+ 0x1f) or a subkey signature (type 0x18), one that refers to the key
+ the flag applies to.
+
+5.2.3.21. Signer's User ID
+
+ 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.
+
+5.2.3.22. 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
+ certificate was revoked.
+
+ The first octet contains a machine-readable code that denotes the
+ reason for the revocation:
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 30]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 0x00 - No reason specified (key revocations or cert revocations)
+ 0x01 - Key is superceded (key revocations)
+ 0x02 - Key material has been compromised (key revocations)
+ 0x03 - Key is no longer used (key revocations)
+ 0x20 - User id information is no longer valid (cert revocations)
+
+ Following the revocation code is a string of octets which gives
+ information about the reason for revocation in human-readable form
+ (UTF-8). The string may be null, that is, of zero length. The length
+ of the subpacket is the length of the reason string plus one.
+
+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.
+
+ The signature data is simple to compute for document signatures
+ (types 0x00 and 0x01), for which the document itself is the data.
+ For standalone signatures, this is a null string.
+
+ When a signature is made over a key, the hash data starts with the
+ octet 0x99, followed by a two-octet length of the key, and then body
+ of the key packet. (Note that this is an old-style packet header for
+ a key packet with two-octet length.) A subkey signature (type 0x18)
+ then hashes the subkey, using the same format as the main key. Key
+ revocation signatures (types 0x20 and 0x28) hash only the key being
+ revoked.
+
+ A certification signature (type 0x10 through 0x13) hashes the user id
+ being bound to the key into the hash context after the above data. A
+ V3 certification hashes the contents of the name packet, without any
+ header. A V4 certification hashes the constant 0xb4 (which is an
+ old-style packet header with the length-of-length set to zero), a
+ four-octet number giving the length of the username, and then the
+ username data.
+
+ Once the data body is hashed, then a trailer is hashed. A V3
+ signature hashes five octets of the packet body, starting from the
+ signature type field. This data is the signature type, followed by
+ the four-octet signature time. A V4 signature hashes the packet body
+ starting from its first field, the version number, through the end of
+ the hashed subpacket data. Thus, the fields hashed are the signature
+ version, the signature type, the public key algorithm, the hash
+ algorithm, the hashed subpacket length, and the hashed subpacket
+ body.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 31]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ V4 signatures also hash in a final trailer of six octets: the version
+ of the signature packet, i.e. 0x04; 0xFF; a four-octet, big-endian
+ number that is the length of the hashed data from the signature
+ packet (note that this number does not include these final six
+ octets.
+
+ After all this has been hashed, the resulting hash field is used in
+ the signature algorithm, and placed at the end of the signature
+ packet.
+
+5.2.4.1. Subpacket Hints
+
+ An implementation SHOULD put the two mandatory subpackets, creation
+ time and issuer, as the first subpackets in the subpacket list,
+ simply to make it easier for the implementer to find them.
+
+ 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
+ signature, but MAY use any conflict resolution scheme that makes more
+ sense. Please note that we are intentionally leaving conflict
+ resolution to the implementer; most conflicts are simply syntax
+ errors, and the wishy-washy 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 an V3 key and a V4 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 subpacket for each key, as a way of explicitly tying those
+ keys to the signature.
+
+5.3. Symmetric-Key Encrypted Session-Key Packets (Tag 3)
+
+ The Symmetric-Key Encrypted Session Key packet holds the symmetric-
+ key encryption of a session key used to encrypt a message. Zero or
+ more Encrypted Session Key packets and/or Symmetric-Key Encrypted
+ Session Key packets may precede 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 or the Symmetric-Key Encrypted
+ Session Key packet.
+
+ If the Symmetrically Encrypted Data Packet 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
+
+
+
+Callas, et. al. Standards Track [Page 32]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ message to be encrypted to a number of public keys, and also to one
+ or more pass phrases. This packet type is new, and is not generated
+ by PGP 2.x or PGP 5.0.
+
+ The body of this packet consists of:
+
+ - A one-octet version number. The only currently defined version
+ is 4.
+
+ - A one-octet number describing the symmetric algorithm used.
+
+ - A string-to-key (S2K) specifier, length as defined above.
+
+ - Optionally, the encrypted session key itself, which is decrypted
+ with the string-to-key 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 file, using the symmetric cipher algorithm 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 one-octet algorithm identifier
+ that specifies the symmetric-key encryption algorithm used to encrypt
+ the following Symmetrically Encrypted Data Packet, 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, either a Salted S2K or an Iterated-
+ Salted S2K. The salt value will insure that the decryption key is
+ not repeated even if the passphrase is reused.
+
+5.4. One-Pass Signature Packets (Tag 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.
+
+ A One-Pass Signature does not interoperate with PGP 2.6.x or earlier.
+
+ The body of this packet consists of:
+
+
+
+
+
+Callas, et. al. Standards Track [Page 33]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ - A one-octet version number. The current version is 3.
+
+ - A one-octet signature type. Signature types are described in
+ section 5.2.1.
+
+ - A one-octet number describing the hash algorithm used.
+
+ - A one-octet number describing the public key algorithm used.
+
+ - An eight-octet number holding the key ID of the signing key.
+
+ - A one-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.
+
+ 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
+ packet and the final signature packet corresponds to the first one-
+ pass packet.
+
+5.5. Key Material Packet
+
+ A key material packet contains all the information about a public or
+ private key. There are four variants of this packet type, and two
+ major versions. Consequently, this section is complex.
+
+5.5.1. Key Packet Variants
+
+5.5.1.1. Public Key Packet (Tag 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 (Tag 14)
+
+ A Public Subkey packet (tag 14) has exactly the same format as a
+ Public Key packet, but denotes a subkey. One or more subkeys may be
+ associated with a top-level key. By convention, the top-level key
+ provides signature services, and the subkeys provide encryption
+ services.
+
+ Note: in PGP 2.6.x, tag 14 was intended to indicate a comment packet.
+ This tag was selected for reuse because no previous version of PGP
+ ever emitted comment packets but they did properly ignore them.
+ Public Subkey packets are ignored by PGP 2.6.x and do not cause it to
+ fail, providing a limited degree of backward compatibility.
+
+
+
+Callas, et. al. Standards Track [Page 34]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+5.5.1.3. Secret Key Packet (Tag 5)
+
+ A Secret Key packet contains all the information that is found in a
+ Public Key packet, including the public key material, but also
+ includes the secret key material after all the public key fields.
+
+5.5.1.4. Secret Subkey Packet (Tag 7)
+
+ A Secret Subkey packet (tag 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 two versions of key-material packets. Version 3 packets
+ were first generated by PGP 2.6. Version 2 packets are identical in
+ format to Version 3 packets, but are generated by PGP 2.5 or before.
+ V2 packets are deprecated and they MUST NOT be generated. PGP 5.0
+ introduced version 4 packets, with new fields and semantics. PGP
+ 2.6.x will not accept key-material packets with versions greater than
+ 3.
+
+ OpenPGP implementations SHOULD create keys with version 4 format. An
+ implementation MAY generate a V3 key to ensure interoperability with
+ old software; note, however, that V4 keys correct some security
+ deficiencies in V3 keys. These deficiencies are described below. An
+ implementation MUST NOT create a V3 key with a public key algorithm
+ other than RSA.
+
+ A version 3 public key or public subkey packet contains:
+
+ - A one-octet version number (3).
+
+ - A four-octet number denoting the time that the key was created.
+
+ - A two-octet number denoting the time in days that this key is
+ valid. If this number is zero, then it does not expire.
+
+ - A one-octet number denoting the public key algorithm of this key
+
+ - A series of multi-precision integers comprising the key
+ material:
+
+ - a multiprecision integer (MPI) of RSA public modulus n;
+
+ - an MPI of RSA public encryption exponent e.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 35]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ V3 keys SHOULD only be used for backward compatibility because of
+ three weaknesses in them. First, it is relatively easy to construct a
+ V3 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. Secondly, because
+ the fingerprint of a V3 key hashes the key material, but not its
+ length, which increases the opportunity for fingerprint collisions.
+ Third, there are minor weaknesses in the MD5 hash algorithm that make
+ developers prefer other algorithms. See below for a fuller discussion
+ of key IDs and fingerprints.
+
+ 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
+ "Enhanced Key Formats."
+
+ A version 4 packet contains:
+
+ - A one-octet version number (4).
+
+ - A four-octet number denoting the time that the key was created.
+
+ - A one-octet number denoting the public key algorithm of this key
+
+ - A series of multi-precision integers comprising the key
+ material. This algorithm-specific portion is:
+
+ Algorithm Specific Fields for RSA public keys:
+
+ - multiprecision integer (MPI) of RSA public modulus n;
+
+ - MPI of RSA public encryption exponent e.
+
+ Algorithm Specific Fields for DSA public keys:
+
+ - 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;
+
+ - MPI of DSA public key value y (= g**x where x is secret).
+
+ Algorithm Specific Fields for Elgamal public keys:
+
+ - MPI of Elgamal prime p;
+
+ - MPI of Elgamal group generator g;
+
+
+
+Callas, et. al. Standards Track [Page 36]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ - MPI of Elgamal public key value y (= g**x where x is
+ secret).
+
+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, in encrypted form.
+
+ The packet contains:
+
+ - A Public Key or Public Subkey packet, as described above
+
+ - One octet indicating string-to-key usage conventions. 0
+ indicates that the secret key data is not encrypted. 255
+ indicates that a string-to-key specifier is being given. Any
+ other value is a symmetric-key encryption algorithm specifier.
+
+ - [Optional] If string-to-key usage octet was 255, a one-octet
+ symmetric encryption algorithm.
+
+ - [Optional] If string-to-key usage octet was 255, a string-to-key
+ specifier. The length of the string-to-key specifier is implied
+ by its type, as described above.
+
+ - [Optional] If secret data is encrypted, eight-octet Initial
+ Vector (IV).
+
+ - Encrypted multi-precision integers comprising the secret key
+ data. These algorithm-specific fields are as described below.
+
+ - Two-octet checksum of the plaintext of the algorithm-specific
+ portion (sum of all octets, mod 65536).
+
+ Algorithm Specific Fields for RSA secret keys:
+
+ - multiprecision integer (MPI) of RSA secret exponent d.
+
+ - MPI of RSA secret prime value p.
+
+ - MPI of RSA secret prime value q (p < q).
+
+ - MPI of u, the multiplicative inverse of p, mod q.
+
+ Algorithm Specific Fields for DSA secret keys:
+
+ - MPI of DSA secret exponent x.
+
+
+
+
+Callas, et. al. Standards Track [Page 37]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ Algorithm Specific Fields for Elgamal secret keys:
+
+ - MPI of Elgamal secret exponent x.
+
+ Secret MPI values can be encrypted using a passphrase. If a string-
+ to-key specifier is given, that describes the algorithm for
+ converting the passphrase to a key, else a simple MD5 hash of the
+ passphrase is used. Implementations SHOULD use a string-to-key
+ specifier; the simple hash is for backward compatibility. The cipher
+ for encrypting the MPIs is specified in the secret key packet.
+
+ Encryption/decryption of the secret data is done in CFB mode using
+ the key created from the passphrase and the Initial Vector from the
+ packet. A different mode is used with V3 keys (which are only RSA)
+ than with other key formats. With V3 keys, the MPI bit count prefix
+ (i.e., 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 V4 keys, a simpler method is used. All secret MPI values are
+ encrypted in CFB mode, including the MPI bitcount prefix.
+
+ The 16-bit checksum that follows the algorithm-specific portion is
+ the algebraic sum, mod 65536, of the plaintext of all the algorithm-
+ specific octets (including MPI prefix and data). With V3 keys, the
+ checksum is stored in the clear. With V4 keys, the checksum is
+ encrypted like the algorithm-specific data. This value is used to
+ check that the passphrase was correct.
+
+5.6. Compressed Data Packet (Tag 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 literal data
+ packets.
+
+ The body of this packet consists of:
+
+ - One octet that gives the algorithm used to compress the packet.
+
+ - The remainder of the packet is compressed data.
+
+ A Compressed Data Packet's body contains an block that compresses
+ some set of packets. See section "Packet Composition" for details on
+ how messages are formed.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 38]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ ZIP-compressed packets are compressed with raw RFC 1951 DEFLATE
+ blocks. Note that PGP V2.6 uses 13 bits of compression. If an
+ implementation uses more bits of compression, PGP V2.6 cannot
+ decompress it.
+
+ ZLIB-compressed packets are compressed with RFC 1950 ZLIB-style
+ blocks.
+
+5.7. Symmetrically Encrypted Data Packet (Tag 9)
+
+ The Symmetrically Encrypted Data packet contains data encrypted with
+ a symmetric-key algorithm. When it has been decrypted, it will
+ typically contain other packets (often literal data packets or
+ compressed data packets).
+
+ The body of this packet consists of:
+
+ - Encrypted data, the output of the selected symmetric-key cipher
+ operating in PGP's variant of Cipher Feedback (CFB) mode.
+
+ The symmetric cipher used may be specified in an Public-Key or
+ Symmetric-Key Encrypted Session Key packet that precedes the
+ Symmetrically Encrypted Data Packet. In that case, the cipher
+ algorithm octet 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.
+
+ The data is encrypted in CFB mode, with a CFB shift size equal to the
+ cipher's block size. The Initial Vector (IV) is specified as all
+ zeros. Instead of using an IV, OpenPGP prefixes a 10-octet string to
+ the data before it is encrypted. The first eight octets are random,
+ and the 9th and 10th octets are copies of the 7th and 8th octets,
+ respectively. After encrypting the first 10 octets, the CFB state is
+ resynchronized if the cipher block size is 8 octets or less. The
+ last 8 octets of ciphertext are passed through the cipher and the
+ block boundary is reset.
+
+ The repetition of 16 bits in the 80 bits of random data prefixed to
+ the message allows the receiver to immediately check whether the
+ session key is incorrect.
+
+5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)
+
+ An experimental version of PGP used this packet as the Literal
+ packet, but no released version of PGP generated Literal packets with
+ this tag. With PGP 5.x, this packet has been re-assigned and is
+ reserved for use as the Marker packet.
+
+
+
+Callas, et. al. Standards Track [Page 39]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ The body of this packet consists of:
+
+ - The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
+
+ Such a packet MUST be ignored when received. It may be placed at the
+ beginning of a message that uses features not available in PGP 2.6.x
+ in order to cause that version to report that newer software is
+ necessary to process the message.
+
+5.9. Literal Data Packet (Tag 11)
+
+ A Literal Data packet contains the body of a message; data that is
+ not to be further interpreted.
+
+ The body of this packet consists of:
+
+ - A one-octet field that describes how the data is formatted.
+
+ If it is a 'b' (0x62), then the literal packet contains binary data.
+ If it is a 't' (0x74), then it contains text data, and thus may need
+ line ends converted to local form, or other text-mode changes. RFC
+ 1991 also defined a value of 'l' as a 'local' mode for machine-local
+ conversions. This use is now deprecated.
+
+ - File name as a string (one-octet length, followed by file name),
+ if the encrypted data should be saved as a file.
+
+ If the special name "_CONSOLE" is used, 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.
+
+ - A four-octet number that indicates the modification date of the
+ file, or the creation time of the packet, or a zero that
+ indicates the present time.
+
+ - The remainder of the packet is literal data.
+
+ Text data is stored with <CR><LF> text endings (i.e. network-normal
+ line endings). These should be converted to native line endings by
+ the receiving software.
+
+5.10. Trust Packet (Tag 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 key holders are trustworthy introducers,
+
+
+
+Callas, et. al. Standards Track [Page 40]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ along with other information that implementing software uses for
+ trust information.
+
+ 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 (Tag 13)
+
+ A User ID packet consists of data that is intended to represent the
+ name and email address of the key holder. By convention, it includes
+ an RFC 822 mail name, but there are no restrictions on its content.
+ The packet length in the header specifies the length of the user id.
+ If it is text, it is encoded in UTF-8.
+
+6. Radix-64 Conversions
+
+ As stated in the introduction, OpenPGP's underlying native
+ 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 native OpenPGP data structures.
+ The OpenPGP standard specifies one such printable encoding scheme to
+ ensure interoperability.
+
+ OpenPGP's Radix-64 encoding is composed of two parts: a base64
+ encoding of the binary data, and a checksum. The base64 encoding is
+ identical to the MIME base64 content-transfer-encoding [RFC2231,
+ Section 6.8]. An OpenPGP implementation MAY use ASCII Armor to
+ protect the raw binary data.
+
+ The checksum is a 24-bit CRC converted to four characters of radix-64
+ encoding by the same MIME base64 transformation, preceded by an
+ equals 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 radix-64, rather than on the converted
+ data. A sample implementation of this algorithm is in the next
+ section.
+
+ The checksum with its leading equal sign MAY appear on the first line
+ after the Base64 encoded data.
+
+ Rationale for CRC-24: The size of 24 bits fits evenly into printable
+ base64. The nonzero initialization can detect more errors than a
+ zero initialization.
+
+
+
+Callas, et. al. Standards Track [Page 41]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+6.1. An Implementation of the CRC-24 in "C"
+
+ #define CRC24_INIT 0xb704ceL
+ #define CRC24_POLY 0x1864cfbL
+
+ typedef 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 ^= CRC24_POLY;
+ }
+ }
+ return crc & 0xffffffL;
+ }
+
+6.2. Forming ASCII Armor
+
+ When OpenPGP encodes data into ASCII Armor, it puts specific headers
+ around the data, so OpenPGP can reconstruct the data later. 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 Armor Checksum
+
+ - The Armor Tail, which depends on the 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 upon the type
+ of data that is being encoded in Armor, and how it is being encoded.
+ Header line texts include the following strings:
+
+
+
+Callas, et. al. Standards Track [Page 42]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ BEGIN PGP MESSAGE
+ Used for signed, encrypted, or compressed files.
+
+ BEGIN PGP PUBLIC KEY BLOCK
+ Used for armoring public keys
+
+ BEGIN PGP PRIVATE KEY BLOCK
+ Used for armoring private keys
+
+ BEGIN PGP MESSAGE, PART X/Y
+ Used for multi-part messages, where the armor is split amongst Y
+ parts, and this is the Xth part out of Y.
+
+ BEGIN PGP MESSAGE, PART X
+ Used for multi-part messages, where this is the Xth part of an
+ unspecified number of parts. Requires the MESSAGE-ID Armor Header
+ to be used.
+
+ BEGIN PGP SIGNATURE
+ Used for detached signatures, OpenPGP/MIME signatures, and
+ natures following clearsigned messages. Note that PGP 2.x s BEGIN
+ PGP MESSAGE for detached signatures.
+
+ 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 a
+ part of 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
+ (':' 0x38) and a single space (0x20) separate the key and value.
+ OpenPGP should consider improperly formatted Armor Headers to be
+ corruption of the ASCII Armor. Unknown keys should be reported to
+ the user, but OpenPGP should continue to process the message.
+
+ Currently defined Armor Header Keys are:
+
+ - "Version", that states the OpenPGP Version used to encode the
+ message.
+
+ - "Comment", a user-defined comment.
+
+ - "MessageID", a 32-character string of printable characters. The
+ string must be the same for all parts of a multi-part message
+ that uses the "PART X" Armor Header. MessageID strings should be
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 43]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ unique enough that the recipient of the mail can associate all
+ the parts of a message with each other. A good checksum or
+ cryptographic hash function is sufficient.
+
+ - "Hash", a comma-separated list of hash algorithms used in this
+ message. This is used only in clear-signed messages.
+
+ - "Charset", a description of the character set that the plaintext
+ is in. Please note that OpenPGP defines text to be in UTF-8 by
+ default. An implementation will get 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.
+
+ The MessageID SHOULD NOT appear unless it is in a multi-part
+ message. If it appears at all, it MUST be computed from the
+ finished (encrypted, signed, etc.) message in a deterministic
+ fashion, rather than contain a purely random value. This is to
+ allow the legitimate recipient to determine that the MessageID
+ cannot serve as a covert means of leaking cryptographic key
+ information.
+
+ 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."
+
+6.3. Encoding Binary in Radix-64
+
+ The encoding process represents 24-bit groups of input bits as output
+ strings of 4 encoded characters. Proceeding from left to right, a
+ 24-bit input group is formed by concatenating three 8-bit input
+ groups. These 24 bits are then treated as four concatenated 6-bit
+ groups, each of which is translated into a single digit in the
+ Radix-64 alphabet. When encoding a bit stream with the Radix-64
+ encoding, the bit stream must be presumed to be ordered with the
+ most-significant-bit first. That is, the first bit in the stream will
+ be the high-order bit in the first 8-bit octet, and the eighth bit
+ will be the low-order bit in the first 8-bit octet, and so on.
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 44]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ +--first octet--+-second octet--+--third octet--+
+ |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
+ +-----------+---+-------+-------+---+-----------+
+ |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
+ +--1.index--+--2.index--+--3.index--+--4.index--+
+
+ Each 6-bit group is used as an index into an array of 64 printable
+ characters from the table below. The character referenced by the
+ index is placed in the output string.
+
+ Value Encoding Value Encoding Value Encoding Value Encoding
+ 0 A 17 R 34 i 51 z
+ 1 B 18 S 35 j 52 0
+ 2 C 19 T 36 k 53 1
+ 3 D 20 U 37 l 54 2
+ 4 E 21 V 38 m 55 3
+ 5 F 22 W 39 n 56 4
+ 6 G 23 X 40 o 57 5
+ 7 H 24 Y 41 p 58 6
+ 8 I 25 Z 42 q 59 7
+ 9 J 26 a 43 r 60 8
+ 10 K 27 b 44 s 61 9
+ 11 L 28 c 45 t 62 +
+ 12 M 29 d 46 u 63 /
+ 13 N 30 e 47 v
+ 14 O 31 f 48 w (pad) =
+ 15 P 32 g 49 x
+ 16 Q 33 h 50 y
+
+ The encoded output stream must be represented in lines of no more
+ than 76 characters each.
+
+ Special processing is performed if fewer than 24 bits are available
+ at the end of the data being encoded. There are three possibilities:
+
+ 1. The last data group has 24 bits (3 octets). No special
+ processing is needed.
+
+ 2. The last data group has 16 bits (2 octets). The first two 6-bit
+ groups are processed as above. The third (incomplete) data group
+ has two zero-value bits added to it, and is processed as above.
+ A pad character (=) is added to the output.
+
+ 3. The last data group has 8 bits (1 octet). The first 6-bit group
+ is processed as above. The second (incomplete) data group has
+ four zero-value bits added to it, and is processed as above. Two
+ pad characters (=) are added to the output.
+
+
+
+
+Callas, et. al. Standards Track [Page 45]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+6.4. Decoding Radix-64
+
+ Any characters outside of the base64 alphabet are ignored in Radix-64
+ data. Decoding software must ignore all line breaks or other
+ characters not found in the table above.
+
+ In Radix-64 data, characters other than those in the table, line
+ breaks, and other white space probably indicate a transmission error,
+ about which a warning message or even a message rejection might be
+ appropriate under some circumstances.
+
+ Because it is used only for padding at the end of the data, the
+ occurrence of any "=" characters may be taken as evidence that the
+ end of the data has been reached (without truncation in transit). No
+ such assurance is possible, however, when the number of octets
+ transmitted was a multiple of three and no "=" characters are
+ present.
+
+6.5. Examples of Radix-64
+
+ Input data: 0x14fb9c03d97e
+ Hex: 1 4 f b 9 c | 0 3 d 9 7 e
+ 8-bit: 00010100 11111011 10011100 | 00000011 11011001
+ 11111110
+ 6-bit: 000101 001111 101110 011100 | 000000 111101 100111
+ 111110
+ Decimal: 5 15 46 28 0 61 37 62
+ Output: F P u c A 9 l +
+
+ Input data: 0x14fb9c03d9
+ Hex: 1 4 f b 9 c | 0 3 d 9
+ 8-bit: 00010100 11111011 10011100 | 00000011 11011001
+ pad with 00
+ 6-bit: 000101 001111 101110 011100 | 000000 111101 100100
+ Decimal: 5 15 46 28 0 61 36
+ pad with =
+ Output: F P u c A 9 k =
+
+ Input data: 0x14fb9c03
+ Hex: 1 4 f b 9 c | 0 3
+ 8-bit: 00010100 11111011 10011100 | 00000011
+ pad with 0000
+ 6-bit: 000101 001111 101110 011100 | 000000 110000
+ Decimal: 5 15 46 28 0 48
+ pad with = =
+ Output: F P u c A w = =
+
+
+
+
+
+Callas, et. al. Standards Track [Page 46]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+6.6. Example of an ASCII Armored Message
+
+
+ -----BEGIN PGP MESSAGE-----
+ Version: OpenPrivacy 0.99
+
+ yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
+ vBSFjNSiVHsuAA==
+ =njUN
+ -----END PGP MESSAGE-----
+
+ Note that this example is indented by two spaces.
+
+7. Cleartext signature framework
+
+ It is desirable to sign a textual octet stream without ASCII armoring
+ the stream itself, so the signed text is still readable without
+ special software. In order to bind a signature to such a cleartext,
+ this framework is used. (Note that RFC 2015 defines another way to
+ clear sign messages for environments that support MIME.)
+
+ The cleartext signed message consists of:
+
+ - The cleartext header '-----BEGIN PGP SIGNED MESSAGE-----' on a
+ single line,
+
+ - One or more "Hash" Armor Headers,
+
+ - Exactly one empty line not included into the message digest,
+
+ - The dash-escaped cleartext that is included into the message
+ digest,
+
+ - The ASCII armored signature(s) including the '-----BEGIN PGP
+ SIGNATURE-----' Armor Header and Armor Tail Lines.
+
+ If the "Hash" armor header is given, the specified message digest
+ algorithm is used for the signature. If there are no such headers,
+ MD5 is used, an implementation MAY omit them for V2.x compatibility.
+ If more than one message digest is used in the signature, the "Hash"
+ armor header contains a comma-delimited list of used message digests.
+
+ Current message digest names are described below with the algorithm
+ IDs.
+
+7.1. Dash-Escaped Text
+
+ The cleartext content of the message must also be dash-escaped.
+
+
+
+Callas, et. al. Standards Track [Page 47]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ Dash escaped cleartext is the ordinary cleartext where every line
+ starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
+ (0x2D) and space ' ' (0x20). This prevents the parser from
+ recognizing armor headers of the cleartext itself. The message digest
+ is computed using the cleartext itself, not the dash escaped form.
+
+ As with binary signatures on text documents, a cleartext signature is
+ calculated on the text using canonical <CR><LF> line endings. The
+ line ending (i.e. the <CR><LF>) before the '-----BEGIN PGP
+ SIGNATURE-----' line that terminates the signed text is not
+ considered part of the signed text.
+
+ Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of
+ any line is ignored when the cleartext signature is calculated.
+
+8. Regular Expressions
+
+ A regular expression is zero or more branches, separated by '|'. It
+ matches anything that matches one of the branches.
+
+ A branch is zero or more pieces, concatenated. It matches a match for
+ the first, followed by a match for the second, etc.
+
+ A piece is 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.
+
+ An atom is a regular expression in parentheses (matching a match for
+ the regular expression), a range (see below), '.' (matching any
+ single character), '^' (matching the null string at the beginning of
+ the input string), '$' (matching the null string at the end of the
+ input string), a '\' followed by a single character (matching that
+ character), or a single character with no other significance
+ (matching that character).
+
+ A range is a sequence of characters enclosed in '[]'. It normally
+ matches any single character from the sequence. If the sequence
+ begins with '^', it matches any single 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 ASCII characters between them
+ (e.g. '[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.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 48]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+9. Constants
+
+ This section describes the constants used in OpenPGP.
+
+ Note that these tables are not exhaustive lists; an implementation
+ MAY implement an algorithm not on these lists.
+
+ See the section "Notes on Algorithms" below for more discussion of
+ the algorithms.
+
+9.1. Public Key Algorithms
+
+ ID Algorithm
+ -- ---------
+ 1 - RSA (Encrypt or Sign)
+ 2 - RSA Encrypt-Only
+ 3 - RSA Sign-Only
+ 16 - Elgamal (Encrypt-Only), see [ELGAMAL]
+ 17 - DSA (Digital Signature Standard)
+ 18 - Reserved for Elliptic Curve
+ 19 - Reserved for ECDSA
+ 20 - Elgamal (Encrypt or Sign)
+
+
+
+
+
+ 21 - Reserved for Diffie-Hellman (X9.42,
+ as defined for IETF-S/MIME)
+ 100 to 110 - Private/Experimental algorithm.
+
+ Implementations MUST implement DSA for signatures, and Elgamal for
+ encryption. Implementations SHOULD implement RSA keys.
+ Implementations MAY implement any other algorithm.
+
+9.2. Symmetric Key Algorithms
+
+ ID Algorithm
+ -- ---------
+ 0 - Plaintext or unencrypted data
+ 1 - IDEA [IDEA]
+ 2 - Triple-DES (DES-EDE, as per spec -
+ 168 bit key derived from 192)
+ 3 - CAST5 (128 bit key, as per RFC 2144)
+ 4 - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
+ 5 - SAFER-SK128 (13 rounds) [SAFER]
+ 6 - Reserved for DES/SK
+ 7 - Reserved for AES with 128-bit key
+
+
+
+Callas, et. al. Standards Track [Page 49]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ 8 - Reserved for AES with 192-bit key
+ 9 - Reserved for AES with 256-bit key
+ 100 to 110 - Private/Experimental algorithm.
+
+ Implementations MUST implement Triple-DES. Implementations SHOULD
+ implement IDEA and CAST5.Implementations MAY implement any other
+ algorithm.
+
+9.3. Compression Algorithms
+
+ ID Algorithm
+ -- ---------
+ 0 - Uncompressed
+ 1 - ZIP (RFC 1951)
+ 2 - ZLIB (RFC 1950)
+ 100 to 110 - Private/Experimental algorithm.
+
+ Implementations MUST implement uncompressed data. Implementations
+ SHOULD implement ZIP. Implementations MAY implement ZLIB.
+
+9.4. Hash Algorithms
+
+ ID Algorithm Text Name
+ -- --------- ---- ----
+ 1 - MD5 "MD5"
+ 2 - SHA-1 "SHA1"
+ 3 - RIPE-MD/160 "RIPEMD160"
+ 4 - Reserved for double-width SHA (experimental)
+ 5 - MD2 "MD2"
+ 6 - Reserved for TIGER/192 "TIGER192"
+ 7 - Reserved for HAVAL (5 pass, 160-bit)
+ "HAVAL-5-160"
+ 100 to 110 - Private/Experimental algorithm.
+
+ Implementations MUST implement SHA-1. Implementations SHOULD
+ implement MD5.
+
+10. Packet 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 describes the rules for how packets
+ should be placed into sequences.
+
+10.1. Transferable Public Keys
+
+ OpenPGP users may transfer public keys. The essential elements of a
+ transferable public key are:
+
+
+
+Callas, et. al. Standards Track [Page 50]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ - One Public Key packet
+
+ - Zero or more revocation signatures
+
+ - One or more User ID packets
+
+ - After each User ID packet, zero or more signature packets
+ (certifications)
+
+ - Zero or more Subkey packets
+
+ - After each Subkey packet, one signature packet, optionally a
+ revocation.
+
+ The Public Key packet occurs first. 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.
+
+ 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 his or her
+ belief that this public key belongs to the user identified by this
+ user ID.
+
+ After the User ID packets there may be one or more Subkey packets.
+ In general, subkeys are provided in cases where the top-level public
+ key is a signature-only key. However, any V4 key may have subkeys,
+ and the subkeys may be encryption-only keys, signature-only keys, or
+ general-purpose keys.
+
+ Each Subkey packet must be followed by one Signature packet, which
+ should be a subkey binding signature issued by 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.
+
+ Transferable public key packet sequences may be concatenated to allow
+ transferring multiple public keys in one operation.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 51]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+10.2. OpenPGP Messages
+
+ An OpenPGP message is a packet or sequence of packets that
+ corresponds to the following grammatical rules (comma represents
+ sequential composition, and 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 Message :- Symmetrically Encrypted Data Packet |
+ ESK Sequence, Symmetrically Encrypted Data Packet.
+
+ One-Pass Signed Message :- One-Pass Signature Packet,
+ OpenPGP Message, Corresponding Signature Packet.
+
+ Signed Message :- Signature Packet, OpenPGP Message |
+ One-Pass Signed Message.
+
+ In addition, decrypting a Symmetrically Encrypted Data packet and
+
+ decompressing a Compressed Data packet must yield a valid OpenPGP
+ Message.
+
+10.3. 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 a signature packet stored separately from the
+ data that they are a signature of.
+
+11. Enhanced Key Formats
+
+11.1. Key Structures
+
+ The format of an OpenPGP V3 key is as follows. Entries in square
+ brackets are optional and ellipses indicate repetition.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 52]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ RSA Public Key
+ [Revocation Self Signature]
+ User ID [Signature ...]
+ [User ID [Signature ...] ...]
+
+ 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.
+
+ The format of an OpenPGP V4 key that uses two public keys is similar
+ except that the other keys are added to the end as 'subkeys' of the
+ primary key.
+
+ Primary-Key
+ [Revocation Self Signature]
+ [Direct Key Self Signature...]
+ User ID [Signature ...]
+ [User ID [Signature ...] ...]
+ [[Subkey [Binding-Signature-Revocation]
+ Primary-Key-Binding-Signature] ...]
+
+ A subkey always has a single signature after it that is issued using
+ the primary key to tie the two keys together. This binding signature
+ may be in either V3 or V4 format, but V4 is preferred, of course.
+
+ In the above diagram, if the binding signature of a subkey has been
+ revoked, the revoked binding signature may be removed, leaving only
+ one signature.
+
+ In a key that has a main key and subkeys, the primary key MUST be a
+ key capable of signing. The subkeys may be keys of any other type.
+ There may be other constructions of V4 keys, too. For example, there
+ may be a single-key RSA key in V4 format, a DSA primary key with an
+ RSA encryption key, or RSA primary key with an Elgamal subkey, etc.
+
+ It is also possible to have a signature-only subkey. This permits a
+ primary key that collects certifications (key signatures) but is used
+ only used for certifying subkeys that are used for encryption and
+ signatures.
+
+11.2. Key IDs and Fingerprints
+
+ For a V3 key, the eight-octet key ID consists of the low 64 bits of
+ the public modulus of the RSA key.
+
+ The fingerprint of a V3 key is formed by hashing the body (but not
+ the two-octet length) of the MPIs that form the key material (public
+ modulus n, followed by exponent e) with MD5.
+
+
+
+Callas, et. al. Standards Track [Page 53]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet
+ Tag, followed by the two-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, with the example of a DSA key:
+
+ a.1) 0x99 (1 octet)
+
+ a.2) high order length octet of (b)-(f) (1 octet)
+
+ a.3) low order length octet of (b)-(f) (1 octet)
+
+ b) version number = 4 (1 octet);
+
+ c) time stamp of key creation (4 octets);
+
+ d) algorithm (1 octet): 17 = DSA (example);
+
+ e) Algorithm specific fields.
+
+ Algorithm Specific Fields for DSA keys (example):
+
+ e.1) MPI of DSA prime p;
+
+ e.2) MPI of DSA group order q (q is a prime divisor of p-1);
+
+ e.3) MPI of DSA group generator g;
+
+ e.4) MPI of DSA public key value y (= g**x where x is secret).
+
+ Note that it is possible for there to be collisions of key IDs -- 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 V3 and V4 format keys share the same RSA key
+ material, they will have different key ids as well as different
+ fingerprints.
+
+12. Notes on Algorithms
+
+12.1. 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 different preferences. For
+ example, Alice may have TripleDES only specified for "alice@work.com"
+ but CAST5, Blowfish, and TripleDES specified for "alice@home.org".
+
+
+
+Callas, et. al. Standards Track [Page 54]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ Note that it is also possible for preferences to be in a subkey's
+ binding signature.
+
+ Since TripleDES is the MUST-implement algorithm, 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 a TripleDES-only implementation.
+
+ 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 the
+ MUST-implement algorithm, TripleDES, ensures that the intersection is
+ not null. The implementation may use any mechanism to pick an
+ algorithm in the intersection.
+
+ If an implementation can decrypt a message that a keyholder doesn't
+ have in their preferences, the implementation SHOULD decrypt the
+ message anyway, but MUST warn the keyholder than protocol has been
+ violated. (For example, suppose that Alice, above, has software 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 software warns her
+ that someone sent her an IDEA-encrypted message, but it would ideally
+ decrypt it anyway.)
+
+ An implementation that is striving for backward compatibility MAY
+ consider a V3 key with a V3 self-signature to be an implicit
+ preference for IDEA, and no ability to do TripleDES. This is
+ technically non-compliant, but an implementation MAY violate the
+ above rule in this case only and use IDEA to encrypt the message,
+ provided that the message creator is warned. Ideally, though, the
+ implementation would follow the rule by actually generating two
+ messages, because it is possible that the OpenPGP user's
+ implementation does not have IDEA, and thus could not read the
+ message. Consequently, an implementation MAY, but SHOULD NOT use IDEA
+ in an algorithm conflict with a V3 key.
+
+12.2. 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 that other comments need to
+ be made about, though, the compression preferences and the hash
+ preferences.
+
+
+
+
+
+Callas, et. al. Standards Track [Page 55]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+12.2.1. Compression Preferences
+
+ Compression has been an integral part of PGP since its first days.
+ OpenPGP and all previous versions of PGP have offered compression.
+ And in this specification, the default is for messages to be
+ compressed, although an implementation is not required to do so.
+ Consequently, the compression preference gives a way for a keyholder
+ to request that messages not be compressed, presumably because they
+ are using a minimal implementation that does not include compression.
+ Additionally, this gives a keyholder a way to state that it can
+ support alternate algorithms.
+
+ Like the algorithm preferences, an implementation MUST NOT use an
+ algorithm that is not in the preference vector. If the preferences
+ are not present, then they are assumed to be [ZIP(1),
+ UNCOMPRESSED(0)].
+
+12.2.2. Hash Algorithm Preferences
+
+ Typically, the choice of a hash algorithm is something the signer
+ does, rather than the verifier, because a signer does not typically
+ know who is going to be verifying the signature. This preference,
+ though, 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 software uses.
+ This preference allows Bob to state in his key which algorithms Alice
+ may use.
+
+12.3. Plaintext
+
+ Algorithm 0, "plaintext", may only be used to denote secret keys that
+ are stored in the clear. Implementations must not use plaintext in
+ Symmetrically Encrypted Data Packets; they must use Literal Data
+ Packets to encode unencrypted or literal data.
+
+12.4. RSA
+
+ There are algorithm types for RSA-signature-only, and RSA-encrypt-
+ only keys. These types are deprecated. The "key flags" subpacket in a
+ signature is a much better way to express the same idea, and
+ generalizes it to all algorithms. An implementation SHOULD NOT create
+ such a key, but MAY interpret it.
+
+ An implementation SHOULD NOT implement RSA keys of size less than 768
+ bits.
+
+
+
+
+Callas, et. al. Standards Track [Page 56]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ It is permissible for an implementation to support RSA merely for
+ backward compatibility; for example, such an implementation would
+ support V3 keys with IDEA symmetric cryptography. Note that this is
+ an exception to the other MUST-implement rules. An implementation
+ that supports RSA in V4 keys MUST implement the MUST-implement
+ features.
+
+12.5. Elgamal
+
+ If an Elgamal key is to be used for both signing and encryption,
+ extra care must be taken in creating the key.
+
+ An ElGamal key consists of a generator g, a prime modulus p, a secret
+ exponent x, and a public value y = g^x mod p.
+
+ The generator and prime must be chosen so that solving the discrete
+ log problem is intractable. The group g should generate the
+ multiplicative group mod p-1 or a large subgroup of it, and the order
+ of g should have at least one large prime factor. A good choice is
+ to use a "strong" Sophie-Germain prime in choosing p, so that both p
+ and (p-1)/2 are primes. In fact, this choice is so good that
+ implementors SHOULD do it, as it avoids a small subgroup attack.
+
+ In addition, a result of Bleichenbacher [BLEICHENBACHER] shows that
+ if the generator g has only small prime factors, and if g divides the
+ order of the group it generates, then signatures can be forged. In
+ particular, choosing g=2 is a bad choice if the group order may be
+ even. On the other hand, a generator of 2 is a fine choice for an
+ encryption-only key, as this will make the encryption faster.
+
+ While verifying Elgamal signatures, note that it is important to test
+ that r and s are less than p. If this test is not done then
+ signatures can be trivially forged by using large r values of
+ approximately twice the length of p. This attack is also discussed
+ in the Bleichenbacher paper.
+
+ Details on safe use of Elgamal signatures may be found in [MENEZES],
+ which discusses all the weaknesses described above.
+
+ If an implementation allows Elgamal signatures, then it MUST use the
+ algorithm identifier 20 for an Elgamal public key that can sign.
+
+ An implementation SHOULD NOT implement Elgamal keys of size less than
+ 768 bits. For long-term security, Elgamal keys should be 1024 bits or
+ longer.
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 57]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+12.6. DSA
+
+ An implementation SHOULD NOT implement DSA keys of size less than 768
+ bits. Note that present DSA is limited to a maximum of 1024 bit keys,
+ which are recommended for long-term use.
+
+12.7. Reserved Algorithm Numbers
+
+ 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 implementor from actually implementing the
+ algorithm. These are marked in the Public Algorithms section as
+ "(reserved for)".
+
+ The reserved public key algorithms, Elliptic Curve (18), ECDSA (19),
+ and X9.42 (21) do not have the necessary parameters, parameter order,
+ or semantics defined.
+
+ The reserved symmetric key algorithm, DES/SK (6), does not have
+ semantics defined.
+
+ The reserved hash algorithms, TIGER192 (6), and HAVAL-5-160 (7), do
+ not have OIDs. The reserved algorithm number 4, reserved for a
+ double-width variant of SHA1, is not presently defined.
+
+ We have reserver three algorithm IDs for the US NIST's Advanced
+ Encryption Standard. This algorithm will work with (at least) 128,
+ 192, and 256-bit keys. We expect that this algorithm will be selected
+ from the candidate algorithms in the year 2000.
+
+12.8. OpenPGP CFB mode
+
+ OpenPGP does symmetric encryption using a variant of Cipher Feedback
+ Mode (CFB mode). This section describes the procedure it uses in
+ detail. This mode is what is used for Symmetrically Encrypted Data
+ Packets; the mechanism used for encrypting secret key material is
+ similar, but described in those sections above.
+
+ OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
+ prefixes the plaintext with ten octets of random data, such that
+ octets 9 and 10 match octets 7 and 8. It does a CFB "resync" after
+ encrypting those ten octets.
+
+ Note that for an algorithm that has a larger block size than 64 bits,
+ the equivalent function will be done with that entire block. For
+ example, a 16-octet block algorithm would operate on 16 octets, and
+ then produce two octets of check, and then work on 16-octet blocks.
+
+
+
+
+Callas, et. al. Standards Track [Page 58]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ Step by step, here is the procedure:
+
+ 1. The feedback register (FR) is set to the IV, which is all zeros.
+
+ 2. FR is encrypted to produce FRE (FR Encrypted). This is the
+ encryption of an all-zero value.
+
+ 3. FRE is xored with the first 8 octets of random data prefixed to
+ the plaintext to produce C1-C8, the first 8 octets of ciphertext.
+
+ 4. FR is loaded with C1-C8.
+
+ 5. FR is encrypted to produce FRE, the encryption of the first 8
+ octets of ciphertext.
+
+ 6. The left two octets of FRE get xored with the next two octets of
+ data that were prefixed to the plaintext. This produces C9-C10,
+ the next two octets of ciphertext.
+
+ 7. (The resync step) FR is loaded with C3-C10.
+
+ 8. FR is encrypted to produce FRE.
+
+ 9. FRE is xored with the first 8 octets of the given plaintext, now
+ that we have finished encrypting the 10 octets of prefixed data.
+ This produces C11-C18, the next 8 octets of ciphertext.
+
+ 10. FR is loaded with C11-C18
+
+ 11. FR is encrypted to produce FRE.
+
+ 12. FRE is xored with the next 8 octets of plaintext, to produce the
+ next 8 octets of ciphertext. These are loaded into FR and the
+ process is repeated until the plaintext is used up.
+
+13. Security Considerations
+
+ As with any technology involving cryptography, you should check the
+ current literature to determine if any algorithms used here have been
+ found to be vulnerable to attack.
+
+ This specification uses Public Key Cryptography technologies.
+ Possession of the private key portion of a public-private key pair is
+ assumed to be controlled by the proper party or parties.
+
+ Certain operations in this specification involve the use of random
+ numbers. An appropriate entropy source should be used to generate
+ these numbers. See RFC 1750.
+
+
+
+Callas, et. al. Standards Track [Page 59]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ The MD5 hash algorithm has been found to have weaknesses (pseudo-
+ collisions in the compress function) that make some people deprecate
+ its use. They consider the SHA-1 algorithm better.
+
+ 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 V4 key
+ format with separate signature and encryption keys. If you as an
+ implementor promote dual-use keys, you should at least be aware of
+ this controversy.
+
+ The DSA algorithm will work with any 160-bit hash, but it is
+ sensitive to the quality of the hash algorithm, if the hash algorithm
+ is broken, it can leak the secret key. The Digital Signature Standard
+ (DSS) specifies that DSA be used with SHA-1. RIPEMD-160 is
+ considered by many cryptographers to be as strong. An implementation
+ should take care which hash algorithms are used with DSA, as a weak
+ hash can not only allow a signature to be forged, but could leak the
+ secret key. These same considerations about the quality of the hash
+ algorithm apply to Elgamal signatures.
+
+ 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 CAST5 is
+ presently considered strong, it has been analyzed less than Triple-
+ DES. Other algorithms may have other controversies surrounding them.
+
+ Some technologies mentioned here may be subject to government control
+ in some countries.
+
+14. Implementation Nits
+
+ This section is a collection of comments to help an implementer,
+ particularly with an eye to backward compatibility. Previous
+ implementations of PGP are not OpenPGP-compliant. Often the
+ differences are small, but small differences are frequently more
+ vexing than large differences. Thus, this list of potential problems
+ and gotchas for a developer who is trying to be backward-compatible.
+
+ * PGP 5.x does not accept V4 signatures for anything other than
+ key material.
+
+ * PGP 5.x does not recognize the "five-octet" lengths in new-format
+ headers or in signature subpacket lengths.
+
+
+
+Callas, et. al. Standards Track [Page 60]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ * PGP 5.0 rejects an encrypted session key if the keylength differs
+ from the S2K symmetric algorithm. This is a bug in its validation
+ function.
+
+ * PGP 5.0 does not handle multiple one-pass signature headers and
+ trailers. Signing one will compress the one-pass signed literal
+ and prefix a V3 signature instead of doing a nested one-pass
+ signature.
+
+ * When exporting a private key, PGP 2.x generates the header "BEGIN
+ PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".
+ All previous versions ignore the implied data type, and look
+ directly at the packet data type.
+
+ * In a clear-signed signature, PGP 5.0 will figure out the correct
+ hash algorithm if there is no "Hash:" header, but it will reject
+ a mismatch between the header and the actual algorithm used. The
+ "standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x
+ rejects the "Hash:" header and assumes MD5. There are a number of
+ enhanced variants of PGP 2.6.x that have been modified for SHA-1
+ signatures.
+
+ * PGP 5.0 can read an RSA key in V4 format, but can only recognize
+ it with a V3 keyid, and can properly use only a V3 format RSA
+ key.
+
+ * Neither PGP 5.x nor PGP 6.0 recognize Elgamal Encrypt and Sign
+ keys. They only handle Elgamal Encrypt-only keys.
+
+ * There are many ways possible for two keys to have the same key
+ material, but different fingerprints (and thus key ids). Perhaps
+ the most interesting is an RSA key that has been "upgraded" to V4
+ format, but since a V4 fingerprint is constructed by hashing the
+ key creation time along with other things, two V4 keys created at
+ different times, yet with the same key material will have
+ different fingerprints.
+
+ * If an implementation is using zlib to interoperate with PGP 2.x,
+ then the "windowBits" parameter should be set to -13.
+
+
+
+
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 61]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+15. Authors and Working Group Chair
+
+ The working group can be contacted via the current chair:
+
+ John W. Noerenberg, II
+ Qualcomm, Inc
+ 6455 Lusk Blvd
+ San Diego, CA 92131 USA
+
+ Phone: +1 619-658-3510
+ EMail: jwn2@qualcomm.com
+
+
+ The principal authors of this memo are:
+
+ Jon Callas
+ Network Associates, Inc.
+ 3965 Freedom Circle
+ Santa Clara, CA 95054, USA
+
+ Phone: +1 408-346-5860
+ EMail: jon@pgp.com, jcallas@nai.com
+
+
+ Lutz Donnerhacke
+ IKS GmbH
+ Wildenbruchstr. 15
+ 07745 Jena, Germany
+
+ Phone: +49-3641-675642
+ EMail: lutz@iks-jena.de
+
+
+ Hal Finney
+ Network Associates, Inc.
+ 3965 Freedom Circle
+ Santa Clara, CA 95054, USA
+
+ EMail: hal@pgp.com
+
+
+ Rodney Thayer
+ EIS Corporation
+ Clearwater, FL 33767, USA
+
+ EMail: rodney@unitran.com
+
+
+
+
+
+Callas, et. al. Standards Track [Page 62]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ This memo also draws on much previous work from a number of other
+ authors who include: Derek Atkins, Charles Breed, Dave Del Torto,
+ Marc Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Raph
+ Levien, Colin Plumb, Will Price, William Stallings, Mark Weaver, and
+ Philip R. Zimmermann.
+
+16. References
+
+ [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating ElGamal
+ signatures without knowing the secret key,"
+ Eurocrypt 96. Note that the version in the
+ proceedings has an error. A revised version is
+ available at the time of writing from
+ <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti/isc
+ /ElGamal.ps>
+
+ [BLOWFISH] Schneier, B. "Description of a New Variable-Length
+ Key, 64-Bit Block Cipher (Blowfish)" Fast Software
+ Encryption, Cambridge Security Workshop Proceedings
+ (December 1993), Springer-Verlag, 1994, pp191-204
+
+ <http://www.counterpane.com/bfsverlag.html>
+
+ [DONNERHACKE] Donnerhacke, L., et. al, "PGP263in - an improved
+ international version of PGP", ftp://ftp.iks-
+ jena.de/mitarb/lutz/crypt/software/pgp/
+
+ [ELGAMAL] T. ElGamal, "A Public-Key Cryptosystem and a
+ Signature Scheme Based on Discrete Logarithms," IEEE
+ Transactions on Information Theory, v. IT-31, n. 4,
+ 1985, pp. 469-472.
+
+ [IDEA] Lai, X, "On the design and security of block
+ ciphers", ETH Series in Information Processing, J.L.
+ Massey (editor), Vol. 1, Hartung-Gorre Verlag
+ Knostanz, Technische Hochschule (Zurich), 1992
+
+ [ISO-10646] ISO/IEC 10646-1:1993. International Standard --
+ Information technology -- Universal Multiple-Octet
+ Coded Character Set (UCS) -- Part 1: Architecture
+ and Basic Multilingual Plane. UTF-8 is described in
+ Annex R, adopted but not yet published. UTF-16 is
+ described in Annex Q, adopted but not yet published.
+
+ [MENEZES] Alfred Menezes, Paul van Oorschot, and Scott
+ Vanstone, "Handbook of Applied Cryptography," CRC
+ Press, 1996.
+
+
+
+
+Callas, et. al. Standards Track [Page 63]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+ [RFC822] Crocker, D., "Standard for the format of ARPA
+ Internet text messages", STD 11, RFC 822, August
+ 1982.
+
+ [RFC1423] Balenson, D., "Privacy Enhancement for Internet
+ Electronic Mail: Part III: Algorithms, Modes, and
+ Identifiers", RFC 1423, October 1993.
+
+ [RFC1641] Goldsmith, D. and M. Davis, "Using Unicode with
+ MIME", RFC 1641, July 1994.
+
+ [RFC1750] Eastlake, D., Crocker, S. and J. Schiller,
+ "Randomness Recommendations for Security", RFC 1750,
+ December 1994.
+
+ [RFC1951] Deutsch, P., "DEFLATE Compressed Data Format
+ Specification version 1.3.", RFC 1951, May 1996.
+
+ [RFC1983] Malkin, G., "Internet Users' Glossary", FYI 18, RFC
+ 1983, August 1996.
+
+ [RFC1991] Atkins, D., Stallings, W. and P. Zimmermann, "PGP
+ Message Exchange Formats", RFC 1991, August 1996.
+
+ [RFC2015] Elkins, M., "MIME Security with Pretty Good Privacy
+ (PGP)", RFC 2015, October 1996.
+
+ [RFC2231] Borenstein, N. and N. Freed, "Multipurpose Internet
+ Mail Extensions (MIME) Part One: Format of Internet
+ Message Bodies.", RFC 2231, November 1996.
+
+ [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Level", BCP 14, RFC 2119, March 1997.
+
+ [RFC2144] Adams, C., "The CAST-128 Encryption Algorithm", RFC
+ 2144, May 1997.
+
+ [RFC2279] Yergeau., F., "UTF-8, a transformation format of
+ Unicode and ISO 10646", RFC 2279, January 1998.
+
+ [RFC2313] Kaliski, B., "PKCS #1: RSA Encryption Standard
+ version 1.5", RFC 2313, March 1998.
+
+ [SAFER] Massey, J.L. "SAFER K-64: One Year Later", B.
+ Preneel, editor, Fast Software Encryption, Second
+ International Workshop (LNCS 1008) pp212-241,
+ Springer-Verlag 1995
+
+
+
+
+Callas, et. al. Standards Track [Page 64]
+
+RFC 2440 OpenPGP Message Format November 1998
+
+
+17. Full Copyright Statement
+
+ Copyright (C) The Internet Society (1998). All Rights Reserved.
+
+ This document and translations of it may be copied and furnished to
+ others, and derivative works that comment on or otherwise explain it
+ or assist in its implementation may be prepared, copied, published
+ and distributed, in whole or in part, without restriction of any
+ kind, provided that the above copyright notice and this paragraph are
+ included on all such copies and derivative works. However, this
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required to translate it into languages other than
+ English.
+
+ The limited permissions granted above are perpetual and will not be
+ revoked by the Internet Society or its successors or assigns.
+
+ This document and the information contained herein is provided on an
+ "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+ TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+ BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Callas, et. al. Standards Track [Page 65]
+