path: root/doc/rfc/rfc8422.txt

Internet Engineering Task Force (IETF)                            Y. Nir
Request for Comments: 8422                                   Check Point
Obsoletes: 4492                                             S. Josefsson
Category: Standards Track                                         SJD AB
ISSN: 2070-1721                                      M. Pegourie-Gonnard
                                                                     ARM
                                                             August 2018


            Elliptic Curve Cryptography (ECC) Cipher Suites
      for Transport Layer Security (TLS) Versions 1.2 and Earlier

Abstract

   This document describes key exchange algorithms based on Elliptic
   Curve Cryptography (ECC) for the Transport Layer Security (TLS)
   protocol.  In particular, it specifies the use of Ephemeral Elliptic
   Curve Diffie-Hellman (ECDHE) key agreement in a TLS handshake and the
   use of the Elliptic Curve Digital Signature Algorithm (ECDSA) and
   Edwards-curve Digital Signature Algorithm (EdDSA) as authentication
   mechanisms.

   This document obsoletes RFC 4492.

Status of This Memo

   This is an Internet Standards Track document.

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

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8422.














Nir, et al.                  Standards Track                    [Page 1]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































Nir, et al.                  Standards Track                    [Page 2]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
   2.  Key Exchange Algorithm  . . . . . . . . . . . . . . . . . . .   4
     2.1.  ECDHE_ECDSA . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  ECDHE_RSA . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  ECDH_anon . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Algorithms in Certificate Chains  . . . . . . . . . . . .   7
   3.  Client Authentication . . . . . . . . . . . . . . . . . . . .   8
     3.1.  ECDSA_sign  . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  TLS Extensions for ECC  . . . . . . . . . . . . . . . . . . .   9
   5.  Data Structures and Computations  . . . . . . . . . . . . . .  10
     5.1.  Client Hello Extensions . . . . . . . . . . . . . . . . .  10
       5.1.1.  Supported Elliptic Curves Extension . . . . . . . . .  11
       5.1.2.  Supported Point Formats Extension . . . . . . . . . .  13
       5.1.3.  The signature_algorithms Extension and EdDSA  . . . .  13
     5.2.  Server Hello Extension  . . . . . . . . . . . . . . . . .  14
     5.3.  Server Certificate  . . . . . . . . . . . . . . . . . . .  15
     5.4.  Server Key Exchange . . . . . . . . . . . . . . . . . . .  16
       5.4.1.  Uncompressed Point Format for NIST Curves . . . . . .  19
     5.5.  Certificate Request . . . . . . . . . . . . . . . . . . .  20
     5.6.  Client Certificate  . . . . . . . . . . . . . . . . . . .  21
     5.7.  Client Key Exchange . . . . . . . . . . . . . . . . . . .  22
     5.8.  Certificate Verify  . . . . . . . . . . . . . . . . . . .  23
     5.9.  Elliptic Curve Certificates . . . . . . . . . . . . . . .  24
     5.10. ECDH, ECDSA, and RSA Computations . . . . . . . . . . . .  24
     5.11. Public Key Validation . . . . . . . . . . . . . . . . . .  26
   6.  Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . .  26
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  27
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     10.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Equivalent Curves (Informative)  . . . . . . . . . .  32
   Appendix B.  Differences from RFC 4492  . . . . . . . . . . . . .  33
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34












Nir, et al.                  Standards Track                    [Page 3]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


1.  Introduction

   This document describes additions to TLS to support ECC that are
   applicable to TLS versions 1.0 [RFC2246], 1.1 [RFC4346], and 1.2
   [RFC5246].  The use of ECC in TLS 1.3 is defined in [TLS1.3] and is
   explicitly out of scope for this document.  In particular, this
   document defines:

   o  the use of the ECDHE key agreement scheme with ephemeral keys to
      establish the TLS premaster secret, and

   o  the use of ECDSA and EdDSA signatures for authentication of TLS
      peers.

   The remainder of this document is organized as follows.  Section 2
   provides an overview of ECC-based key exchange algorithms for TLS.
   Section 3 describes the use of ECC certificates for client
   authentication.  TLS extensions that allow a client to negotiate the
   use of specific curves and point formats are presented in Section 4.
   Section 5 specifies various data structures needed for an ECC-based
   handshake, their encoding in TLS messages, and the processing of
   those messages.  Section 6 defines ECC-based cipher suites and
   identifies a small subset of these as recommended for all
   implementations of this specification.  Section 8 discusses security
   considerations.  Section 9 describes IANA considerations for the name
   spaces created by this document's predecessor.  Appendix B provides
   differences from [RFC4492], the document that this one replaces.

   Implementation of this specification requires familiarity with TLS,
   TLS extensions [RFC4366], and ECC.

1.1.  Conventions Used in This Document

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

2.  Key Exchange Algorithm

   This document defines three new ECC-based key exchange algorithms for
   TLS.  All of them use Ephemeral ECDH (ECDHE) to compute the TLS
   premaster secret, and they differ only in the mechanism (if any) used
   to authenticate them.  The derivation of the TLS master secret from
   the premaster secret and the subsequent generation of bulk
   encryption/MAC keys and initialization vectors is independent of the
   key exchange algorithm and not impacted by the introduction of ECC.



Nir, et al.                  Standards Track                    [Page 4]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Table 1 summarizes the new key exchange algorithms.  All of these key
   exchange algorithms provide forward secrecy if and only if fresh
   ephemeral keys are generated and used, and also destroyed after use.

     +-------------+------------------------------------------------+
     | Algorithm   | Description                                    |
     +-------------+------------------------------------------------+
     | ECDHE_ECDSA | Ephemeral ECDH with ECDSA or EdDSA signatures. |
     | ECDHE_RSA   | Ephemeral ECDH with RSA signatures.            |
     | ECDH_anon   | Anonymous ephemeral ECDH, no signatures.       |
     +-------------+------------------------------------------------+

                   Table 1: ECC Key Exchange Algorithms

   These key exchanges are analogous to DHE_DSS, DHE_RSA, and DH_anon,
   respectively.

   With ECDHE_RSA, a server can reuse its existing RSA certificate and
   easily comply with a constrained client's elliptic curve preferences
   (see Section 4).  However, the computational cost incurred by a
   server is higher for ECDHE_RSA than for the traditional RSA key
   exchange, which does not provide forward secrecy.

   The anonymous key exchange algorithm does not provide authentication
   of the server or the client.  Like other anonymous TLS key exchanges,
   it is subject to man-in-the-middle attacks.  Applications using TLS
   with this algorithm SHOULD provide authentication by other means.
























Nir, et al.                  Standards Track                    [Page 5]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


          Client                                        Server
          ------                                        ------
          ClientHello          -------->
                                                   ServerHello
                                                  Certificate*
                                            ServerKeyExchange*
                                          CertificateRequest*+
                               <--------       ServerHelloDone
          Certificate*+
          ClientKeyExchange
          CertificateVerify*+
          [ChangeCipherSpec]
          Finished             -------->
                                            [ChangeCipherSpec]
                               <--------              Finished
          Application Data     <------->      Application Data

               * message is not sent under some conditions
               + message is not sent unless client authentication
                 is desired

            Figure 1: Message Flow in a Full TLS 1.2 Handshake

   Figure 1 shows all messages involved in the TLS key establishment
   protocol (aka full handshake).  The addition of ECC has direct impact
   only on the ClientHello, the ServerHello, the server's Certificate
   message, the ServerKeyExchange, the ClientKeyExchange, the
   CertificateRequest, the client's Certificate message, and the
   CertificateVerify.  Next, we describe the ECC key exchange algorithm
   in greater detail in terms of the content and processing of these
   messages.  For ease of exposition, we defer discussion of client
   authentication and associated messages (identified with a '+' in
   Figure 1) until Section 3 and of the optional ECC-specific extensions
   (which impact the Hello messages) until Section 4.

2.1.  ECDHE_ECDSA

   In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA- or
   EdDSA-capable public key.

   The server sends its ephemeral ECDH public key and a specification of
   the corresponding curve in the ServerKeyExchange message.  These
   parameters MUST be signed with ECDSA or EdDSA using the private key
   corresponding to the public key in the server's Certificate.

   The client generates an ECDH key pair on the same curve as the
   server's ephemeral ECDH key and sends its public key in the
   ClientKeyExchange message.



Nir, et al.                  Standards Track                    [Page 6]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Both client and server perform an ECDH operation (see Section 5.10)
   and use the resultant shared secret as the premaster secret.

2.2.  ECDHE_RSA

   This key exchange algorithm is the same as ECDHE_ECDSA except that
   the server's certificate MUST contain an RSA public key authorized
   for signing and the signature in the ServerKeyExchange message must
   be computed with the corresponding RSA private key.

2.3.  ECDH_anon

   NOTE: Despite the name beginning with "ECDH_" (no E), the key used in
   ECDH_anon is ephemeral just like the key in ECDHE_RSA and
   ECDHE_ECDSA.  The naming follows the example of DH_anon, where the
   key is also ephemeral but the name does not reflect it.

   In ECDH_anon, the server's Certificate, the CertificateRequest, the
   client's Certificate, and the CertificateVerify messages MUST NOT be
   sent.

   The server MUST send an ephemeral ECDH public key and a specification
   of the corresponding curve in the ServerKeyExchange message.  These
   parameters MUST NOT be signed.

   The client generates an ECDH key pair on the same curve as the
   server's ephemeral ECDH key and sends its public key in the
   ClientKeyExchange message.

   Both client and server perform an ECDH operation and use the
   resultant shared secret as the premaster secret.  All ECDH
   calculations are performed as specified in Section 5.10.

2.4.  Algorithms in Certificate Chains

   This specification does not impose restrictions on signature schemes
   used anywhere in the certificate chain.  The previous version of this
   document required the signatures to match, but this restriction,
   originating in previous TLS versions, is lifted here as it had been
   in RFC 5246.











Nir, et al.                  Standards Track                    [Page 7]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


3.  Client Authentication

   This document defines a client authentication mechanism named after
   the type of client certificate involved: ECDSA_sign.  The ECDSA_sign
   mechanism is usable with any of the non-anonymous ECC key exchange
   algorithms described in Section 2 as well as other non-anonymous
   (non-ECC) key exchange algorithms defined in TLS.

   Note that client certificates with EdDSA public keys also use this
   mechanism.

   The server can request ECC-based client authentication by including
   this certificate type in its CertificateRequest message.  The client
   must check if it possesses a certificate appropriate for the method
   suggested by the server and is willing to use it for authentication.

   If these conditions are not met, the client SHOULD send a client
   Certificate message containing no certificates.  In this case, the
   ClientKeyExchange MUST be sent as described in Section 2, and the
   CertificateVerify MUST NOT be sent.  If the server requires client
   authentication, it may respond with a fatal handshake failure alert.

   If the client has an appropriate certificate and is willing to use it
   for authentication, it must send that certificate in the client's
   Certificate message (as per Section 5.6) and prove possession of the
   private key corresponding to the certified key.  The process of
   determining an appropriate certificate and proving possession is
   different for each authentication mechanism and is described below.

   NOTE: It is permissible for a server to request (and the client to
   send) a client certificate of a different type than the server
   certificate.

3.1.  ECDSA_sign

   To use this authentication mechanism, the client MUST possess a
   certificate containing an ECDSA- or EdDSA-capable public key.

   The client proves possession of the private key corresponding to the
   certified key by including a signature in the CertificateVerify
   message as described in Section 5.8.










Nir, et al.                  Standards Track                    [Page 8]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


4.  TLS Extensions for ECC

   Two TLS extensions are defined in this specification: (i) the
   Supported Elliptic Curves Extension and (ii) the Supported Point
   Formats Extension.  These allow negotiating the use of specific
   curves and point formats (e.g., compressed vs. uncompressed,
   respectively) during a handshake starting a new session.  These
   extensions are especially relevant for constrained clients that may
   only support a limited number of curves or point formats.  They
   follow the general approach outlined in [RFC4366]; message details
   are specified in Section 5.  The client enumerates the curves it
   supports and the point formats it can parse by including the
   appropriate extensions in its ClientHello message.  The server
   similarly enumerates the point formats it can parse by including an
   extension in its ServerHello message.

   A TLS client that proposes ECC cipher suites in its ClientHello
   message SHOULD include these extensions.  Servers implementing ECC
   cipher suites MUST support these extensions, and when a client uses
   these extensions, servers MUST NOT negotiate the use of an ECC cipher
   suite unless they can complete the handshake while respecting the
   choice of curves specified by the client.  This eliminates the
   possibility that a negotiated ECC handshake will be subsequently
   aborted due to a client's inability to deal with the server's EC key.

   The client MUST NOT include these extensions in the ClientHello
   message if it does not propose any ECC cipher suites.  A client that
   proposes ECC cipher suites may choose not to include these
   extensions.  In this case, the server is free to choose any one of
   the elliptic curves or point formats listed in Section 5.  That
   section also describes the structure and processing of these
   extensions in greater detail.

   In the case of session resumption, the server simply ignores the
   Supported Elliptic Curves Extension and the Supported Point Formats
   Extension appearing in the current ClientHello message.  These
   extensions only play a role during handshakes negotiating a new
   session.













Nir, et al.                  Standards Track                    [Page 9]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


5.  Data Structures and Computations

   This section specifies the data structures and computations used by
   ECC-based key mechanisms specified in the previous three sections.
   The presentation language used here is the same as that used in TLS.
   Since this specification extends TLS, these descriptions should be
   merged with those in the TLS specification and any others that extend
   TLS.  This means that enum types may not specify all possible values,
   and structures with multiple formats chosen with a select() clause
   may not indicate all possible cases.

5.1.  Client Hello Extensions

   This section specifies two TLS extensions that can be included with
   the ClientHello message as described in [RFC4366]: the Supported
   Elliptic Curves Extension and the Supported Point Formats Extension.

   When these extensions are sent:

   The extensions SHOULD be sent along with any ClientHello message that
   proposes ECC cipher suites.

   Meaning of these extensions:

   These extensions allow a client to enumerate the elliptic curves it
   supports and/or the point formats it can parse.

   Structure of these extensions:

   The general structure of TLS extensions is described in [RFC4366],
   and this specification adds two types to ExtensionType.

      enum {
          elliptic_curves(10),
          ec_point_formats(11)
      } ExtensionType;

   o  elliptic_curves (Supported Elliptic Curves Extension): Indicates
      the set of elliptic curves supported by the client.  For this
      extension, the opaque extension_data field contains
      NamedCurveList.  See Section 5.1.1 for details.

   o  ec_point_formats (Supported Point Formats Extension): Indicates
      the set of point formats that the client can parse.  For this
      extension, the opaque extension_data field contains
      ECPointFormatList.  See Section 5.1.2 for details.





Nir, et al.                  Standards Track                   [Page 10]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Actions of the sender:

   A client that proposes ECC cipher suites in its ClientHello message
   appends these extensions (along with any others), enumerating the
   curves it supports and the point formats it can parse.  Clients
   SHOULD send both the Supported Elliptic Curves Extension and the
   Supported Point Formats Extension.  If the Supported Point Formats
   Extension is indeed sent, it MUST contain the value 0 (uncompressed)
   as one of the items in the list of point formats.

   Actions of the receiver:

   A server that receives a ClientHello containing one or both of these
   extensions MUST use the client's enumerated capabilities to guide its
   selection of an appropriate cipher suite.  One of the proposed ECC
   cipher suites must be negotiated only if the server can successfully
   complete the handshake while using the curves and point formats
   supported by the client (cf. Sections 5.3 and 5.4).

   NOTE: A server participating in an ECDHE_ECDSA key exchange may use
   different curves for the ECDSA or EdDSA key in its certificate and
   for the ephemeral ECDH key in the ServerKeyExchange message.  The
   server MUST consider the extensions in both cases.

   If a server does not understand the Supported Elliptic Curves
   Extension, does not understand the Supported Point Formats Extension,
   or is unable to complete the ECC handshake while restricting itself
   to the enumerated curves and point formats, it MUST NOT negotiate the
   use of an ECC cipher suite.  Depending on what other cipher suites
   are proposed by the client and supported by the server, this may
   result in a fatal handshake failure alert due to the lack of common
   cipher suites.

5.1.1.  Supported Elliptic Curves Extension

   RFC 4492 defined 25 different curves in the NamedCurve registry (now
   renamed the "TLS Supported Groups" registry, although the enumeration
   below is still named NamedCurve) for use in TLS.  Only three have
   seen much use.  This specification is deprecating the rest (with
   numbers 1-22).  This specification also deprecates the explicit











Nir, et al.                  Standards Track                   [Page 11]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   curves with identifiers 0xFF01 and 0xFF02.  It also adds the new
   curves defined in [RFC7748].  The end result is as follows:

           enum {
               deprecated(1..22),
               secp256r1 (23), secp384r1 (24), secp521r1 (25),
               x25519(29), x448(30),
               reserved (0xFE00..0xFEFF),
               deprecated(0xFF01..0xFF02),
               (0xFFFF)
           } NamedCurve;

   Note that other specifications have since added other values to this
   enumeration.  Some of those values are not curves at all, but finite
   field groups.  See [RFC7919].

   secp256r1, etc: Indicates support of the corresponding named curve or
   groups.  The named curves secp256r1, secp384r1, and secp521r1 are
   specified in SEC 2 [SECG-SEC2].  These curves are also recommended in
   ANSI X9.62 [ANSI.X9-62.2005] and FIPS 186-4 [FIPS.186-4].  The rest
   of this document refers to these three curves as the "NIST curves"
   because they were originally standardized by the National Institute
   of Standards and Technology.  The curves x25519 and x448 are defined
   in [RFC7748].  Values 0xFE00 through 0xFEFF are reserved for private
   use.

   The predecessor of this document also supported explicitly defined
   prime and char2 curves, but these are deprecated by this
   specification.

   The NamedCurve name space (now titled "TLS Supported Groups") is
   maintained by IANA.  See Section 9 for information on how new value
   assignments are added.

           struct {
               NamedCurve named_curve_list<2..2^16-1>
           } NamedCurveList;

   Items in named_curve_list are ordered according to the client's
   preferences (favorite choice first).

   As an example, a client that only supports secp256r1 (aka NIST P-256;
   value 23 = 0x0017) and secp384r1 (aka NIST P-384; value 24 = 0x0018)
   and prefers to use secp256r1 would include a TLS extension consisting
   of the following octets.  Note that the first two octets indicate the
   extension type (Supported Elliptic Curves Extension):

           00 0A 00 06 00 04 00 17 00 18



Nir, et al.                  Standards Track                   [Page 12]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


5.1.2.  Supported Point Formats Extension

           enum {
               uncompressed (0),
               deprecated (1..2),
               reserved (248..255)
           } ECPointFormat;
           struct {
               ECPointFormat ec_point_format_list<1..2^8-1>
           } ECPointFormatList;

   Three point formats were included in the definition of ECPointFormat
   above.  This specification deprecates all but the uncompressed point
   format.  Implementations of this document MUST support the
   uncompressed format for all of their supported curves and MUST NOT
   support other formats for curves defined in this specification.  For
   backwards compatibility purposes, the point format list extension MAY
   still be included and contain exactly one value: the uncompressed
   point format (0).  RFC 4492 specified that if this extension is
   missing, it means that only the uncompressed point format is
   supported, so interoperability with implementations that support the
   uncompressed format should work with or without the extension.

   If the client sends the extension and the extension does not contain
   the uncompressed point format, and the client has used the Supported
   Groups extension to indicate support for any of the curves defined in
   this specification, then the server MUST abort the handshake and
   return an illegal_parameter alert.

   The ECPointFormat name space (now titled "TLS EC Point Formats") is
   maintained by IANA.  See Section 9 for information on how new value
   assignments are added.

   A client compliant with this specification that supports no other
   curves MUST send the following octets; note that the first two octets
   indicate the extension type (Supported Point Formats Extension):

           00 0B 00 02 01 00

5.1.3.  The signature_algorithms Extension and EdDSA

   The signature_algorithms extension, defined in Section 7.4.1.4.1 of
   [RFC5246], advertises the combinations of signature algorithm and
   hash function that the client supports.  The pure (non-prehashed)
   forms of EdDSA do not hash the data before signing it.  For this
   reason, it does not make sense to combine them with a hash function
   in the extension.




Nir, et al.                  Standards Track                   [Page 13]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   For bits-on-the-wire compatibility with TLS 1.3, we define a new
   dummy value in the "TLS HashAlgorithm" registry that we call
   "Intrinsic" (value 8), meaning that hashing is intrinsic to the
   signature algorithm.

   To represent ed25519 and ed448 in the signature_algorithms extension,
   the value shall be (8,7) and (8,8), respectively.

5.2.  Server Hello Extension

   This section specifies a TLS extension that can be included with the
   ServerHello message as described in [RFC4366], the Supported Point
   Formats Extension.

   When this extension is sent:

   The Supported Point Formats Extension is included in a ServerHello
   message in response to a ClientHello message containing the Supported
   Point Formats Extension when negotiating an ECC cipher suite.

   Meaning of this extension:

   This extension allows a server to enumerate the point formats it can
   parse (for the curve that will appear in its ServerKeyExchange
   message when using the ECDHE_ECDSA, ECDHE_RSA, or ECDH_anon key
   exchange algorithm.

   Structure of this extension:

   The server's Supported Point Formats Extension has the same structure
   as the client's Supported Point Formats Extension (see
   Section 5.1.2).  Items in ec_point_format_list here are ordered
   according to the server's preference (favorite choice first).  Note
   that the server MAY include items that were not found in the client's
   list.  However, without extensions, this specification allows exactly
   one point format, so there is not really any opportunity for
   mismatches.

   Actions of the sender:

   A server that selects an ECC cipher suite in response to a
   ClientHello message including a Supported Point Formats Extension
   appends this extension (along with others) to its ServerHello
   message, enumerating the point formats it can parse.  The Supported
   Point Formats Extension, when used, MUST contain the value 0
   (uncompressed) as one of the items in the list of point formats.





Nir, et al.                  Standards Track                   [Page 14]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Actions of the receiver:

   A client that receives a ServerHello message containing a Supported
   Point Formats Extension MUST respect the server's choice of point
   formats during the handshake (cf.  Sections 5.6 and 5.7).  If no
   Supported Point Formats Extension is received with the ServerHello,
   this is equivalent to an extension allowing only the uncompressed
   point format.

5.3.  Server Certificate

   When this message is sent:

   This message is sent in all non-anonymous, ECC-based key exchange
   algorithms.

   Meaning of this message:

   This message is used to authentically convey the server's static
   public key to the client.  The following table shows the server
   certificate type appropriate for each key exchange algorithm.  ECC
   public keys MUST be encoded in certificates as described in
   Section 5.9.

   NOTE: The server's Certificate message is capable of carrying a chain
   of certificates.  The restrictions mentioned in Table 2 apply only to
   the server's certificate (first in the chain).

   +-------------+-----------------------------------------------------+
   | Algorithm   | Server Certificate Type                             |
   +-------------+-----------------------------------------------------+
   | ECDHE_ECDSA | Certificate MUST contain an ECDSA- or EdDSA-capable |
   |             | public key.                                         |
   | ECDHE_RSA   | Certificate MUST contain an RSA public key.         |
   +-------------+-----------------------------------------------------+

                     Table 2: Server Certificate Types

   Structure of this message:

   Identical to the TLS Certificate format.

   Actions of the sender:

   The server constructs an appropriate certificate chain and conveys it
   to the client in the Certificate message.  If the client has used a
   Supported Elliptic Curves Extension, the public key in the server's




Nir, et al.                  Standards Track                   [Page 15]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   certificate MUST respect the client's choice of elliptic curves.  A
   server that cannot satisfy this requirement MUST NOT choose an ECC
   cipher suite in its ServerHello message.)

   Actions of the receiver:

   The client validates the certificate chain, extracts the server's
   public key, and checks that the key type is appropriate for the
   negotiated key exchange algorithm.  (A possible reason for a fatal
   handshake failure is that the client's capabilities for handling
   elliptic curves and point formats are exceeded; cf. Section 5.1.)

5.4.  Server Key Exchange

   When this message is sent:

   This message is sent when using the ECDHE_ECDSA, ECDHE_RSA, and
   ECDH_anon key exchange algorithms.

   Meaning of this message:

   This message is used to convey the server's ephemeral ECDH public key
   (and the corresponding elliptic curve domain parameters) to the
   client.

   The ECCurveType enum used to have values for explicit prime and for
   explicit char2 curves.  Those values are now deprecated, so only one
   value remains:

   Structure of this message:

           enum {
               deprecated (1..2),
               named_curve (3),
               reserved(248..255)
           } ECCurveType;

   The value named_curve indicates that a named curve is used.  This
   option is now the only remaining format.

   Values 248 through 255 are reserved for private use.

   The ECCurveType name space (now titled "TLS EC Curve Types") is
   maintained by IANA.  See Section 9 for information on how new value
   assignments are added.






Nir, et al.                  Standards Track                   [Page 16]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   RFC 4492 had a specification for an ECCurve structure and an
   ECBasisType structure.  Both of these are omitted now because they
   were only used with the now deprecated explicit curves.

           struct {
               opaque point <1..2^8-1>;
           } ECPoint;

   point: This is the byte string representation of an elliptic curve
   point following the conversion routine in Section 4.3.6 of
   [ANSI.X9-62.2005].  This byte string may represent an elliptic curve
   point in uncompressed, compressed, or hybrid format, but this
   specification deprecates all but the uncompressed format.  For the
   NIST curves, the format is repeated in Section 5.4.1 for convenience.
   For the X25519 and X448 curves, the only valid representation is the
   one specified in [RFC7748], a 32- or 56-octet representation of the u
   value of the point.  This structure MUST NOT be used with Ed25519 and
   Ed448 public keys.

           struct {
               ECCurveType    curve_type;
               select (curve_type) {
                   case named_curve:
                       NamedCurve namedcurve;
               };
           } ECParameters;

   curve_type: This identifies the type of the elliptic curve domain
   parameters.

   namedCurve: Specifies a recommended set of elliptic curve domain
   parameters.  All those values of NamedCurve are allowed that refer to
   a curve capable of Diffie-Hellman.  With the deprecation of the
   explicit curves, this now includes all of the NamedCurve values.

           struct {
               ECParameters    curve_params;
               ECPoint         public;
           } ServerECDHParams;

   curve_params: Specifies the elliptic curve domain parameters
   associated with the ECDH public key.

   public: The ephemeral ECDH public key.







Nir, et al.                  Standards Track                   [Page 17]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   The ServerKeyExchange message is extended as follows.

           enum {
               ec_diffie_hellman
           } KeyExchangeAlgorithm;

   o  ec_diffie_hellman: Indicates the ServerKeyExchange message
      contains an ECDH public key.

      select (KeyExchangeAlgorithm) {
          case ec_diffie_hellman:
              ServerECDHParams    params;
              Signature           signed_params;
      } ServerKeyExchange;

   o  params: Specifies the ECDH public key and associated domain
      parameters.

   o  signed_params: A hash of the params, with the signature
      appropriate to that hash applied.  The private key corresponding
      to the certified public key in the server's Certificate message is
      used for signing.

        enum {
            ecdsa(3),
            ed25519(7)
            ed448(8)
        } SignatureAlgorithm;
        select (SignatureAlgorithm) {
           case ecdsa:
                digitally-signed struct {
                    opaque sha_hash[sha_size];
                };
           case ed25519,ed448:
                digitally-signed struct {
                    opaque rawdata[rawdata_size];
                };
        } Signature;
      ServerKeyExchange.signed_params.sha_hash
          SHA(ClientHello.random + ServerHello.random +
                                 ServerKeyExchange.params);
      ServerKeyExchange.signed_params.rawdata
          ClientHello.random + ServerHello.random +
                                 ServerKeyExchange.params;

   NOTE: SignatureAlgorithm is "rsa" for the ECDHE_RSA key exchange
   algorithm and "anonymous" for ECDH_anon.  These cases are defined in
   TLS.  SignatureAlgorithm is "ecdsa" or "eddsa" for ECDHE_ECDSA.



Nir, et al.                  Standards Track                   [Page 18]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   ECDSA signatures are generated and verified as described in
   Section 5.10.  SHA, in the above template for sha_hash, may denote a
   hash algorithm other than SHA-1.  As per ANSI X9.62, an ECDSA
   signature consists of a pair of integers, r and s.  The digitally-
   signed element is encoded as an opaque vector <0..2^16-1>, the
   contents of which are the DER encoding corresponding to the following
   ASN.1 notation.

              Ecdsa-Sig-Value ::= SEQUENCE {
                  r       INTEGER,
                  s       INTEGER
              }

   EdDSA signatures in both the protocol and in certificates that
   conform to [RFC8410] are generated and verified according to
   [RFC8032].  The digitally-signed element is encoded as an opaque
   vector <0..2^16-1>, the contents of which include the octet string
   output of the EdDSA signing algorithm.

   Actions of the sender:

   The server selects elliptic curve domain parameters and an ephemeral
   ECDH public key corresponding to these parameters according to the
   ECKAS-DH1 scheme from IEEE 1363 [IEEE.P1363].  It conveys this
   information to the client in the ServerKeyExchange message using the
   format defined above.

   Actions of the receiver:

   The client verifies the signature (when present) and retrieves the
   server's elliptic curve domain parameters and ephemeral ECDH public
   key from the ServerKeyExchange message.  (A possible reason for a
   fatal handshake failure is that the client's capabilities for
   handling elliptic curves and point formats are exceeded; cf.
   Section 5.1.)

5.4.1.  Uncompressed Point Format for NIST Curves

   The following represents the wire format for representing ECPoint in
   ServerKeyExchange records.  The first octet of the representation
   indicates the form, which may be compressed, uncompressed, or hybrid.
   This specification supports only the uncompressed format for these
   curves.  This is followed by the binary representation of the X value
   in "big-endian" or "network" format, followed by the binary
   representation of the Y value in "big-endian" or "network" format.
   There are no internal length markers, so each number representation
   occupies as many octets as implied by the curve parameters.  For




Nir, et al.                  Standards Track                   [Page 19]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   P-256 this means that each of X and Y use 32 octets, padded on the
   left by zeros if necessary.  For P-384, they take 48 octets each, and
   for P-521, they take 66 octets each.

   Here's a more formal representation:

             enum {
                 uncompressed(4),
                 (255)
               } PointConversionForm;

             struct {
                 PointConversionForm  form;
                 opaque               X[coordinate_length];
                 opaque               Y[coordinate_length];
             } UncompressedPointRepresentation;

5.5.  Certificate Request

   When this message is sent:

   This message is sent when requesting client authentication.

   Meaning of this message:

   The server uses this message to suggest acceptable client
   authentication methods.

   Structure of this message:

   The TLS CertificateRequest message is extended as follows.

           enum {
               ecdsa_sign(64),
               deprecated1(65),  /* was rsa_fixed_ecdh */
               deprecated2(66),  /* was ecdsa_fixed_ecdh */
               (255)
           } ClientCertificateType;

   o  ecdsa_sign: Indicates that the server would like to use the
      corresponding client authentication method specified in Section 3.

   Note that RFC 4492 also defined RSA and ECDSA certificates that
   included a fixed ECDH public key.  These mechanisms saw very little
   implementation, so this specification is deprecating them.






Nir, et al.                  Standards Track                   [Page 20]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Actions of the sender:

   The server decides which client authentication methods it would like
   to use and conveys this information to the client using the format
   defined above.

   Actions of the receiver:

   The client determines whether it has a suitable certificate for use
   with any of the requested methods and whether to proceed with client
   authentication.

5.6.  Client Certificate

   When this message is sent:

   This message is sent in response to a CertificateRequest when a
   client has a suitable certificate and has decided to proceed with
   client authentication.  (Note that if the server has used a Supported
   Point Formats Extension, a certificate can only be considered
   suitable for use with the ECDSA_sign authentication method if the
   public key point specified in it is uncompressed, as that is the only
   point format still supported.

   Meaning of this message:

   This message is used to authentically convey the client's static
   public key to the server.  ECC public keys must be encoded in
   certificates as described in Section 5.9.  The certificate MUST
   contain an ECDSA- or EdDSA-capable public key.

   NOTE: The client's Certificate message is capable of carrying a chain
   of certificates.  The restrictions mentioned above apply only to the
   client's certificate (first in the chain).

   Structure of this message:

   Identical to the TLS client Certificate format.

   Actions of the sender:

   The client constructs an appropriate certificate chain and conveys it
   to the server in the Certificate message.








Nir, et al.                  Standards Track                   [Page 21]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Actions of the receiver:

   The TLS server validates the certificate chain, extracts the client's
   public key, and checks that the key type is appropriate for the
   client authentication method.

5.7.  Client Key Exchange

   When this message is sent:

   This message is sent in all key exchange algorithms.  It contains the
   client's ephemeral ECDH public key.

   Meaning of the message:

   This message is used to convey ephemeral data relating to the key
   exchange belonging to the client (such as its ephemeral ECDH public
   key).

   Structure of this message:

   The TLS ClientKeyExchange message is extended as follows.

           enum {
               implicit,
               explicit
           } PublicValueEncoding;

   o  implicit, explicit: For ECC cipher suites, this indicates whether
      the client's ECDH public key is in the client's certificate
      ("implicit") or is provided, as an ephemeral ECDH public key, in
      the ClientKeyExchange message ("explicit").  The implicit encoding
      is deprecated and is retained here for backward compatibility
      only.

           struct {
               ECPoint ecdh_Yc;
           } ClientECDiffieHellmanPublic;

   ecdh_Yc: Contains the client's ephemeral ECDH public key as a byte
   string ECPoint.point, which may represent an elliptic curve point in
   uncompressed format.

           struct {
               select (KeyExchangeAlgorithm) {
                   case ec_diffie_hellman: ClientECDiffieHellmanPublic;
               } exchange_keys;
           } ClientKeyExchange;



Nir, et al.                  Standards Track                   [Page 22]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Actions of the sender:

   The client selects an ephemeral ECDH public key corresponding to the
   parameters it received from the server.  The format is the same as in
   Section 5.4.

   Actions of the receiver:

   The server retrieves the client's ephemeral ECDH public key from the
   ClientKeyExchange message and checks that it is on the same elliptic
   curve as the server's ECDH key.

5.8.  Certificate Verify

   When this message is sent:

   This message is sent when the client sends a client certificate
   containing a public key usable for digital signatures.

   Meaning of the message:

   This message contains a signature that proves possession of the
   private key corresponding to the public key in the client's
   Certificate message.

   Structure of this message:

   The TLS CertificateVerify message and the underlying signature type
   are defined in the TLS base specifications, and the latter is
   extended here in Section 5.4.  For the "ecdsa" and "eddsa" cases, the
   signature field in the CertificateVerify message contains an ECDSA or
   EdDSA (respectively) signature computed over handshake messages
   exchanged so far, exactly similar to CertificateVerify with other
   signing algorithms:

           CertificateVerify.signature.sha_hash
               SHA(handshake_messages);
           CertificateVerify.signature.rawdata
               handshake_messages;

   ECDSA signatures are computed as described in Section 5.10, and SHA
   in the above template for sha_hash accordingly may denote a hash
   algorithm other than SHA-1.  As per ANSI X9.62, an ECDSA signature
   consists of a pair of integers, r and s.  The digitally-signed
   element is encoded as an opaque vector <0..2^16-1>, the contents of
   which are the DER encoding [X.690] corresponding to the following
   ASN.1 notation [X.680].




Nir, et al.                  Standards Track                   [Page 23]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


           Ecdsa-Sig-Value ::= SEQUENCE {
               r       INTEGER,
               s       INTEGER
           }

   EdDSA signatures are generated and verified according to [RFC8032].
   The digitally-signed element is encoded as an opaque vector
   <0..2^16-1>, the contents of which include the octet string output of
   the EdDSA signing algorithm.

   Actions of the sender:

   The client computes its signature over all handshake messages sent or
   received starting at client hello and up to but not including this
   message.  It uses the private key corresponding to its certified
   public key to compute the signature, which is conveyed in the format
   defined above.

   Actions of the receiver:

   The server extracts the client's signature from the CertificateVerify
   message and verifies the signature using the public key it received
   in the client's Certificate message.

5.9.  Elliptic Curve Certificates

   X.509 certificates containing ECC public keys or signed using ECDSA
   MUST comply with [RFC3279] or another RFC that replaces or extends
   it.  X.509 certificates containing ECC public keys or signed using
   EdDSA MUST comply with [RFC8410].  Clients SHOULD use the elliptic
   curve domain parameters recommended in ANSI X9.62, FIPS 186-4, and
   SEC 2 [SECG-SEC2], or in [RFC8032].

   EdDSA keys using the Ed25519 algorithm MUST use the ed25519 signature
   algorithm, and Ed448 keys MUST use the ed448 signature algorithm.
   This document does not define use of Ed25519ph and Ed448ph keys with
   TLS.  Ed25519, Ed25519ph, Ed448, and Ed448ph keys MUST NOT be used
   with ECDSA.

5.10.  ECDH, ECDSA, and RSA Computations

   All ECDH calculations for the NIST curves (including parameter and
   key generation as well as the shared secret calculation) are
   performed according to [IEEE.P1363] using the ECKAS-DH1 scheme with
   the identity map as the Key Derivation Function (KDF) so that the
   premaster secret is the x-coordinate of the ECDH shared secret
   elliptic curve point represented as an octet string.  Note that this
   octet string (Z in IEEE 1363 terminology), as output by FE2OSP (Field



Nir, et al.                  Standards Track                   [Page 24]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   Element to Octet String Conversion Primitive), has constant length
   for any given field; leading zeros found in this octet string MUST
   NOT be truncated.

   (Note that this use of the identity KDF is a technicality.  The
   complete picture is that ECDH is employed with a non-trivial KDF
   because TLS does not directly use the premaster secret for anything
   other than for computing the master secret.  In TLS 1.0 and 1.1, this
   means that the MD5- and SHA-1-based TLS Pseudorandom Function (PRF)
   serves as a KDF; in TLS 1.2, the KDF is determined by ciphersuite,
   and it is conceivable that future TLS versions or new TLS extensions
   introduced in the future may vary this computation.)

   An ECDHE key exchange using X25519 (curve x25519) goes as follows:
   (1) each party picks a secret key d uniformly at random and computes
   the corresponding public key x = X25519(d, G); (2) parties exchange
   their public keys and compute a shared secret as x_S = X25519(d,
   x_peer); and (3), if either party obtains all-zeroes x_S, it MUST
   abort the handshake (as required by definition of X25519 and X448).
   ECDHE for X448 works similarly, replacing X25519 with X448 and x25519
   with x448.  The derived shared secret is used directly as the
   premaster secret, which is always exactly 32 bytes when ECDHE with
   X25519 is used and 56 bytes when ECDHE with X448 is used.

   All ECDSA computations MUST be performed according to ANSI X9.62 or
   its successors.  Data to be signed/verified is hashed, and the result
   runs directly through the ECDSA algorithm with no additional hashing.
   A secure hash function such as SHA-256, SHA-384, or SHA-512 from
   [FIPS.180-4] MUST be used.

   All EdDSA computations MUST be performed according to [RFC8032] or
   its successors.  Data to be signed/verified is run through the EdDSA
   algorithm with no hashing (EdDSA will internally run the data through
   the "prehash" function PH).  The context parameter for Ed448 MUST be
   set to the empty string.

   RFC 4492 anticipated the standardization of a mechanism for
   specifying the required hash function in the certificate, perhaps in
   the parameters field of the subjectPublicKeyInfo.  Such
   standardization never took place, and as a result, SHA-1 is used in
   TLS 1.1 and earlier (except for EdDSA, which uses identity function).
   TLS 1.2 added a SignatureAndHashAlgorithm parameter to the
   DigitallySigned struct, thus allowing agility in choosing the
   signature hash.  EdDSA signatures MUST have HashAlgorithm of 8
   (Intrinsic).

   All RSA signatures must be generated and verified according to
   Section 7.2 of [RFC8017].



Nir, et al.                  Standards Track                   [Page 25]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


5.11.  Public Key Validation

   With the NIST curves, each party MUST validate the public key sent by
   its peer in the ClientKeyExchange and ServerKeyExchange messages.  A
   receiving party MUST check that the x and y parameters from the
   peer's public value satisfy the curve equation, y^2 = x^3 + ax + b
   mod p.  See Section 2.3 of [Menezes] for details.  Failing to do so
   allows attackers to gain information about the private key to the
   point that they may recover the entire private key in a few requests
   if that key is not really ephemeral.

   With X25519 and X448, a receiving party MUST check whether the
   computed premaster secret is the all-zero value and abort the
   handshake if so, as described in Section 6 of [RFC7748].

   Ed25519 and Ed448 internally do public key validation as part of
   signature verification.

6.  Cipher Suites

   The table below defines ECC cipher suites that use the key exchange
   algorithms specified in Section 2.

       +-----------------------------------------+----------------+
       | CipherSuite                             | Identifier     |
       +-----------------------------------------+----------------+
       | TLS_ECDHE_ECDSA_WITH_NULL_SHA           | { 0xC0, 0x06 } |
       | TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA   | { 0xC0, 0x08 } |
       | TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA    | { 0xC0, 0x09 } |
       | TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA    | { 0xC0, 0x0A } |
       | TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | { 0xC0, 0x2B } |
       | TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 | { 0xC0, 0x2C } |
       |                                         |                |
       | TLS_ECDHE_RSA_WITH_NULL_SHA             | { 0xC0, 0x10 } |
       | TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA     | { 0xC0, 0x12 } |
       | TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA      | { 0xC0, 0x13 } |
       | TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA      | { 0xC0, 0x14 } |
       | TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256   | { 0xC0, 0x2F } |
       | TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384   | { 0xC0, 0x30 } |
       |                                         |                |
       | TLS_ECDH_anon_WITH_NULL_SHA             | { 0xC0, 0x15 } |
       | TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA     | { 0xC0, 0x17 } |
       | TLS_ECDH_anon_WITH_AES_128_CBC_SHA      | { 0xC0, 0x18 } |
       | TLS_ECDH_anon_WITH_AES_256_CBC_SHA      | { 0xC0, 0x19 } |
       +-----------------------------------------+----------------+

                      Table 3: TLS ECC Cipher Suites




Nir, et al.                  Standards Track                   [Page 26]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   The key exchange method, cipher, and hash algorithm for each of these
   cipher suites are easily determined by examining the name.  Ciphers
   (other than AES ciphers) and hash algorithms are defined in [RFC2246]
   and [RFC4346].  AES ciphers are defined in [RFC5246], and AES-GCM
   ciphersuites are in [RFC5289].

   Server implementations SHOULD support all of the following cipher
   suites, and client implementations SHOULD support at least one of
   them:

   o  TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   o  TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA

   o  TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256

   o  TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA

7.  Implementation Status

   Both ECDHE and ECDSA with the NIST curves are widely implemented and
   supported in all major browsers and all widely used TLS libraries.
   ECDHE with Curve25519 is by now implemented in several browsers and
   several TLS libraries including OpenSSL.  Curve448 and EdDSA have
   working interoperable implementations, but they are not yet as widely
   deployed.

8.  Security Considerations

   Security issues are discussed throughout this memo.

   For TLS handshakes using ECC cipher suites, the security
   considerations in Appendix D of each of the three TLS base documents
   apply accordingly.

   Security discussions specific to ECC can be found in [IEEE.P1363] and
   [ANSI.X9-62.2005].  One important issue that implementers and users
   must consider is elliptic curve selection.  Guidance on selecting an
   appropriate elliptic curve size is given in Table 1.  Security
   considerations specific to X25519 and X448 are discussed in Section 7
   of [RFC7748].

   Beyond elliptic curve size, the main issue is elliptic curve
   structure.  As a general principle, it is more conservative to use
   elliptic curves with as little algebraic structure as possible.
   Thus, random curves are more conservative than special curves such as
   Koblitz curves, and curves over F_p with p random are more
   conservative than curves over F_p with p of a special form, and



Nir, et al.                  Standards Track                   [Page 27]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   curves over F_p with p random are considered more conservative than
   curves over F_2^m as there is no choice between multiple fields of
   similar size for characteristic 2.

   Another issue is the potential for catastrophic failures when a
   single elliptic curve is widely used.  In this case, an attack on the
   elliptic curve might result in the compromise of a large number of
   keys.  Again, this concern may need to be balanced against efficiency
   and interoperability improvements associated with widely used curves.
   Substantial additional information on elliptic curve choice can be
   found in [IEEE.P1363], [ANSI.X9-62.2005], and [FIPS.186-4].

   The Introduction of [RFC8032] lists the security, performance, and
   operational advantages of EdDSA signatures over ECDSA signatures
   using the NIST curves.

   All of the key exchange algorithms defined in this document provide
   forward secrecy.  Some of the deprecated key exchange algorithms do
   not.

9.  IANA Considerations

   [RFC4492], the predecessor of this document, defined the IANA
   registries for the following:

   o  Supported Groups (Section 5.1)

   o  EC Point Format (Section 5.1)

   o  EC Curve Type (Section 5.4)

   IANA has prepended "TLS" to the names of these three registries.

   For each name space, this document defines the initial value
   assignments and defines a range of 256 values (NamedCurve) or eight
   values (ECPointFormat and ECCurveType) reserved for Private Use.  The
   policy for any additional assignments is "Specification Required".
   (RFC 4492 required IETF review.)

   All existing entries in the "ExtensionType Values", "TLS
   ClientCertificateType Identifiers", "TLS Cipher Suites", "TLS
   Supported Groups", "TLS EC Point Format", and "TLS EC Curve Type"
   registries that referred to RFC 4492 have been updated to refer to
   this document.

   IANA has assigned the value 29 to x25519 and the value 30 to x448 in
   the "TLS Supported Groups" registry.




Nir, et al.                  Standards Track                   [Page 28]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   IANA has assigned two values in the "TLS SignatureAlgorithm" registry
   for ed25519 (7) and ed448 (8) with this document as reference.  This
   keeps compatibility with TLS 1.3.

   IANA has assigned one value from the "TLS HashAlgorithm" registry for
   Intrinsic (8) with DTLS-OK set to true (Y) and this document as
   reference.  This keeps compatibility with TLS 1.3.

10.  References

10.1.  Normative References

   [ANSI.X9-62.2005]
              American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry: The
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI X9.62, November 2005.

   [FIPS.186-4]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-4.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, DOI 10.17487/RFC2246, January 1999,
              <https://www.rfc-editor.org/info/rfc2246>.

   [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
              2002, <https://www.rfc-editor.org/info/rfc3279>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <https://www.rfc-editor.org/info/rfc4346>.







Nir, et al.                  Standards Track                   [Page 29]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,
              <https://www.rfc-editor.org/info/rfc4366>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
              256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
              DOI 10.17487/RFC5289, August 2008,
              <https://www.rfc-editor.org/info/rfc5289>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519 and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.

   [SECG-SEC2]
              Certicom Research, "SEC 2: Recommended Elliptic Curve
              Domain Parameters", Standards for Efficient Cryptography 2
              (SEC 2), Version 2.0, January 2010,
              <http://www.secg.org/sec2-v2.pdf>.

   [X.680]    ITU-T, "Abstract Syntax Notation One (ASN.1):
              Specification of basic notation", ITU-T Recommendation
              X.680, ISO/IEC 8824-1, August 2015.



Nir, et al.                  Standards Track                   [Page 30]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


   [X.690]    ITU-T, "Information technology-ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1, August
              2015.

10.2.  Informative References

   [FIPS.180-4]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, DOI
              10.6028/NIST.FIPS.180-4, August 2015,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [IEEE.P1363]
              IEEE, "Standard Specifications for Public Key
              Cryptography", IEEE Std P1363,
              <http://ieeexplore.ieee.org/document/891000/>.

   [Menezes]  Menezes, A. and B. Ustaoglu, "On reusing ephemeral keys in
              Diffie-Hellman key agreement protocols", International
              Journal of Applied Cryptography, Vol. 2, Issue 2,
              DOI 10.1504/IJACT.2010.038308, January 2010.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <https://www.rfc-editor.org/info/rfc4492>.

   [RFC7919]  Gillmor, D., "Negotiated Finite Field Diffie-Hellman
              Ephemeral Parameters for Transport Layer Security (TLS)",
              RFC 7919, DOI 10.17487/RFC7919, August 2016,
              <https://www.rfc-editor.org/info/rfc7919>.

   [TLS1.3]   Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", Work in Progress, draft-ietf-tls-tls13-28,
              March 2018.












Nir, et al.                  Standards Track                   [Page 31]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


Appendix A.  Equivalent Curves (Informative)

   All of the NIST curves [FIPS.186-4] and several of the ANSI curves
   [ANSI.X9-62.2005] are equivalent to curves listed in Section 5.1.1.
   The following table displays the curve names chosen by different
   standards organizations; multiple names in one row represent aliases
   for the same curve.

                  +-----------+------------+------------+
                  | SECG      | ANSI X9.62 | NIST       |
                  +-----------+------------+------------+
                  | sect163k1 |            | NIST K-163 |
                  | sect163r1 |            |            |
                  | sect163r2 |            | NIST B-163 |
                  | sect193r1 |            |            |
                  | sect193r2 |            |            |
                  | sect233k1 |            | NIST K-233 |
                  | sect233r1 |            | NIST B-233 |
                  | sect239k1 |            |            |
                  | sect283k1 |            | NIST K-283 |
                  | sect283r1 |            | NIST B-283 |
                  | sect409k1 |            | NIST K-409 |
                  | sect409r1 |            | NIST B-409 |
                  | sect571k1 |            | NIST K-571 |
                  | sect571r1 |            | NIST B-571 |
                  | secp160k1 |            |            |
                  | secp160r1 |            |            |
                  | secp160r2 |            |            |
                  | secp192k1 |            |            |
                  | secp192r1 | prime192v1 | NIST P-192 |
                  | secp224k1 |            |            |
                  | secp224r1 |            | NIST P-224 |
                  | secp256k1 |            |            |
                  | secp256r1 | prime256v1 | NIST P-256 |
                  | secp384r1 |            | NIST P-384 |
                  | secp521r1 |            | NIST P-521 |
                  +-----------+------------+------------+

        Table 4: Equivalent Curves Defined by SECG, ANSI, and NIST












Nir, et al.                  Standards Track                   [Page 32]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


Appendix B.  Differences from RFC 4492

   o  Renamed EllipticCurveList to NamedCurveList.

   o  Added TLS 1.2.

   o  Merged errata.

   o  Removed the ECDH key exchange algorithms: ECDH_RSA and ECDH_ECDSA

   o  Deprecated a bunch of ciphersuites:

         TLS_ECDH_ECDSA_WITH_NULL_SHA

         TLS_ECDH_ECDSA_WITH_RC4_128_SHA

         TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA

         TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA

         TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA

         TLS_ECDH_RSA_WITH_NULL_SHA

         TLS_ECDH_RSA_WITH_RC4_128_SHA

         TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA

         TLS_ECDH_RSA_WITH_AES_128_CBC_SHA

         TLS_ECDH_RSA_WITH_AES_256_CBC_SHA

         All the other RC4 ciphersuites

   o  Removed unused curves and all but the uncompressed point format.

   o  Added X25519 and X448.

   o  Deprecated explicit curves.

   o  Removed restriction on signature algorithm in certificate.










Nir, et al.                  Standards Track                   [Page 33]
^L
RFC 8422                ECC Cipher Suites for TLS            August 2018


Acknowledgements

   Most of the text in this document is taken from [RFC4492], the
   predecessor of this document.  The authors of that document were:

   o  Simon Blake-Wilson
   o  Nelson Bolyard
   o  Vipul Gupta
   o  Chris Hawk
   o  Bodo Moeller

   In the predecessor document, the authors acknowledged the
   contributions of Bill Anderson and Tim Dierks.

   The authors would like to thank Nikos Mavrogiannopoulos, Martin
   Thomson, and Tanja Lange for contributions to this document.

Authors' Addresses

   Yoav Nir
   Check Point Software Technologies Ltd.
   5 Hasolelim st.
   Tel Aviv  6789735
   Israel

   Email: ynir.ietf@gmail.com


   Simon Josefsson
   SJD AB

   Email: simon@josefsson.org


   Manuel Pegourie-Gonnard
   ARM

   Email: mpg@elzevir.fr













Nir, et al.                  Standards Track                   [Page 34]
^L