    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.

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    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.

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    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

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    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.

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       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.

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              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.

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       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.

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    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.

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    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.

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    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.

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       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

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       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

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    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.

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       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.

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       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

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       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.

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       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.

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       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.

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       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

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       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.

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       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.

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       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;

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       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].

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               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

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       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].

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    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

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       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

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       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.

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       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>.

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    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.

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    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.

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    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

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    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.

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    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

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