    Network Working Group                                    S. Blake-Wilson
    Request for Comments: 4492                                       SafeNet
    Category: Informational                                       N. Bolyard
                                                            Sun Microsystems
                                                                    V. Gupta
                                                                    Sun Labs
                                                                     C. Hawk
                                                                   Corriente
                                                                  B. Moeller
                                                             Ruhr-Uni Bochum
                                                                    May 2006

                Elliptic Curve Cryptography (ECC) Cipher Suites
                       for Transport Layer Security (TLS)

    Status of This Memo

       This memo provides information for the Internet community.  It does
       not specify an Internet standard of any kind.  Distribution of this
       memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2006).

    Abstract

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

    Blake-Wilson, et al.         Informational                      [Page 1]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    Table of Contents

       1. Introduction ....................................................3
       2. Key Exchange Algorithms .........................................4
          2.1. ECDH_ECDSA .................................................6
          2.2. ECDHE_ECDSA ................................................6
          2.3. ECDH_RSA ...................................................7
          2.4. ECDHE_RSA ..................................................7
          2.5. ECDH_anon ..................................................7
       3. Client Authentication ...........................................8
          3.1. ECDSA_sign .................................................8
          3.2. ECDSA_fixed_ECDH ...........................................9
          3.3. RSA_fixed_ECDH .............................................9
       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 ................12
               5.1.2. Supported Point Formats Extension ..................13
          5.2. Server Hello Extension ....................................14
          5.3. Server Certificate ........................................15
          5.4. Server Key Exchange .......................................17
          5.5. Certificate Request .......................................21
          5.6. Client Certificate ........................................22
          5.7. Client Key Exchange .......................................23
          5.8. Certificate Verify ........................................25
          5.9. Elliptic Curve Certificates ...............................26
          5.10. ECDH, ECDSA, and RSA Computations ........................26
       6. Cipher Suites ..................................................27
       7. Security Considerations ........................................28
       8. IANA Considerations ............................................29
       9. Acknowledgements ...............................................29
       10. References ....................................................30
          10.1. Normative References .....................................30
          10.2. Informative References ...................................31
       Appendix A.  Equivalent Curves (Informative) ......................32

    Blake-Wilson, et al.         Informational                      [Page 2]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    1.  Introduction

       Elliptic Curve Cryptography (ECC) is emerging as an attractive
       public-key cryptosystem, in particular for mobile (i.e., wireless)
       environments.  Compared to currently prevalent cryptosystems such as
       RSA, ECC offers equivalent security with smaller key sizes.  This is
       illustrated in the following table, based on [18], which gives
       approximate comparable key sizes for symmetric- and asymmetric-key
       cryptosystems based on the best-known algorithms for attacking them.

                        Symmetric  |   ECC   |  DH/DSA/RSA
                       ------------+---------+-------------
                            80     |   163   |     1024
                           112     |   233   |     2048
                           128     |   283   |     3072
                           192     |   409   |     7680
                           256     |   571   |    15360

                      Table 1: Comparable Key Sizes (in bits)

       Smaller key sizes result in savings for power, memory, bandwidth, and
       computational cost that make ECC especially attractive for
       constrained environments.

       This document describes additions to TLS to support ECC, applicable
       both to TLS Version 1.0 [2] and to TLS Version 1.1 [3].  In
       particular, it defines

       o  the use of the Elliptic Curve Diffie-Hellman (ECDH) key agreement
          scheme with long-term or ephemeral keys to establish the TLS
          premaster secret, and

       o  the use of fixed-ECDH certificates and ECDSA 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 new ECC-based cipher suites and
       identifies a small subset of these as recommended for all
       implementations of this specification.  Section 7 discusses security
       considerations.  Section 8 describes IANA considerations for the name
       spaces created by this document.  Section 9 gives acknowledgements.

    Blake-Wilson, et al.         Informational                      [Page 3]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       This is followed by the lists of normative and informative references
       cited in this document, the authors' contact information, and
       statements on intellectual property rights and copyrights.

       Implementation of this specification requires familiarity with TLS
       [2][3], TLS extensions [4], and ECC [5][6][7][11][17].

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in RFC 2119 [1].

    2.  Key Exchange Algorithms

       This document introduces five new ECC-based key exchange algorithms
       for TLS.  All of them use ECDH to compute the TLS premaster secret,
       and they differ only in the lifetime of ECDH keys (long-term or
       ephemeral) and 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.

       The table below summarizes the new key exchange algorithms, which
       mimic DH_DSS, DHE_DSS, DH_RSA, DHE_RSA, and DH_anon (see [2] and
       [3]), respectively.

              Key
              Exchange
              Algorithm           Description
              ---------           -----------

              ECDH_ECDSA          Fixed ECDH with ECDSA-signed certificates.

              ECDHE_ECDSA         Ephemeral ECDH with ECDSA signatures.

              ECDH_RSA            Fixed ECDH with RSA-signed certificates.

              ECDHE_RSA           Ephemeral ECDH with RSA signatures.

              ECDH_anon           Anonymous ECDH, no signatures.

                         Table 2: ECC Key Exchange Algorithms

       The ECDHE_ECDSA and ECDHE_RSA key exchange mechanisms provide forward
       secrecy.  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

    Blake-Wilson, et al.         Informational                      [Page 4]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       incurred by a server is higher for ECDHE_RSA than for the traditional
       RSA key exchange, which does not provide forward secrecy.

       The ECDH_RSA mechanism requires a server to acquire an ECC
       certificate, but the certificate issuer can still use an existing RSA
       key for signing.  This eliminates the need to update the keys of
       trusted certification authorities accepted by TLS clients.  The
       ECDH_ECDSA mechanism requires ECC keys for the server as well as the
       certification authority and is best suited for constrained devices
       unable to support RSA.

       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.  Implementations of this
       algorithm SHOULD provide authentication by other means.

       Note that there is no structural difference between ECDH and ECDSA
       keys.  A certificate issuer may use X.509 v3 keyUsage and
       extendedKeyUsage extensions to restrict the use of an ECC public key
       to certain computations [15].  This document refers to an ECC key as
       ECDH-capable if its use in ECDH is permitted.  ECDSA-capable is
       defined similarly.

                  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 handshake

    Blake-Wilson, et al.         Informational                      [Page 5]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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 each 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.  ECDH_ECDSA

       In ECDH_ECDSA, the server's certificate MUST contain an ECDH-capable
       public key and be signed with ECDSA.

       A ServerKeyExchange MUST NOT be sent (the server's certificate
       contains all the necessary keying information required by the client
       to arrive at the premaster secret).

       The client generates an ECDH key pair on the same curve as the
       server's long-term public key and sends its public key in the
       ClientKeyExchange message (except when using client authentication
       algorithm ECDSA_fixed_ECDH or RSA_fixed_ECDH, in which case the
       modifications from Section 3.2 or Section 3.3 apply).

       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.

    2.2.  ECDHE_ECDSA

       In ECDHE_ECDSA, the server's certificate MUST contain an ECDSA-
       capable public key and be signed with ECDSA.

       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 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.

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

    Blake-Wilson, et al.         Informational                      [Page 6]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    2.3.  ECDH_RSA

       This key exchange algorithm is the same as ECDH_ECDSA except that the
       server's certificate MUST be signed with RSA rather than ECDSA.

    2.4.  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 that the signature in the ServerKeyExchange message
       must be computed with the corresponding RSA private key.  The server
       certificate MUST be signed with RSA.

    2.5.  ECDH_anon

       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.

       Note that while the ECDH_ECDSA, ECDHE_ECDSA, ECDH_RSA, and ECDHE_RSA
       key exchange algorithms require the server's certificate to be signed
       with a particular signature scheme, this specification (following the
       similar cases of DH_DSS, DHE_DSS, DH_RSA, and DHE_RSA in [2] and [3])
       does not impose restrictions on signature schemes used elsewhere in
       the certificate chain.  (Often such restrictions will be useful, and
       it is expected that this will be taken into account in certification
       authorities' signing practices.  However, such restrictions are not
       strictly required in general: Even if it is beyond the capabilities
       of a client to completely validate a given chain, the client may be
       able to validate the server's certificate by relying on a trusted
       certification authority whose certificate appears as one of the
       intermediate certificates in the chain.)

    Blake-Wilson, et al.         Informational                      [Page 7]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    3.  Client Authentication

       This document defines three new client authentication mechanisms,
       each named after the type of client certificate involved: ECDSA_sign,
       ECDSA_fixed_ECDH, and RSA_fixed_ECDH.  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 [2][3].  The ECDSA_fixed_ECDH and
       RSA_fixed_ECDH mechanisms are usable with ECDH_ECDSA and ECDH_RSA.
       Their use with ECDHE_ECDSA and ECDHE_RSA is prohibited because the
       use of a long-term ECDH client key would jeopardize the forward
       secrecy property of these algorithms.

       The server can request ECC-based client authentication by including
       one or more of these certificate types in its CertificateRequest
       message.  The server must not include any certificate types that are
       prohibited for the negotiated key exchange algorithm.  The client
       must check if it possesses a certificate appropriate for any of the
       methods 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 should be sent as described in Section 2, and the
       CertificateVerify should 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 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-capable public key and signed with
       ECDSA.

       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.

    Blake-Wilson, et al.         Informational                      [Page 8]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    3.2.  ECDSA_fixed_ECDH

       To use this authentication mechanism, the client MUST possess a
       certificate containing an ECDH-capable public key, and that
       certificate MUST be signed with ECDSA.  Furthermore, the client's
       ECDH key MUST be on the same elliptic curve as the server's long-term
       (certified) ECDH key.  This might limit use of this mechanism to
       closed environments.  In situations where the client has an ECC key
       on a different curve, it would have to authenticate using either
       ECDSA_sign or a non-ECC mechanism (e.g., RSA).  Using fixed ECDH for
       both servers and clients is computationally more efficient than
       mechanisms providing forward secrecy.

       When using this authentication mechanism, the client MUST send an
       empty ClientKeyExchange as described in Section 5.7 and MUST NOT send
       the CertificateVerify message.  The ClientKeyExchange is empty since
       the client's ECDH public key required by the server to compute the
       premaster secret is available inside the client's certificate.  The
       client's ability to arrive at the same premaster secret as the server
       (demonstrated by a successful exchange of Finished messages) proves
       possession of the private key corresponding to the certified public
       key, and the CertificateVerify message is unnecessary.

    3.3.  RSA_fixed_ECDH

       This authentication mechanism is identical to ECDSA_fixed_ECDH except
       that the client's certificate MUST be signed with RSA.

       Note that while the ECDSA_sign, ECDSA_fixed_ECDH, and RSA_fixed_ECDH
       client authentication mechanisms require the client's certificate to
       be signed with a particular signature scheme, this specification does
       not impose restrictions on signature schemes used elsewhere in the
       certificate chain.  (Often such restrictions will be useful, and it
       is expected that this will be taken into account in certification
       authorities' signing practices.  However, such restrictions are not
       strictly required in general: Even if it is beyond the capabilities
       of a server to completely validate a given chain, the server may be
       able to validate the clients certificate by relying on a trust anchor
       that appears as one of the intermediate certificates in the chain.)

    4.  TLS Extensions for ECC

       Two new 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

    Blake-Wilson, et al.         Informational                      [Page 9]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       only support a limited number of curves or point formats.  They
       follow the general approach outlined in [4]; 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 and compression techniques 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.

    5.  Data Structures and Computations

       This section specifies the data structures and computations used by
       ECC-based key mechanisms specified in Sections 2, 3, and 4.  The
       presentation language used here is the same as that used in TLS
       [2][3].  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 [4], the Supported Elliptic
       Curves Extension and the Supported Point Formats Extension.

    Blake-Wilson, et al.         Informational                     [Page 10]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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 [4], and this
       specification adds two new types to ExtensionType.

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

       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
          EllipticCurveList.  See Section 5.1.1 for details.

       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.

       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).

    Blake-Wilson, et al.         Informational                     [Page 11]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       NOTE: A server participating in an ECDHE-ECDSA key exchange may use
       different curves for (i) the ECDSA key in its certificate, and (ii)
       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

            enum {
                sect163k1 (1), sect163r1 (2), sect163r2 (3),
                sect193r1 (4), sect193r2 (5), sect233k1 (6),
                sect233r1 (7), sect239k1 (8), sect283k1 (9),
                sect283r1 (10), sect409k1 (11), sect409r1 (12),
                sect571k1 (13), sect571r1 (14), secp160k1 (15),
                secp160r1 (16), secp160r2 (17), secp192k1 (18),
                secp192r1 (19), secp224k1 (20), secp224r1 (21),
                secp256k1 (22), secp256r1 (23), secp384r1 (24),
                secp521r1 (25),
                reserved (0xFE00..0xFEFF),
                arbitrary_explicit_prime_curves(0xFF01),
                arbitrary_explicit_char2_curves(0xFF02),
                (0xFFFF)
            } NamedCurve;

       sect163k1, etc:   Indicates support of the corresponding named curve
          or class of explicitly defined curves.  The named curves defined
          here are those specified in SEC 2 [13].  Note that many of these
          curves are also recommended in ANSI X9.62 [7] and FIPS 186-2 [11].
          Values 0xFE00 through 0xFEFF are reserved for private use.  Values
          0xFF01 and 0xFF02 indicate that the client supports arbitrary
          prime and characteristic-2 curves, respectively (the curve
          parameters must be encoded explicitly in ECParameters).

       The NamedCurve name space is maintained by IANA.  See Section 8 for
       information on how new value assignments are added.

            struct {
                NamedCurve elliptic_curve_list<1..2^16-1>
            } EllipticCurveList;

    Blake-Wilson, et al.         Informational                     [Page 12]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

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

       As an example, a client that only supports secp192r1 (aka NIST P-192;
       value 19 = 0x0013) and secp224r1 (aka NIST P-224; value 21 = 0x0015)
       and prefers to use secp192r1 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 13 00 15

       A client that supports arbitrary explicit characteristic-2 curves
       (value 0xFF02) would include an extension consisting of the following
       octets:

            00 0A 00 04 00 02 FF 02

    5.1.2.  Supported Point Formats Extension

            enum { uncompressed (0), ansiX962_compressed_prime (1),
                   ansiX962_compressed_char2 (2), reserved (248..255)
            } ECPointFormat;

            struct {
                ECPointFormat ec_point_format_list<1..2^8-1>
            } ECPointFormatList;

       Three point formats are included in the definition of ECPointFormat
       above.  The uncompressed point format is the default format in that
       implementations of this document MUST support it for all of their
       supported curves.  Compressed point formats reduce bandwidth by
       including only the x-coordinate and a single bit of the y-coordinate
       of the point.  Implementations of this document MAY support the
       ansiX962_compressed_prime and ansiX962_compressed_char2 formats,
       where the former applies only to prime curves and the latter applies
       only to characteristic-2 curves.  (These formats are specified in
       [7].)  Values 248 through 255 are reserved for private use.

       The ECPointFormat name space is maintained by IANA.  See Section 8
       for information on how new value assignments are added.

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

    Blake-Wilson, et al.         Informational                     [Page 13]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       A client that can parse only the uncompressed point format (value 0)
       includes an extension consisting of the following octets; note that
       the first two octets indicate the extension type (Supported Point
       Formats Extension):

            00 0B 00 02 01 00

       A client that in the case of prime fields prefers the compressed
       format (ansiX962_compressed_prime, value 1) over the uncompressed
       format (value 0), but in the case of characteristic-2 fields prefers
       the uncompressed format (value 0) over the compressed format
       (ansiX962_compressed_char2, value 2), may indicate these preferences
       by including an extension consisting of the following octets:

            00 0B 00 04 03 01 00 02

    5.2.  Server Hello Extension

       This section specifies a TLS extension that can be included with the
       ServerHello message as described in [4], 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, or for the curve that is used in the server's
       public key that will appear in its Certificate message when using the
       ECDH_ECDSA or ECDH_RSA 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 elliptic_curve_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 (e.g., the server may prefer to receive points in compressed
       format even when a client cannot parse this format: the same client
       may nevertheless be capable of outputting points in compressed
       format).

    Blake-Wilson, et al.         Informational                     [Page 14]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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.

       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 3 apply only to
       the server's certificate (first in the chain).

    Blake-Wilson, et al.         Informational                     [Page 15]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

              Key Exchange Algorithm  Server Certificate Type
              ----------------------  -----------------------

              ECDH_ECDSA              Certificate MUST contain an
                                      ECDH-capable public key.  It
                                      MUST be signed with ECDSA.

              ECDHE_ECDSA             Certificate MUST contain an
                                      ECDSA-capable public key.  It
                                      MUST be signed with ECDSA.

              ECDH_RSA                Certificate MUST contain an
                                      ECDH-capable public key.  It
                                      MUST be signed with RSA.

              ECDHE_RSA               Certificate MUST contain an
                                      RSA public key authorized for
                                      use in digital signatures.  It
                                      MUST be signed with RSA.

                        Table 3: 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
       certificate MUST respect the client's choice of elliptic curves; in
       particular, the public key MUST employ a named curve (not the same
       curve as an explicit curve) unless the client has indicated support
       for explicit curves of the appropriate type.  If the client has used
       a Supported Point Formats Extension, both the server's public key
       point and (in the case of an explicit curve) the curve's base point
       MUST respect the client's choice of point formats.  (A server that
       cannot satisfy these requirements MUST NOT choose an ECC cipher suite
       in its ServerHello message.)

    Blake-Wilson, et al.         Informational                     [Page 16]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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.

       Structure of this message:

            enum { explicit_prime (1), explicit_char2 (2),
                   named_curve (3), reserved(248..255) } ECCurveType;

       explicit_prime:   Indicates the elliptic curve domain parameters are
          conveyed verbosely, and the underlying finite field is a prime
          field.

       explicit_char2:   Indicates the elliptic curve domain parameters are
          conveyed verbosely, and the underlying finite field is a
          characteristic-2 field.

       named_curve:   Indicates that a named curve is used.  This option
          SHOULD be used when applicable.

       Values 248 through 255 are reserved for private use.

       The ECCurveType name space is maintained by IANA.  See Section 8 for
       information on how new value assignments are added.

            struct {
                opaque a <1..2^8-1>;
                opaque b <1..2^8-1>;
            } ECCurve;

    Blake-Wilson, et al.         Informational                     [Page 17]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       a, b:   These parameters specify the coefficients of the elliptic
          curve.  Each value contains the byte string representation of a
          field element following the conversion routine in Section 4.3.3 of
          ANSI X9.62 [7].

            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 [7].  This byte string may represent an elliptic curve point
          in uncompressed or compressed format; it MUST conform to what the
          client has requested through a Supported Point Formats Extension
          if this extension was used.

            enum { ec_basis_trinomial, ec_basis_pentanomial } ECBasisType;

       ec_basis_trinomial:   Indicates representation of a characteristic-2
          field using a trinomial basis.

       ec_basis_pentanomial:   Indicates representation of a
          characteristic-2 field using a pentanomial basis.

            struct {
                ECCurveType    curve_type;
                select (curve_type) {
                    case explicit_prime:
                        opaque      prime_p <1..2^8-1>;
                        ECCurve     curve;
                        ECPoint     base;
                        opaque      order <1..2^8-1>;
                        opaque      cofactor <1..2^8-1>;
                    case explicit_char2:
                        uint16      m;
                        ECBasisType basis;
                        select (basis) {
                            case ec_trinomial:
                                opaque  k <1..2^8-1>;
                            case ec_pentanomial:
                                opaque  k1 <1..2^8-1>;
                                opaque  k2 <1..2^8-1>;
                                opaque  k3 <1..2^8-1>;
                        };
                        ECCurve     curve;
                        ECPoint     base;
                        opaque      order <1..2^8-1>;
                        opaque      cofactor <1..2^8-1>;

    Blake-Wilson, et al.         Informational                     [Page 18]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

                    case named_curve:
                        NamedCurve namedcurve;
                };
            } ECParameters;

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

       prime_p:   This is the odd prime defining the field Fp.

       curve:   Specifies the coefficients a and b of the elliptic curve E.

       base:   Specifies the base point G on the elliptic curve.

       order:   Specifies the order n of the base point.

       cofactor:   Specifies the cofactor h = #E(Fq)/n, where #E(Fq)
          represents the number of points on the elliptic curve E defined
          over the field Fq (either Fp or F2^m).

       m:   This is the degree of the characteristic-2 field F2^m.

       k:   The exponent k for the trinomial basis representation x^m + x^k
          +1.

       k1, k2, k3:   The exponents for the pentanomial representation x^m +
          x^k3 + x^k2 + x^k1 + 1 (such that k3 > k2 > k1).

       namedcurve:   Specifies a recommended set of elliptic curve domain
          parameters.  All those values of NamedCurve are allowed that refer
          to a specific curve.  Values of NamedCurve that indicate support
          for a class of explicitly defined curves are not allowed here
          (they are only permissible in the ClientHello extension); this
          applies to arbitrary_explicit_prime_curves(0xFF01) and
          arbitrary_explicit_char2_curves(0xFF02).

            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.

    Blake-Wilson, et al.         Informational                     [Page 19]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       The ServerKeyExchange message is extended as follows.

            enum { ec_diffie_hellman } KeyExchangeAlgorithm;

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

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

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

       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 } SignatureAlgorithm;

              select (SignatureAlgorithm) {
                  case ecdsa:
                      digitally-signed struct {
                          opaque sha_hash[sha_size];
                      };
              } Signature;

            ServerKeyExchange.signed_params.sha_hash
                SHA(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 [2][3].  SignatureAlgorithm is "ecdsa" for ECDHE_ECDSA.  ECDSA
       signatures are generated and verified 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 [9] corresponding to the
       following ASN.1 notation [8].

    Blake-Wilson, et al.         Informational                     [Page 20]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

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

       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 [6].  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.)

    5.5.  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), rsa_fixed_ecdh(65),
                ecdsa_fixed_ecdh(66), (255)
            } ClientCertificateType;

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

    Blake-Wilson, et al.         Informational                     [Page 21]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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, RSA_fixed_ECDH, and
       ECDSA_fixed_ECDH authentication methods if the public key point
       specified in it respects the server's choice of point formats.  If no
       Supported Point Formats Extension has been used, a certificate can
       only be considered suitable for use with these authentication methods
       if the point is represented in uncompressed point format.)

       Meaning of this message:

       This message is used to authentically convey the client's static
       public key to the server.  The following table summarizes what client
       certificate types are appropriate for the ECC-based client
       authentication mechanisms described in Section 3.  ECC public keys
       must be encoded in certificates as described in Section 5.9.

       NOTE: The client's Certificate message is capable of carrying a chain
       of certificates.  The restrictions mentioned in Table 4 apply only to
       the client's certificate (first in the chain).

    Blake-Wilson, et al.         Informational                     [Page 22]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

              Client
              Authentication Method   Client Certificate Type
              ---------------------   -----------------------

              ECDSA_sign              Certificate MUST contain an
                                      ECDSA-capable public key and
                                      be signed with ECDSA.

              ECDSA_fixed_ECDH        Certificate MUST contain an
                                      ECDH-capable public key on the
                                      same elliptic curve as the server's
                                      long-term ECDH key.  This certificate
                                      MUST be signed with ECDSA.

              RSA_fixed_ECDH          Certificate MUST contain an
                                      ECDH-capable public key on the
                                      same elliptic curve as the server's
                                      long-term ECDH key.  This certificate
                                      MUST be signed with RSA.

                         Table 4: Client Certificate Types

       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.

       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.  If client
       authentication with ECDSA_fixed_ECDH or RSA_fixed_ECDH is used, this
       message is empty.  Otherwise, it contains the client's ephemeral ECDH
       public key.

    Blake-Wilson, et al.         Informational                     [Page 23]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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;

       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").  (This is "explicit"
          in ECC cipher suites except when the client uses the
          ECDSA_fixed_ECDH or RSA_fixed_ECDH client authentication
          mechanism.)

            struct {
                select (PublicValueEncoding) {
                    case implicit: struct { };
                    case explicit: ECPoint ecdh_Yc;
                } ecdh_public;
            } 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 or compressed format.  Here, the format MUST
          conform to what the server has requested through a Supported Point
          Formats Extension if this extension was used, and MUST be
          uncompressed if this extension was not used.

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

       Actions of the sender:

       The client selects an ephemeral ECDH public key corresponding to the
       parameters it received from the server according to the ECKAS-DH1
       scheme from IEEE 1363 [6].  It conveys this information to the client
       in the ClientKeyExchange message using the format defined above.

    Blake-Wilson, et al.         Informational                     [Page 24]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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, e.g., when the
       client is authenticated using the ECDSA_sign mechanism.

       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 [2] and [3], and the latter is extended here in
       Section 5.4.  For the ecdsa case, the signature field in the
       CertificateVerify message contains an ECDSA signature computed over
       handshake messages exchanged so far, exactly similar to
       CertificateVerify with other signing algorithms in [2] and [3]:

            CertificateVerify.signature.sha_hash
                SHA(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 [9] corresponding to the following ASN.1
       notation [8].

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

    Blake-Wilson, et al.         Informational                     [Page 25]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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 [14] or another RFC that replaces or extends it.
       Clients SHOULD use the elliptic curve domain parameters recommended
       in ANSI X9.62 [7], FIPS 186-2 [11], and SEC 2 [13].

    5.10.  ECDH, ECDSA, and RSA Computations

       All ECDH calculations (including parameter and key generation as well
       as the shared secret calculation) are performed according to [6]
       using the ECKAS-DH1 scheme with the identity map as 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, the Field 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.  As of TLS 1.0 [2] and
       1.1 [3], this means that the MD5- and SHA-1-based TLS PRF serves as a
       KDF; it is conceivable that future TLS versions or new TLS extensions
       introduced in the future may vary this computation.)

       All ECDSA computations MUST be performed according to ANSI X9.62 [7]
       or its successors.  Data to be signed/verified is hashed, and the
       result run directly through the ECDSA algorithm with no additional
       hashing.  The default hash function is SHA-1 [10], and sha_size (see
       Sections 5.4 and 5.8) is 20.  However, an alternative hash function,
       such as one of the new SHA hash functions specified in FIPS 180-2
       [10], may be used instead if the certificate containing the EC public

    Blake-Wilson, et al.         Informational                     [Page 26]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       key explicitly requires use of another hash function.  (The mechanism
       for specifying the required hash function has not been standardized,
       but this provision anticipates such standardization and obviates the
       need to update this document in response.  Future PKIX RFCs may
       choose, for example, to specify the hash function to be used with a
       public key in the parameters field of subjectPublicKeyInfo.)

       All RSA signatures must be generated and verified according to PKCS#1
       [12] block type 1.

    6.  Cipher Suites

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

         CipherSuite TLS_ECDH_ECDSA_WITH_NULL_SHA           = { 0xC0, 0x01 }
         CipherSuite TLS_ECDH_ECDSA_WITH_RC4_128_SHA        = { 0xC0, 0x02 }
         CipherSuite TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA   = { 0xC0, 0x03 }
         CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA    = { 0xC0, 0x04 }
         CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA    = { 0xC0, 0x05 }

         CipherSuite TLS_ECDHE_ECDSA_WITH_NULL_SHA          = { 0xC0, 0x06 }
         CipherSuite TLS_ECDHE_ECDSA_WITH_RC4_128_SHA       = { 0xC0, 0x07 }
         CipherSuite TLS_ECDHE_ECDSA_WITH_3DES_EDE_CBC_SHA  = { 0xC0, 0x08 }
         CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA   = { 0xC0, 0x09 }
         CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA   = { 0xC0, 0x0A }

         CipherSuite TLS_ECDH_RSA_WITH_NULL_SHA             = { 0xC0, 0x0B }
         CipherSuite TLS_ECDH_RSA_WITH_RC4_128_SHA          = { 0xC0, 0x0C }
         CipherSuite TLS_ECDH_RSA_WITH_3DES_EDE_CBC_SHA     = { 0xC0, 0x0D }
         CipherSuite TLS_ECDH_RSA_WITH_AES_128_CBC_SHA      = { 0xC0, 0x0E }
         CipherSuite TLS_ECDH_RSA_WITH_AES_256_CBC_SHA      = { 0xC0, 0x0F }

         CipherSuite TLS_ECDHE_RSA_WITH_NULL_SHA            = { 0xC0, 0x10 }
         CipherSuite TLS_ECDHE_RSA_WITH_RC4_128_SHA         = { 0xC0, 0x11 }
         CipherSuite TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA    = { 0xC0, 0x12 }
         CipherSuite TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA     = { 0xC0, 0x13 }
         CipherSuite TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA     = { 0xC0, 0x14 }

         CipherSuite TLS_ECDH_anon_WITH_NULL_SHA            = { 0xC0, 0x15 }
         CipherSuite TLS_ECDH_anon_WITH_RC4_128_SHA         = { 0xC0, 0x16 }
         CipherSuite TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA    = { 0xC0, 0x17 }
         CipherSuite TLS_ECDH_anon_WITH_AES_128_CBC_SHA     = { 0xC0, 0x18 }
         CipherSuite TLS_ECDH_anon_WITH_AES_256_CBC_SHA     = { 0xC0, 0x19 }

                            Table 5: TLS ECC cipher suites

    Blake-Wilson, et al.         Informational                     [Page 27]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       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 [2] and
       [3].  AES ciphers are defined in [19].

       Server implementations SHOULD support all of the following cipher
       suites, and client implementations SHOULD support at least one of
       them: TLS_ECDH_ECDSA_WITH_3DES_EDE_CBC_SHA,
       TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA,
       TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA, and
       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA.

    7.  Security Considerations

       Security issues are discussed throughout this memo.

       For TLS handshakes using ECC cipher suites, the security
       considerations in appendices D.2 and D.3 of [2] and [3] apply
       accordingly.

       Security discussions specific to ECC can be found in [6] and [7].
       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.

       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
       curves over F_p with p random might be considered more conservative
       than curves over F_2^m as there is no choice between multiple fields
       of similar size for characteristic 2).  Note, however, that algebraic
       structure can also lead to implementation efficiencies, and
       implementers and users may, therefore, need to balance conservatism
       against a need for efficiency.  Concrete attacks are known against
       only very few special classes of curves, such as supersingular
       curves, and these classes are excluded from the ECC standards that
       this document references [6], [7].

       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 [5], [6], [7], and [11].

    Blake-Wilson, et al.         Informational                     [Page 28]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       Implementers and users must also consider whether they need forward
       secrecy.  Forward secrecy refers to the property that session keys
       are not compromised if the static, certified keys belonging to the
       server and client are compromised.  The ECDHE_ECDSA and ECDHE_RSA key
       exchange algorithms provide forward secrecy protection in the event
       of server key compromise, while ECDH_ECDSA and ECDH_RSA do not.
       Similarly, if the client is providing a static, certified key,
       ECDSA_sign client authentication provides forward secrecy protection
       in the event of client key compromise, while ECDSA_fixed_ECDH and
       RSA_fixed_ECDH do not.  Thus, to obtain complete forward secrecy
       protection, ECDHE_ECDSA or ECDHE_RSA must be used for key exchange,
       with ECDSA_sign used for client authentication if necessary.  Here
       again the security benefits of forward secrecy may need to be
       balanced against the improved efficiency offered by other options.

    8.  IANA Considerations

       This document describes three new name spaces for use with the TLS
       protocol:

       o  NamedCurve (Section 5.1)

       o  ECPointFormat (Section 5.1)

       o  ECCurveType (Section 5.4)

       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.  Any
       additional assignments require IETF Consensus action [16].

    9.  Acknowledgements

       The authors wish to thank Bill Anderson and Tim Dierks.

    Blake-Wilson, et al.         Informational                     [Page 29]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    10.  References

    10.1.  Normative References

       [1]   Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
             Levels", RFC 2119, March 1997.

       [2]   Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
             RFC 2246, January 1999.

       [3]   Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
             Protocol Version 1.1", RFC 4346, April 2006.

       [4]   Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
             T. Wright, "Transport Layer Security (TLS) Extensions", RFC
             4366, April 2006.

       [5]   SECG, "Elliptic Curve Cryptography", SEC 1, 2000,
             <http://www.secg.org/>.

       [6]   IEEE, "Standard Specifications for Public Key Cryptography",
             IEEE 1363, 2000.

       [7]   ANSI, "Public Key Cryptography For The Financial Services
             Industry: The Elliptic Curve Digital Signature Algorithm
             (ECDSA)", ANSI X9.62, 1998.

       [8]   International Telecommunication Union, "Information technology
             - Abstract Syntax Notation One (ASN.1): Specification of basic
             notation", ITU-T Recommendation X.680, 2002.

       [9]   International Telecommunication Union, "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, 2002.

       [10]  NIST, "Secure Hash Standard", FIPS 180-2, 2002.

       [11]  NIST, "Digital Signature Standard", FIPS 186-2, 2000.

       [12]  RSA Laboratories, "PKCS#1: RSA Encryption Standard version
             1.5", PKCS 1, November 1993.

       [13]  SECG, "Recommended Elliptic Curve Domain Parameters", SEC 2,
             2000, <http://www.secg.org/>.

    Blake-Wilson, et al.         Informational                     [Page 30]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       [14]  Polk, T., Housley, R., and L. Bassham, "Algorithms and
             Identifiers for the Internet X.509 Public Key Infrastructure
             Certificate and Certificate Revocation List (CRL) Profile",
             RFC 3279, April 2002.

       [15]  Housley, R., Polk, T., Ford, W., and D. Solo, "Internet X.509
             Public Key Infrastructure Certificate and Certificate
             Revocation List (CRL) Profile", RFC 3280, April 2002.

       [16]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
             Considerations Section in RFCs", RFC 2434, October 1998.

    10.2.  Informative References

       [17]  Harper, G., Menezes, A., and S. Vanstone, "Public-Key
             Cryptosystems with Very Small Key Lengths", Advances in
             Cryptology -- EUROCRYPT '92, LNCS 658, 1993.

       [18]  Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
             Sizes", Journal of Cryptology 14 (2001) 255-293,
             <http://www.cryptosavvy.com/>.

       [19]  Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
             Transport Layer Security (TLS)", RFC 3268, June 2002.

    Blake-Wilson, et al.         Informational                     [Page 31]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    Appendix A.  Equivalent Curves (Informative)

       All of the NIST curves [11] and several of the ANSI curves [7] are
       equivalent to curves listed in Section 5.1.1.  In the following
       table, multiple names in one row represent aliases for the same
       curve.

                 ------------------------------------------
                           Curve names chosen by
                      different standards organizations
                 ------------+---------------+-------------
                 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 6: Equivalent curves defined by SECG, ANSI, and NIST

    Blake-Wilson, et al.         Informational                     [Page 32]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    Authors' Addresses

       Simon Blake-Wilson
       SafeNet Technologies BV
       Amstelveenseweg 88-90
       1075 XJ, Amsterdam
       NL

       Phone: +31 653 899 836
       EMail: sblakewilson@safenet-inc.com

       Nelson Bolyard
       Sun Microsystems Inc.
       4170 Network Circle
       MS SCA17-201
       Santa Clara, CA  95054
       US

       Phone: +1 408 930 1443
       EMail: nelson@bolyard.com

       Vipul Gupta
       Sun Microsystems Laboratories
       16 Network Circle
       MS UMPK16-160
       Menlo Park, CA  94025
       US

       Phone: +1 650 786 7551
       EMail: vipul.gupta@sun.com

       Chris Hawk
       Corriente Networks LLC
       1563 Solano Ave., #484
       Berkeley, CA  94707
       US

       Phone: +1 510 527 0601
       EMail: chris@corriente.net

    Blake-Wilson, et al.         Informational                     [Page 33]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

       Bodo Moeller
       Ruhr-Uni Bochum
       Horst-Goertz-Institut, Lehrstuhl fuer Kommunikationssicherheit
       IC 4/139
       44780 Bochum
       DE

       Phone: +49 234 32 26795
       EMail: bodo@openssl.org

    Blake-Wilson, et al.         Informational                     [Page 34]

    RFC 4492               ECC Cipher Suites for TLS                May 2006

    Full Copyright Statement

       Copyright (C) The Internet Society (2006).

       This document is subject to the rights, licenses and restrictions
       contained in BCP 78, and except as set forth therein, the authors
       retain all their rights.

       This document and the information contained herein are provided on an
       "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
       OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
       ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
       INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
       INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
       WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Intellectual Property

       The IETF takes no position regarding the validity or scope of any
       Intellectual Property Rights or other rights that might be claimed to
       pertain to the implementation or use of the technology described in
       this document or the extent to which any license under such rights
       might or might not be available; nor does it represent that it has
       made any independent effort to identify any such rights.  Information
       on the procedures with respect to rights in RFC documents can be
       found in BCP 78 and BCP 79.

       Copies of IPR disclosures made to the IETF Secretariat and any
       assurances of licenses to be made available, or the result of an
       attempt made to obtain a general license or permission for the use of
       such proprietary rights by implementers or users of this
       specification can be obtained from the IETF on-line IPR repository at
       http://www.ietf.org/ipr.

       The IETF invites any interested party to bring to its attention any
       copyrights, patents or patent applications, or other proprietary
       rights that may cover technology that may be required to implement
       this standard.  Please address the information to the IETF at
       ietf-ipr@ietf.org.

    Acknowledgement

       Funding for the RFC Editor function is provided by the IETF
       Administrative Support Activity (IASA).

    Blake-Wilson, et al.         Informational                     [Page 35]