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rfc:rfc4492

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.
  1. —————————————–

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

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

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