TLS Working Group                                         Mohamad Badra
Internet Draft                                         LIMOS Laboratory
Intended status: Informational                         November 1, 2008
Expires: May 2009



         ECDHE_PSK Ciphersuites for Transport Layer Security (TLS)
                      draft-ietf-tls-ecdhe-psk-05.txt


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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   This document extends RFC 4279, RFC 4492 and RFC 4785, and specifies
   a set of cipher suites that use a pre-shared key (PSK) to
   authenticate an Elliptic Curve Diffie-Hellman exchange (ECDH).  These
   cipher suites provide Perfect Forward Secrecy (PFS).





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   Table of Contents

   1. Introduction...................................................3
      1.1. Applicability Statement...................................3
      1.2. Conventions used in this document.........................3
   2. ECDHE_PSK Key Exchange Algorithm...............................3
   3. ECDHE_PSK Based Cipher Suites..................................4
      3.1. ECDHE_PSK Cipher Suites Using the SHA-1 Hash..............4
      3.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes................5
   4. ECDHE_PSK Based Cipher Suites with NULL Encryption.............5
      4.1. ECDHE_PSK Cipher Suite Using the SHA-1 Hash with NULL
      Encryption.....................................................5
      4.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with NULL
      Encryption.....................................................6
   5. Security Considerations........................................6
   6. IANA Considerations............................................6
   7. Acknowledgments................................................7
   8. References.....................................................7
      8.1. Normative References......................................7
   Author's Addresses................................................7
   Intellectual Property Statement...................................7
   Disclaimer of Validity............................................8



























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. Introduction

   RFC 4279 specifies cipher suites for supporting TLS using pre-shared
   symmetric keys which (a) use only symmetric key operations for
   authentication, (b) use a Diffie-Hellman exchange authenticated with
   a pre-shared key, or (c) combine public key authentication of the
   server with pre-shared key authentication of the client.

   RFC 4785 specifies authentication-only cipher suites (with no
   encryption).  These cipher suites are useful when authentication and
   integrity protection is desired, but confidentiality is not needed or
   not permitted.

   RFC 4492 defines a set of ECC-based cipher suites for TLS and
   describes the use of ECC certificates for client authentication.  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.

   This document specifies a set of cipher suites that use a PSK to
   authenticate an ECDH exchange.  These cipher suites provide Perfect
   Forward Secrecy.  One of these cipher suites provides authentication-
   only.

   The reader is expected to become familiar with RFC 4279, RFC 4492,
   and RFC 4785 prior to studying this document.

1.1. Applicability Statement

   The cipher suites defined in this document can be negotiated,
   whatever the negotiated TLS version is.

1.2. Conventions used in this document

   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 [RFC2119].

. ECDHE_PSK Key Exchange Algorithm

   The cipher suites described in this document make use of the EC
   parameter negotiation mechanism defined in RFC 4492.  When the cipher
   suites defined in this document are used, the 'ec_diffie_hellman_psk'
   case inside the ServerKeyExchange and ClientKeyExchange structure
   MUST be used instead of the 'psk' case defined in [RFC4279] (i.e.,
   the ServerKeyExchange and ClientKeyExchange messages include the


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   Diffie-Hellman parameters).  The PSK identity and identity hint
   fields have the same meaning and encoding as specified in [RFC4279]
   (note that the ServerKeyExchange message is always sent, even if no
   PSK identity hint is provided).

   The format of the ServerKeyExchange and ClientKeyExchange messages is
   shown below.

         struct {
             select (KeyExchangeAlgorithm) {
                 /* other cases for rsa, diffie_hellman, etc. */
                 case ec_diffie_hellman_psk:  /* NEW */
                     opaque psk_identity_hint<0..2^16-1>;
                     ServerECDHParams params;
             };
         } ServerKeyExchange;

         struct {
             select (KeyExchangeAlgorithm) {
                 /* other cases for rsa, diffie_hellman, etc. */
                 case ec_diffie_hellman_psk:   /* NEW */
                     opaque psk_identity<0..2^16-1>;
                     ClientECDiffieHellmanPublic public;
             } exchange_keys;
         } ClientKeyExchange;

   The premaster secret is formed as follows.  First, perform the ECDH
   computation as described in Section 5.10 of [RFC4492].  Let Z be the
   octet string produced by this computation.  Next, concatenate a
   uint16 containing the length of Z (in octets), Z itself, a uint16
   containing the length of the PSK (in octets), and the PSK itself.

   This corresponds to the general structure for the premaster secrets
   (see Note 1 in Section 2 of [RFC4279]), with "other_secret"
   containing Z.

         struct {
              opaque other_secret<0..2^16-1>;
              opaque psk<0..2^16-1>;
         };

. ECDHE_PSK Based Cipher Suites

3.1. ECDHE_PSK Cipher Suites Using the SHA-1 Hash

      CipherSuite TLS_ECDHE_PSK_WITH_RC4_128_SHA          = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA     = {0xXX,0xXX};


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      CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA      = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA      = {0xXX,0xXX};

   The above four cipher suites match the cipher suites defined in
   [RFC4279], except that they use an Elliptic Curve Diffie-Hellman
   exchange [RFC4492] authenticated with a PSK, and that:

           -  The MAC is HMAC [RFC2104] with SHA-1 as the hash
              function.

           -  When negotiated in a version of TLS prior to 1.2, the PRF
              from that version is used; otherwise the PRF is the TLS
              PRF [RFC5246] with SHA-256 as the hash function.

3.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes

      CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256   = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384   = {0xXX,0xXX};

   The above two cipher suites are the same as the corresponding AES
   cipher suites in section 3.1 above, except for the hash and PRF
   algorithms, which SHALL be as follows:

        O For the cipher suites TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256:

           -  The MAC is HMAC [RFC2104] with SHA-256 as the hash
              function.

           -  When negotiated in a version of TLS prior to 1.2, the PRF
              from that version is used; otherwise the PRF is the TLS
              PRF [RFC5246] with SHA-256 as the hash function.

        o For the cipher suite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384:

           -  The MAC is HMAC [RFC2104] with SHA-384 as the hash
              function.

           -  When negotiated in a version of TLS prior to 1.2, the PRF
              from that version is used; otherwise the PRF is the TLS
              PRF [RFC5246] with SHA-384 as the hash function.

. ECDHE_PSK Based Cipher Suites with NULL Encryption

4.1. ECDHE_PSK Cipher Suite Using the SHA-1 Hash with NULL Encryption

   The following cipher suite matches the cipher suites defined in
   section 3.1, except that we define a suite with NULL encryption.


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      CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA             = {0xXX,0xXX};

4.2. ECDHE_PSK Cipher Suites Using SHA-2 Hashes with NULL Encryption

   The following two cipher suites are the same as the corresponding
   cipher suites in section 3.2, but with NULL encryption (instead of
   AES).

      CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA256          = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA384          = {0xXX,0xXX};

. Security Considerations

   The security considerations described throughout [RFC5246],
   [RFC4785], [RFC4492], and [RFC4279] apply here as well.  In
   particular, as authentication-only cipher suites (with no encryption)
   defined here do not support confidentiality, care should be taken not
   to send sensitive information (such as passwords) over connections
   protected with one of the cipher suites with NULL encryption defined
   in this document.

   Given the current state of published to date crypto attacks, HMAC-
   SHA1 apparently is not (yet) so bad that we need to risk breaking
   interoperability with previous versions of TLS.  However,
   implementers and administrators should monitor the general statements
   on recommended cryptographic algorithms published from time to time
   by various forums including the IETF, as a base for the portfolio
   they support and the policies for strength of function acceptable for
   the cipher suites they set.

. IANA Considerations

   This document defines the following new cipher suites, whose values
   are to be assigned from the TLS Cipher Suite registry defined in
   [RFC5246].

      CipherSuite TLS_ECDHE_PSK_WITH_RC4_128_SHA          = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA     = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA      = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA      = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA256   = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA384   = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA             = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA256          = {0xXX,0xXX};
      CipherSuite TLS_ECDHE_PSK_WITH_NULL_SHA384          = {0xXX,0xXX};




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. Acknowledgments

   The author appreciates Alfred Hoenes for his detailed review and
   effort on issues resolving discussion.  The author would like to
   acknowledge Bodo Moeller, Simon Josefsson, Uri Blumenthal, Pasi
   Eronen, Paul Hoffman, Joseph Salowey, Mark Tillinghast, and the TLS
   mailing list members for their comments on the document.

. References

8.1. Normative References

   [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed
             Hashing for Message Authentication", RFC 2104, February
             1997.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
             for Transport Layer Security (TLS)", RFC 4279, December
             2005.

   [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C. and B.
             Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
             for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [RFC4785] Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
             Ciphersuites with NULL Encryption for Transport Layer
             Security (TLS)", RFC 4785, January 2007.

   [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

Author's Addresses

   Mohamad Badra
   LIMOS Laboratory - UMR6158, CNRS
   France

   Email: badra@isima.fr


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