
 

Internet Engineering Task Force                                M. Badra 
INTERNET DRAFT                                         LIMOS Laboratory 
Updates: 4785, 4279 
 
Expires: November 2007                                        July 2007 
    
                      ECDHE_PSK Ciphersuites for TLS 
                    <draft-badra-ecdhe-tls-psk-00.txt> 
    
    
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Copyright Notice 
    
   Copyright (C) The IETF Trust (2007). 
    
Abstract 
    
   This document updates RFC 4785 and RFC 4279 and specifies a set of 
   ciphersuites that use an Elliptic Curve Diffie-Hellman exchange 
   authenticated with a pre-shared key. These ciphersuites provides 
   Perfect Forward Secrecy. It specifies as well one authentication-
   only ciphersuites (with no encryption). This ciphersuite is useful 
   when authentication and integrity protection is desired, but 
   confidentiality is not needed or not permitted. 
    
   The reader is expected to become familiar with RFC 4785 and RFC 4279 
   prior to studying this document. 
    

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1. Introduction 
    
   RFC 4279 specifies ciphersuites for supporting TLS using pre-shared 
   symmetric keys and they (a) use only symmetric key operations for 
   authentication, (b) use a Diffie-Hellman exchange  authenticated 
   with a pre-shared key, or (c) combines public key authentication of 
   the server with pre-shared key authentication of the client. 
    
   RFC 4785 specifies authentication-only ciphersuites (with no 
   encryption).  
    
   This document specifies a set of ciphersuites that use an Elliptic 
   Curve Diffie-Hellman exchange authenticated with a pre-shared key. 
   These ciphersuites provides Perfect Forward Secrecy. It specifies as 
   well one authentication-only ciphersuites (with no encryption). This 
   ciphersuite is useful when authentication and integrity protection 
   is desired, but confidentiality is not needed or not permitted. 
    
2. Updating RFC4279 
    
   The new ciphersuites proposed here match the ciphersuites defined in 
   [RFC4279], except that they use an Elliptic Curve Diffie-Hellman 
   exchange [RFC4492] authenticated with a pre-shared key. They are 
   defined as follow: 
    
   CipherSuite                          Key Exchange  Cipher       Hash 
    
   TLS_ECDHE_PSK_WITH_RC4_128_SHA       ECDHE_PSK     RC4_128       SHA 
   TLS_ECDHE_PSK_WITH_3DES_EDE_CBC_SHA  ECDHE_PSK     3DES_EDE_CBC  SHA 
   TLS_ECDHE_PSK_WITH_AES_128_CBC_SHA   ECDHE_PSK     AES_128_CBC   SHA 
   TLS_ECDHE_PSK_WITH_AES_256_CBC_SHA   ECDHE_PSK     AES_256_CBC   SHA 
    
   When these ciphersuites are used, the ServerKeyExchange and 
   ClientKeyExchange messages also include the Diffie-Hellman 
   parameters.  The PSK identity and identity hint fields have the same 
   meaning as in the previous section (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; 
    

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      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 
   Elliptic Curve Diffie-Hellman computation in the same way as for 
   other Diffie-Hellman-based ciphersuites in [TLS1.0] or [TLS1.1]. Let 
   Z be the value produced by this computation.  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 in RFC 4279) in [RFC4279], with 
   "other_secret" containing Z: 
    
       struct { 
           opaque other_secret<0..2^16-1>; 
           opaque psk<0..2^16-1>; 
       }; 
    
   Here "other_secret" comes from the Elliptic Curve Diffie-Hellman 
   exchange (ECDHE_PSK). 
    
3. Updating RFC4785 
    
   The new ciphersuite proposed here match the ciphersuites defined in 
   [RFC4785], except that it uses an Elliptic Curve Diffie-Hellman 
   exchange authenticated with a pre-shared key  
    
      CipherSuite                     Key Exchange   Cipher      Hash 
    
      TLS_ECDHE_PSK_WITH_NULL_SHA     ECDHE_PSK      NULL        SHA 
    
4. Security Considerations 
    
   The security considerations described throughout [TLS1.0], 
   [TLSv1.1], RFC 4785 and RFC 4279 apply here as well. 
    
5. IANA Considerations  
    
   This document defines the following new ciphersuites, whose values 
   are to be assigned from the TLS Cipher Suite registry defined in 
   [TLS1.1]. 
    
   CipherSuite   TLS_ECDHE_PSK_WITH_RC4_128_SHA       = { 0xXX, 0xXX }; 

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   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_NULL_SHA          = { 0xXX, 0xXX }; 
    
6. References 
    
    
6.1. Normative References 
    
   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 
             Requirement Levels", BCP 14, RFC 2119, March 1997.  
    
   [TLS1.0]  T., Dierks, C., Allen, "The TLS Protocol Version 1.0",  
             RFC 2246, January 1999. 
    
   [TLS1.1]  Dierks, T., Rescorla, E., "The TLS Protocol Version 1.1",  
             RFC 4346, April 200P. 
    
   [RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites 
             for Transport Layer Security (TLS)", RFC 4279, December  
             2005. 
    
   [RFC4785] Blumenthal, U., Goel, P., "Pre-Shared Key (PSK)  
             Ciphersuites with NULL Encryption for Transport Layer  
             Security (TLS)", RFC 4785, January 2007. 
    
   [RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C.,  
             Moeller, B., "Elliptic Curve Cryptography (ECC) Cipher  
             Suites for Transport Layer Security (TLS)", RFC 4492, May  
             2006. 
    
Acknowledgements  
    
   The authors would like to thank Bodo Moeller for comments on the 
   document. 
    
Author's Addresses 
    
   Mohamad Badra 
   LIMOS Laboratory - UMR (6158), CNRS 
   France                    Email: badra@isima.fr 
    
   Full Copyright Statement 
    
   Copyright (C) The IETF Trust (2007). 
    
   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. 

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