path: root/doc/protocol/draft-ietf-tls-ecdhe-psk-01.txt

TLS Working Group                                         Mohamad Badra 
Internet Draft                                         LIMOS Laboratory 
Intended status: Informational                            April 2, 2008 
Expires: October 2008 
                                    
 
                                      
         ECDHE_PSK Ciphersuites for Transport Layer Security (TLS)  
                      draft-ietf-tls-ecdhe-psk-01.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 ciphersuites that use a pre-shared key (PSK) to authenticate 
   an Elliptic Curve Diffie-Hellman exchange (ECDH).  These ciphersuites 
   provide Perfect Forward Secrecy (PFS). 


 
 
 
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Table of Contents 

   1. Introduction...................................................3 
      1.1. Conventions used in this document.........................3 
   2. ECDHE_PSK Key Exchange Algorithm...............................3 
   3. ECDHE_PSK Key Exchange Algorithm with NULL Encryption..........5 
   4. Security Considerations........................................5 
   5. IANA Considerations............................................5 
   6. Acknowledgments................................................5 
   7. References.....................................................5 
      7.1. Normative References......................................5 
   Author's Addresses................................................6 
   Intellectual Property Statement...................................6 
   Disclaimer of Validity............................................6 
    
































 
 
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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).  These ciphersuites 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 ciphersuites 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 ciphersuites that use a PSK to 
   authenticate an ECDH exchange.  These ciphersuites provide Perfect 
   Forward Secrecy.  One of these ciphersuite 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. 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]. 

2. ECDHE_PSK Key Exchange Algorithm 

   The ciphersuites in this section match the ciphersuites defined in 
   [RFC4279], except that they use an Elliptic Curve Diffie-Hellman 
   exchange [RFC4492] authenticated with a PSK.  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 
 
 
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   These ciphersuites make use of the EC parameter negotiation mechanism 
   defined in RFC 4492.  When the ciphersuites 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 Diffie-
   Hellman parameters).  The PSK identity and identity hint fields have 
   the same meaning and encoding 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 an ECDH 
   operation (See section 5.10 of [RFC4492]) to compute the shared 
   secret.  Next, concatenate a uint16 containing the length of the 
   shared secret (in octets), the shared secret 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 the shared secret: 

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

 
 
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3. ECDHE_PSK Key Exchange Algorithm with NULL Encryption 

   The ciphersuite in this section matches the ciphersuites defined in 
   section 2, except that we define a suite with null encryption. 

     CipherSuite                     Key Exchange   Cipher      Hash  

     TLS_ECDHE_PSK_WITH_NULL_SHA     ECDHE_PSK      NULL        SHA  

4. Security Considerations 

   The security considerations described throughout [RFC4279], 
   [RFC4346], [RFC4492], and [RFC4785] 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 
   [RFC4346]. 

     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_NULL_SHA           = { 0xXX, 0xXX }; 

6. Acknowledgments 

   The author would like to thank Bodo Moeller, Simon Josefsson, Uri 
   Blumenthal, Pasi Eronen, Alfred Hoenes, Paul Hoffman, Joseph Salowey, 
   and the TLS mailing list members for their comments on the document. 

7. References 

7.1. Normative References 

   [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.  

   [RFC4346] Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1", 
             RFC 4346, April 2006. 


 
 
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   [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. 

Author's Addresses 

   Mohamad Badra 
   LIMOS Laboratory - UMR6158, CNRS 
   France 
    
   Email: badra@isima.fr 
    

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