


Transport Layer Security Working                               D. Taylor
Group                                             Forge Research Pty Ltd
Internet-Draft                                           August 21, 2002
Expires: February 19, 2003


                    Using SRP for TLS Authentication
                         draft-ietf-tls-srp-02

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 19, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This memo presents a technique for using the SRP [2] (Secure Remote
   Password) protocol as an authentication method for the TLS
   [1](Transport Layer Security) protocol.











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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    SRP Authentication in TLS  . . . . . . . . . . . . . . . . .  4
   2.1   Modifications to the TLS Handshake Sequence  . . . . . . . .  4
   2.1.1 Message Sequence . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.2 Session Re-use . . . . . . . . . . . . . . . . . . . . . . .  4
   2.2   SRP Verifier Message Digest Selection  . . . . . . . . . . .  5
   2.3   Changes to the Handshake Message Contents  . . . . . . . . .  5
   2.3.1 Client hello . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.3.2 Server hello . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.3.3 Server certificate . . . . . . . . . . . . . . . . . . . . .  5
   2.3.4 Client key exchange  . . . . . . . . . . . . . . . . . . . .  6
   2.3.5 Server key exchange  . . . . . . . . . . . . . . . . . . . .  6
   2.4   Calculating the Pre-master Secret  . . . . . . . . . . . . .  6
   2.5   Cipher Suite Definitions . . . . . . . . . . . . . . . . . .  6
   2.6   New Message Structures . . . . . . . . . . . . . . . . . . .  7
   2.6.1 ExtensionType  . . . . . . . . . . . . . . . . . . . . . . .  7
   2.6.2 Client Hello . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.6.3 Server Hello . . . . . . . . . . . . . . . . . . . . . . . .  8
   2.6.4 Client Key Exchange  . . . . . . . . . . . . . . . . . . . .  8
   2.6.5 Server Key Exchange  . . . . . . . . . . . . . . . . . . . .  8
   3.    Security Considerations  . . . . . . . . . . . . . . . . . . 10
         References . . . . . . . . . . . . . . . . . . . . . . . . . 11
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 11
   A.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 13
























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1. Introduction

   At the time of writing, TLS uses public key certificiates with RSA/
   DSA digital signatures, or Kerberos, for authentication.

   These authentication methods do not seem well suited to the
   applications now being adapted to use TLS (IMAP [3], FTP [5], or
   TELNET [6], for example).  Given these protocols (and others like
   them) are designed to use the user name and password method of
   authentication, being able to safely use user names and passwords to
   authenticate the TLS connection provides a much easier route to
   additional security than implementing a public key infrastructure in
   certain situations.

   SRP is an authentication method that allows the use of user names and
   passwords over unencrypted channels without revealing the password to
   an eavesdropper.  SRP also supplies a shared secret at the end of the
   authetication sequence that can be used to generate encryption keys.

   This document describes the use of the SRP authentication method for
   TLS.

   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.


























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2. SRP Authentication in TLS

2.1 Modifications to the TLS Handshake Sequence

   The SRP protocol can not be implemented using the sequence of
   handshake messages defined in [1] due to the sequence in which the
   SRP messages must be sent.

   This document presents a new sequence of handshake messages for
   handshakes using the SRP authentication method.

2.1.1 Message Sequence

   Handshake Message Flow for SRP Authentication

          Client                                 Server
            |                                      |
       Client Hello (U) ------------------------>  |
            |  <---------------------------- Server Hello (g, N, s)
            |  <---------------------------- Certificate*
       Client Key Exchange (A) ----------------->  |
            |  <---------------------------- Server Key Exchange (B)
            |  <---------------------------- Server Hello Done
       change cipher spec                          |
       Finished -------------------------------->  |
            |                                change cipher spec
            |  <---------------------------- Finished
            |                                      |

   * Indicates optional or situation-dependent messages that are not
   always sent.

   The identifiers given after each message name refer to the SRP
   variables included in that message.  The variables U, g, N, s, A, and
   B are defined in [2].

   Extended client and server hello messages, as defined in [7], are
   used to to send the initial client and server values.

   The client key exchange message is sent during the sequence of server
   messages.  This modification is required because the client must send
   its public key (A) before it receives the servers public key (B), as
   stated in Section 3 of [2].

2.1.2 Session Re-use

   The short handshake mechanism for re-using sessions for new
   connections, and renegotiating keys for existing connections will



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   still work with the SRP authentication mechanism and handshake.

   When a client attemps to re-use a session that uses SRP
   authentication, it MUST still include the SRP extension carrying the
   user name (U) in the client hello message, in case the server cannot
   or will not allow re-use of the session, meaning a full handshake
   sequence is required.

   If a client requests an existing session and the server agrees to use
   it (meaning the short handshake will be used), the server MAY omit
   the SRP extension from the server hello message, as the information
   it contains is not used in the short handshake.

2.2 SRP Verifier Message Digest Selection

   The cipher suites defined in this document use the SHA-1 message
   digest with the SRP algorithm, as specified in [2].  Implementations
   conforming to this document MUST use the SHA-1 message digest.

   Future documents may define other cipher suites that use different
   message digests, or other similar functions, with the SRP algorithm.

2.3 Changes to the Handshake Message Contents

   This section describes the changes to the TLS handshake message
   contents when SRP is being used for authentication.  The definitions
   of the new message contents and the on-the-wire changes are given in
   Section 2.6.

2.3.1 Client hello

   The user name is appended to the standard client hello message using
   the hello message extension mechanism defined in [7].

2.3.2 Server hello

   The the generator (g), the prime (N), and the salt value (s) read
   from the SRP password file are appended to the server hello message
   using the hello message extension mechanism defined in [7].

2.3.3 Server certificate

   The server MUST send a certificate if it agrees to an SRP cipher
   suite that requires the server to provide additional authentication
   in the form of a digital signature.  See Section 2.5 for details of
   which ciphersuites defined in this document require a server
   certificate to be sent.




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   Because the server's certificate is only used for generating a
   digital signature in SRP cipher suites, the certificate sent MUST
   contain a public key that can be used for generating digital
   signatures.

2.3.4 Client key exchange

   The client key exchange message carries the client's public key (A),
   which is calculated using both information known locally, and
   information received in the server hello message.  This message MUST
   be sent before the server key exchange message.

2.3.5 Server key exchange

   The server key exchange message contains the server's public key (B).
   The server key exchange message MUST be sent after the client key
   exchange message.

   If the server has sent a certificate message, the server key exchange
   message MUST be signed.

2.4 Calculating the Pre-master Secret

   The shared secret resulting from the SRP calculations (S) (defined in
   [2]) is used as the pre-master secret.

   The finished messages perform the same function as the client and
   server evidence messages specified in [2].  If either the client or
   the server calculate an incorrect value, the finished messages will
   not be understood, and the connection will be dropped as specified in
   [1].

2.5 Cipher Suite Definitions

   The following cipher suites are added by this draft.  The usage of
   AES ciphersuites is as defined in [4].

      CipherSuite TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA     = { 0x00,0x50 };

      CipherSuite TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x51 };

      CipherSuite TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x52 };

      CipherSuite TLS_SRP_SHA_WITH_AES_128_CBC_SHA      = { 0x00,0x53 };

      CipherSuite TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA  = { 0x00,0x54 };

      CipherSuite TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA  = { 0x00,0x55 };



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      CipherSuite TLS_SRP_SHA_WITH_AES_256_CBC_SHA      = { 0x00,0x56 };

      CipherSuite TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA  = { 0x00,0x57 };

      CipherSuite TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA  = { 0x00,0x58 };

   Cipher suites that do not include a digitial signature algorithm
   identifier assume the server is authenticated by its possesion of the
   SRP database.

   Cipher suites that begin with TLS_SRP_SHA_RSA or TLS_SRP_SHA_DSS
   require the server to send a certificate message containing a
   certificate with the specified type of public key, and to sign the
   server key exchange message using a matching private key.

   Implementations conforming to this specification MUST implement the
   TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA ciphersuite, SHOULD implement the
   TLS_SRP_SHA_WITH_AES_128_CBC_SHA and TLS_SRP_SHA_WITH_AES_256_CBC_SHA
   ciphersuites, and MAY implement the remaining ciphersuites.

2.6 New Message Structures

   This section shows the structure of the messages passed during a
   handshake that uses SRP for authentication.  The representation
   language used is the same as that used in [1].

   When encoding the numbers g, N, A, and B as opaque types, if the most
   significant bit is set, an extra byte of value 0x00 (all bits
   cleared) MUST be added as the most significant byte.  This is done as
   a safeguard against implementations that do not assume these numbers
   are positive.

2.6.1 ExtensionType

   A new value, "srp(6)", has been added to the enumerated
   ExtensionType, defined in [7].  This value is used as the extension
   number for the extensions in both the client hello message and the
   server hello message.

2.6.2 Client Hello

   The user name (U) is encoded in an SRPExtension structure, and sent
   in an extended client hello message, using an extension of type
   "srp".







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   enum { client, server } ClientOrServerExtension;

   struct {
      select(ClientOrServerExtension) {
         case client:
            opaque srp_U<1..2^8-1>;
         case server:
            opaque srp_s<1..2^8-1>
            opaque srp_N<1..2^16-1>;
            opaque srp_g<1..2^16-1>;
      }
   } SRPExtension;


2.6.3 Server Hello

   The generator (g), the prime (N), and the salt value (s) are encoded
   in an SRPExtension structure, which is sent in an extended server
   hello message, using an extension of type "srp".

2.6.4 Client Key Exchange

   When the value of KeyExchangeAlgorithm is set to "srp", the client's
   ephemeral public key (A) is sent in the client key exchange message,
   encoded in an ClientSRPPublic structure.

   An extra value, srp, has been added to the enumerated
   KeyExchangeAlgorithm, originally defined in TLS [1].

   struct {
      select (KeyExchangeAlgorithm) {
         case rsa: EncryptedPreMasterSecret;
         case diffie_hellman: ClientDiffieHellmanPublic;
         case srp: ClientSRPPublic;   /* new entry */
      } exchange_keys;
   } ClientKeyExchange;

   enum { rsa, diffie_hellman, srp } KeyExchangeAlgorithm;

   struct {
      opaque srp_A<1..2^16-1>;
   } ClientSRPPublic;


2.6.5 Server Key Exchange

   When the value of KeyExchangeAlgorithm is set to "srp", the server's
   ephemeral public key (B) is sent in the server key exchange message,



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   encoded in an ServerSRPPublic structure.

   The following table gives the SignatureAlgorithm value to be used for
   each ciphersuite.

      Ciphersuite                            SignatureAlgorithm

      TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA         anonymous

      TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA     rsa

      TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA     dsa

      TLS_SRP_SHA_WITH_AES_128_CBC_SHA          anonymous

      TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA      rsa

      TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA      dsa

      TLS_SRP_SHA_WITH_AES_256_CBC_SHA          anonymous

      TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA      rsa

      TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA      dsa


   struct {
      select (KeyExchangeAlgorithm) {
         case diffie_hellman:
            ServerDHParams params;
            Signature signed_params;
         case rsa:
            ServerRSAParams params;
            Signature signed_params;
         case srp:   /* new entry */
            ServerSRPPublic params;
            Signature signed_params;
      };
   } ServerKeyExchange;

   struct {
      opaque srp_B<1..2^16-1>;
   } ServerSRPPublic;     /* SRP parameters */








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3. Security Considerations

   If an attacker is able to steal the SRP verifier file, the attacker
   can masquerade as the real host.  Filesystem based X.509 certificate
   installations are vulnerable to a similar attack unless the server's
   certificate is issued from a PKI that maintains revocation lists, and
   the client TLS code can both contact the PKI and make use of the
   revocation list.











































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References

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

   [2]  Wu, T., "The SRP Authentication and Key Exchange System", RFC
        2945, September 2000.

   [3]  Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June
        1999.

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

   [5]  Ford-Hutchinson, P., Carpenter, M., Hudson, T., Murray, E. and
        V. Wiegand, "Securing FTP with TLS", draft-murray-auth-ftp-ssl-
        09 (work in progress), April 2002.

   [6]  Boe, M. and J. Altman, "TLS-based Telnet Security", draft-ietf-
        tn3270e-telnet-tls-06 (work in progress), April 2002.

   [7]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T.
        Wright, "TLS Extensions", draft-ietf-tls-extensions-05 (work in
        progress), July 2002.


Author's Address

   David Taylor
   Forge Research Pty Ltd

   EMail: DavidTaylor@forge.com.au
   URI:   http://www.forge.com.au/


















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Appendix A. Acknowledgements

   Thanks to all on the IETF tls mailing list for ideas and analysis.
















































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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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