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Network Working Group                                          D. Taylor
Internet-Draft                                    Forge Research Pty Ltd
Expires: August 6, 2001                                 February 5, 2001

                    Using SRP for TLS Authentication

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
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as

   Internet-Drafts are draft documents valid for a maximum of six
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   at any time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on August 6, 2001.

Copyright Notice

   Copyright (C) The Internet Society (2001). All Rights Reserved.


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

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1. Introduction

   At the time of writing, TLS[1] 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[4], or
   TELNET[5], for example). Given these protocols (and others like
   them) are designed to use the user name and password method of
   authentication, being able to use user names and passwords to
   authenticate the TLS connection seems to be a useful feature.

   SRP[2] is an authentication method that allows the use of user names
   and passwords in a safe manner.

   This document describes the use of the SRP authentication method for

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

   The identifiers given after each message name refer to variables
   defined in [2] that are sent in that message.

   This new handshake sequence has a number of differences from the
   standard TLS handshake sequence:

   o  The client hello message has the user name appended to the
      message. This is allowable as stated in section of [1].

   o  The client cannot generate its its public key (A) until after it
      has received the (g) and (N) paramters from the server, and the
      client must send its public key before it receives the servers
      public key (B) (as stated in section 3 of [2]). This means the
      client must wait for a server key exchange message containing (g)
      and (N), send a client key exchange message containing (A), and
      then wait for another server key exchange message containing (B).

   o  There is no server identification in this version of a TLS
      handshake. If an attacker gets the SRP password file, they can
      masquerade as the real system.

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2.2 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 details of
   the on-the-wire changes are given in Section 2.5.

2.2.1 The Client Hello Message

   The user name is appended to the standard client hello message. The
   extra data is included in the handshake message hashes.

2.2.2 The First Server Key Exchange Message

   The server key exchange message in the first round contains the
   generator (g), the prime (N), and the salt value (s) read from the
   SRP password file.

2.2.3 The Client Key Exchange Message

   The client key exchange message carries the clients public key (A),
   which is calculated using both information known locally, and
   information received in the first server key exchange message. This
   message MUST be sent between the first and second server key
   exchange messages.

2.2.4 The Second Server Key Exchange Message

   The server key exchange message in the second round contains the
   servers public key (B).

2.3 Calculating the Pre-master Secret

   The shared secret resulting from the SRP calculations (S) 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].

2.4 Cipher Suite Definitions

   The following cipher suites are added by this draft. The numbers
   have been left blank until a suitable range has been selected.

      CipherSuite     TLS_SRP_WITH_3DES_EDE_CBC_SHA       = { ?,? };

      CipherSuite     TLS_SRP_WITH_RC4_128_SHA            = { ?,? };

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      CipherSuite     TLS_SRP_WITH_IDEA_CBC_SHA           = { ?,? };

      CipherSuite     TLS_SRP_WITH_3DES_EDE_CBC_MD5       = { ?,? };

      CipherSuite     TLS_SRP_WITH_RC4_128_MD5            = { ?,? };

      CipherSuite     TLS_SRP_WITH_IDEA_CBC_MD5           = { ?,? };

2.5 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 that used in [1].

   opaque Username<1..2^8-1>;

   enum { non_srp, srp } CipherSuiteType;

   struct {
      ProtocolVersion client_version;
      Random random;
      SessionID session_id;
      CipherSuite cipher_suites<2..2^16-1>;

      /* Need a better way to show the optional user_name field */
      select (CipherSuiteType) {
         case non_srp:
            CompressionMethod compression_methods<1..2^8-1>;
         case srp:
            CompressionMethod compression_methods<1..2^8-1>;
            Username user_name;   /* new entry */
   } ClientHello;

   enum { rsa, diffie_hellman, srp } KeyExchangeAlgorithm;

   enum { first, second } KeyExchangeRound;

   struct {
      select (KeyExchangeRound) {
         case first:
            opaque srp_s<1..2^8-1>
            opaque srp_N<1..2^16-1>;
            opaque srp_g<1..2^16-1>;
         case second:
            opaque srp_B<1..2^16-1>;
   } ServerSRPParams;     /* SRP parameters */

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

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

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

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3. Security Considerations

   There is no server identification in this version of a TLS
   handshake. If an attacker gets the SRP password file, they can
   masquerade as the real system.

   What are the security issues of this new handshake sequence? Are the
   SRP parameters passed in a safe order? Is it a problem having the
   username appended to the client hello message?

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

   [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]  Ford-Hutchinson, P., Carpenter, M., Hudson, T., Murray, E. and
        V. Wiegand, "Securing FTP with TLS",
        draft-murray-auth-ftp-ssl-06 (work in progress), September 2000.

   [5]  Boe, M. and J. Altman, "TLS-based Telnet Security",
        draft-ietf-tn3270e-telnet-tls-05 (work in progress), October

Author's Address

   David Taylor
   Forge Research Pty Ltd

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

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Full Copyright Statement

   Copyright (C) The Internet Society (2001). All Rights Reserved.

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   Internet Society.

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