TLS Working Group                                              P. Eronen
Internet-Draft                                                     Nokia
Expires: November 22, 2004                                 H. Tschofenig
                                                            May 24, 2004

     Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document specifies new ciphersuites for the Transport Layer
   Security (TLS) protocol to support authentication based on pre-shared
   keys.  These pre-shared keys are symmetric keys, shared in advance
   among the communicating parties, and do not require any public key

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1.  Introduction

   Usually TLS uses public key certificates [TLS] or Kerberos [TLS-KRB]
   for authentication.  This document describes how to use symmetric
   keys (later called pre-shared keys or PSKs), shared in advance among
   the communicating parties, to establish a TLS connection.

   There are basically two reasons why one might want to do this:

   o  First, TLS may be used in performance-constrained environments
      where the CPU power needed for public key operations is not

   o  Second, pre-shared keys may be more convenient from a key
      management point of view.  For instance, in closed environments
      where the connections are mostly configured manually in advance,
      it may be easier to configure a PSK than to use certificates.
      Another case is when the parties already have a mechanism for
      setting up a shared secret key, and that mechanism could be used
      to "bootstrap" a key for authenticating a TLS connection.

   This document specifies a number of new ciphersuites for TLS.  These
   ciphersuites use a new authentication and key exchange algorithm for
   PSKs, and re-use existing cipher and MAC algorithms from [TLS] and

1.1  Applicability statement

   The ciphersuites defined in this document are intended for a rather
   limited set of applications, usually involving only a very small
   number of clients and servers.  Even in such environments, other
   alternatives may be more appropriate.

   If the main goal is to avoid PKIs, another possibility worth
   considering is to use self-signed certificates with public key
   fingerprints.  Instead of manually configuring a shared secret in,
   for instance, some configuration file, a fingerprint (hash) of the
   other party's public key (or certificate) could be placed there

   It is also possible to use the SRP (Secure Remote Password)
   ciphersuites for shared secret authentication [TLS-SRP].  While SRP
   protects against dictionary attacks, it requires more computational

1.2  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in [KEYWORDS].

2.  Protocol

   It is assumed that the reader is familiar with ordinary TLS
   handshake, shown below.  The elements in parenthesis are not included
   in PSK-based ciphersuites.

      Client                                               Server
      ------                                               ------

      ClientHello                  -------->
                                   <--------      ServerHelloDone
      Finished                     -------->
                                   <--------             Finished
      Application Data             <------->     Application Data

   The client indicates its willingness to use pre-shared key
   authentication by including one or more PSK-based ciphersuites in the
   ClientHello message.  The following ciphersuites are defined in this

      CipherSuite TLS_PSK_WITH_RC4_128_SHA        = { 0x00, 0xTBD };
      CipherSuite TLS_PSK_WITH_3DES_EDE_CBC_SHA   = { 0x00, 0xTBD };
      CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA    = { 0x00, 0xTBD };
      CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA    = { 0x00, 0xTBD };

   Note that this document defines only a new authentication and key
   exchange algorithm; see [TLS] and [TLS-AES] for description of the
   cipher and MAC algorithms.

   If the TLS server also wants to use pre-shared keys, it selects one
   of the PSK ciphersuites, places the selected ciphersuite in the
   ServerHello message, and includes an appropriate ServerKeyExchange
   message (see below).  The Certificate and CertificateRequest payloads
   are omitted from the response.

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   Both clients and servers may have pre-shared keys with several
   different parties.  The client indicates which key to use by
   including a "PSK identity" in the ClientKeyExchange message (note
   that unlike in [TLS-SHAREDKEYS], the session_id field in ClientHello
   message keeps its usual meaning).  To help the client in selecting
   which identity to use, the server can provide a "PSK identity hint"
   in the ServerKeyExchange message (note that if no hint is provided, a
   ServerKeyExchange message is still sent).

   This document does not specify the format of the PSK identity or PSK
   identity hint; neither is specified how exactly the client uses the
   hint (if it uses it at all).  The parties have to agree on the
   identities when the shared secret is configured (however, see Section
   4 for related security considerations).  In situations where the
   identity is entered by a person, it is RECOMMENDED that the input is
   processed using an appropriate stringprep [STRINGPREP] profile and
   encoded in octets using UTF-8 encoding [UTF8].  One possible
   stringprep profile is described in [SASLPREP].

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

      struct {
          select (KeyExchangeAlgorithm) {
              case diffie_hellman:
                  ServerDHParams params;
                  Signature signed_params;
              case rsa:
                  ServerRSAParams params;
                  Signature signed_params;
              case psk:  /* NEW */
                  opaque psk_identity_hint<0..2^16-1>;
      } ServerKeyExchange;

      struct {
          select (KeyExchangeAlgorithm) {
              case rsa: EncryptedPreMasterSecret;
              case diffie_hellman: ClientDiffieHellmanPublic;
              case psk: opaque psk_identity<0..2^16-1>;  /* NEW */
          } exchange_keys;
      } ClientKeyExchange;

   The premaster secret is formed as follows: concatenate 24 zero
   octets, followed by SHA-1 hash [FIPS180-2] of the PSK itself,
   followed by 4 zero octets.

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      Note: This effectively means that only the HMAC-SHA1 part of the
      TLS PRF is used, and the HMAC-MD5 part is not used.  See
      [Krawczyk20040113] for a more detailed rationale.  The PSK is
      first hashed so that PSKs longer than 24 octets can be used; this
      is similar to what is done in [HMAC] if the key length is longer
      than the hash block size.

   If the server does not recognize the PSK identity, it SHOULD respond
   with a decrypt_error alert message.  This alert is also sent if
   validating the Finished message fails.  The use of the same alert
   message makes it more difficult to find out which PSK identities are
   known to the server.

3.  IANA considerations

   This document does not define any new namespaces to be managed by
   IANA.  It does require assignment of several new ciphersuite numbers,
   but it is unclear how this is done, since the TLS spec does not say
   who is responsible for assigning them :-)

4.  Security Considerations

   As with all schemes involving shared keys, special care should be
   taken to protect the shared values and to limit their exposure over

   The ciphersuites defined in this document do not provide Perfect
   Forward Secrecy (PFS).  That is, if the shared secret key is somehow
   compromised, an attacker can decrypt old conversations.  (Note that
   the most popular TLS key exchange algorithm, RSA, does not provide
   PFS either.)

   Use of a fixed shared secret of limited entropy (such as a password)
   allows an attacker to perform a brute-force or dictionary attack to
   recover the secret.  This may be either an off-line attack (against a
   captured TLS conversation), or an on-line attack where the attacker
   attempts to connect to the server and tries different keys.  An
   attacker can also get the information required for an off-line attack
   if a valid client attempts to connect with the attacker.  It is
   RECOMMENDED that implementations that allow the administrator to
   manually configure the PSK also provide a functionality for
   generating a new random PSK, taking [RANDOMNESS] into account.

   The PSK identity is sent in cleartext.  While using a user name or
   other similar string as the PSK identity is the most straightforward
   option, it may lead to problems in some environments since an
   eavesdropper is able to identify the communicating parties.  Even
   when the identity does not reveal any information itself, reusing the

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   same identity over time may eventually allow an attacker to perform
   traffic analysis to the identify parties.  It should be noted that
   this is no worse than client certificates, since they are also sent
   in cleartext.

5.  Acknowledgments

   The protocol defined in this document is heavily based on work by Tim
   Dierks and Peter Gutmann, and borrows some text from [TLS-SHAREDKEYS]
   and [TLS-AES].  Valuable feedback was also provided by Philip
   Ginzboorg, Peter Gutmann, David Jablon, Nikos Mavroyanopoulos, Bodo
   Moeller, and Mika Tervonen.

   When the first version of this draft was almost ready, the authors
   learned that something similar had been proposed already in 1996
   [TLS-PASSAUTH].  However, this draft is not intended for web password
   authentication, but rather for other uses of TLS.

6.  References

6.1  Normative References

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

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

   [TLS]      Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, January 1999.

              Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

              National Institute of Standards and Technology,
              "Specifications for the Secure Hash Standard",  Federal
              Information Processing Standard (FIPS) Publication 180-2,
              August 2002.

6.2  Informative References

              Gutmann, P., "Use of Shared Keys in the TLS Protocol",
              draft-ietf-tls-sharedkeys-02 (expired), October 2003.

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

              Krawczyk, H., "Re: TLS shared keys PRF",  message on
     mailing list 2004-01-13,

              Zeilenga, K., "SASLprep: Stringprep profile for user names
              and passwords", draft-ietf-sasl-saslprep-09 (work in
              progress), April 2004.

              Hoffman, P. and M. Blanchet, "Preparation of
              Internationalized Strings ("stringprep")", RFC 3454,
              December 2002.

   [TLS-KRB]  Medvinsky, A. and M. Hur, "Addition of Kerberos Cipher
              Suites to Transport Layer Security (TLS)", RFC 2712,
              October 1999.

              Simon, D., "Addition of Shared Key Authentication to
              Transport Layer Security (TLS)",
              draft-ietf-tls-passauth-00 (expired), November 1996.

   [TLS-SRP]  Taylor, D., Wu, T., Mavroyanopoulos, N. and T. Perrin,
              "Using SRP for TLS Authentication", draft-ietf-tls-srp-06
              (work in progress), January 2004.

   [UTF8]     Yergeau, F., "UTF-8, a transformation format of ISO
              10646", RFC 3629, November 2003.

Authors' Addresses

   Pasi Eronen
   Nokia Research Center
   P.O. Box 407
   FIN-00045 Nokia Group


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   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bayern  81739


Appendix A.  Changelog

   (This section should be removed by the RFC Editor before

   Changes from draft-eronen-tls-psk-00 to draft-ietf-tls-psk-00:

   o  Updated dictionary attack considerations based on comments from
      David Jablon.

   o  Added a recommendation about using UTF-8 in the identity field.

   o  Removed Appendix A comparing this document with

   o  Removed IPR comment about SPR.

   o  Minor editorial changes.

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