TLS Working Group                                         P. Eronen Eronen, Ed.
Internet-Draft                                                     Nokia
Expires: February 16, March 29, 2005                               H. Tschofenig Tschofenig, Ed.
                                                         August 18,
                                                      September 28, 2004

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

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, I certify each
   author represents that any applicable patent or other IPR claims of
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   which I he or she become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

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


   This document specifies three sets of 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.  The first set of
   ciphersuites uses only symmetric key operations for authentication.
   The second set uses a Diffie-Hellman exchange authenticated with a
   pre-shared key; and the third set combines public key authentication
   of the server with pre-shared key authentication of the client.

1.  Introduction

   Usually TLS uses public key certificates [3] or Kerberos [11] 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 three sets of new ciphersuites for TLS.
   These ciphersuites use new key exchange algorithms, and re-use
   existing cipher and MAC algorithms from [3] and [2].  A summary of
   these ciphersuites is shown below.

      CipherSuite                        Key Exchange  Cipher       Hash

      TLS_PSK_WITH_RC4_128_SHA           PSK           RC4_128       SHA
      TLS_PSK_WITH_AES_128_CBC_SHA       PSK           AES_128_CBC   SHA
      TLS_PSK_WITH_AES_256_CBC_SHA       PSK           AES_256_CBC   SHA
      TLS_DHE_PSK_WITH_RC4_128_SHA       DHE_PSK       RC4_128       SHA
      TLS_RSA_PSK_WITH_RC4_128_SHA       RSA_PSK       RC4_128       SHA

   The first set of ciphersuites (with PSK key exchange algorithm),
   defined in Section 2 use only symmetric key algorithms, and are thus
   especially suitable for performance-constrained environments.

   The ciphersuites in Section 3 (with DHE_PSK key exchange algorithm)
   use a PSK to authenticate a Diffie-Hellman exchange.  These
   ciphersuites give some additional protection protect against dictionary
   attacks, attacks by passive
   eavesdroppers (but not active attackers), and also provide Perfect
   Forward Secrecy (PFS).

   The third set of ciphersuites (with RSA_PSK key exchange algorithm),
   defined in Section 4, combine public key based authentication of the
   server (using RSA and certificates) with mutual authentication using
   a PSK.

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 [13].  SRP was designed
   to be used with passwords, and incorporates protection against
   dictionary attacks.  However, it is computationally more expensive
   than the PSK ciphersuites in Section 2.

1.2  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [1].

2.  PSK key exchange algorithm

   This section defines the PSK key exchange algorithm and associated
   ciphersuites.  These ciphersuites use only symmetric key algorithms.

   It is assumed that the reader is familiar with ordinary TLS
   handshake, shown below.  The elements in parenthesis are not included
   when PSK key exchange algorithm is used.

      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 ciphersuites in the
   ClientHello message.  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.

   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 [6], 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
   6 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 [9] profile and encoded in
   octets using UTF-8 encoding [14].  One possible stringprep profile is
   described in [8].

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

      struct {
          select (KeyExchangeAlgorithm) {
              /* other cases for rsa, diffie_hellman, etc. */
              case psk:  /* NEW */
                  opaque psk_identity_hint<0..2^16-1>;
      } ServerKeyExchange;

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

   The premaster secret is formed as follows: If the PSK is N octets
   long, concatenate N zero octets and the PSK.

      Note: This effectively means that only the HMAC-SHA1 part (but not
      the HMAC-MD5 part) of the TLS PRF is used, and used when constructing the HMAC-MD5 part is not used.
      master secret.  See [7] for a more detailed rationale.

   The TLS handshake is authenticated using the Finished messages as

   If the server does not recognize the PSK identity, it MAY respond
   with an "unknown_psk_identity" alert message.  Alternatively, if the
   server wishes to hide the fact that the PSK identity was not known,
   it MAY continue the protocol as if the PSK identity existed but the
   key was incorrect: that is, respond with a "decrypt_error" alert.

3.  DHE_PSK key exchange algorithm

   This section defines additional ciphersuites that use a PSK to
   authenticate a Diffie-Hellman exchange.  These ciphersuites give some
   additional protection against dictionary attacks, and also provide
   Perfect Forward Secrecy (PFS).  See Section 6 for discussion of
   related security considerations.

   When these ciphersuites are used, the ServerKeyExchange and
   ClientKeyExchange also include the Diffie-Hellman parameters.  The
   PSK identity and identity hint fields have the same meaning as in the
   previous section.

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

      struct {
          select (KeyExchangeAlgorithm) {
              /* other cases for rsa, diffie_hellman, etc. */
              case diffie_hellman_psk:  /* NEW */
                  opaque psk_identity_hint<0..2^16-1>;
                  ServerDHParams params;
      } ServerKeyExchange;

      struct {
          select (KeyExchangeAlgorithm) {
              /* other cases for rsa, diffie_hellman, etc. */
              case diffie_hellman_psk:   /* NEW */
                  opaque psk_identity<0..2^16-1>;
                  ClientDiffieHellmanPublic public;
          } exchange_keys;
      } ClientKeyExchange;

   The premaster secret is formed as follows: concatenate the value
   produced by the Diffie-Hellman exchange (with leading zero bytes
   stripped as in other Diffie-Hellman based ciphersuites) and the PSK,
   in this order.

4.  RSA_PSK key exchange algorithm

   The ciphersuites in this section use RSA and certificates to
   authenticate the server, in addition to using a PSK.

   As in normal RSA ciphersuites, the server must send a Certificate
   message.  The format of the ServerKeyExchange and ClientKeyExchange
   messages is shown below.

      struct {
          select (KeyExchangeAlgorithm) {
              /* other cases for rsa, diffie_hellman, etc. */
              case rsa_psk:  /* NEW */
                  opaque psk_identity_hint<0..2^16-1>;
      } ServerKeyExchange;

      struct {
          select (KeyExchangeAlgorithm) {
              /* other cases for rsa, diffie_hellman, etc. */
              case rsa_psk:   /* NEW */
                  opaque psk_identity<0..2^16-1>;
          } exchange_keys;
      } ClientKeyExchange;

   The premaster secret is formed as follows: concatenate the 48-byte
   value generated by the client (and sent to the server in
   ClientKeyExchange message) and the PSK, in this order.

   Neither the normal RSA ciphersuites nor these RSA_PSK ciphersuites
   themselves specify what the certificates contain (in addition to the
   RSA public key), or how the certificates are to be validated.  In
   particular, it is possible to use the RSA_PSK ciphersuites with
   unvalidated self-signed certificates to provide somewhat similar
   protection against dictionary attacks as the DHE_PSK ciphersuites
   defined in Section 4.

5.  IANA considerations

   (This depends on whether this document is published before or after
   TLS 1.1.)

   (If after 1.1) This document does not create any new namespaces to be
   maintained by IANA, but it requires new values in the ciphersuite
   namespace defined in TLS 1.1 specification.

   (If before 1.1) There are no IANA actions associated with this
   document.  For easier reference in the future, the ciphersuite
   numbers defined in this document are summarized below.

      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 };
      CipherSuite TLS_DHE_PSK_WITH_RC4_128_SHA      = { 0x00, 0xTBD };
      CipherSuite TLS_DHE_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0xTBD };
      CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA  = { 0x00, 0xTBD };
      CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA  = { 0x00, 0xTBD };
      CipherSuite TLS_RSA_PSK_WITH_RC4_128_SHA      = { 0x00, 0xTBD };
      CipherSuite TLS_RSA_PSK_WITH_3DES_EDE_CBC_SHA = { 0x00, 0xTBD };
      CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA  = { 0x00, 0xTBD };
      CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA  = { 0x00, 0xTBD };

   This document also defines a new TLS alert message,
   unknown_psk_identity(TBD).  Since IANA does not maintain a registry
   of TLS alert messages, no IANA action is needed for this.

6.  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

6.1  Perfect forward secrecy (PFS)

   The PSK and RSA_PSK ciphersuites defined in this document do not
   provide Perfect Forward Secrecy (PFS).  That is, if the shared secret
   key (in PSK ciphersuites), or both the shared secret key and the RSA
   private key (in RSA_PSK ciphersuites), is somehow compromised, an
   attacker can decrypt old conversations.

   The DHE_PSK ciphersuites provide Perfect Forward Secrecy if a fresh
   DH private key is generated for each handshake.

6.2  Brute-force and dictionary attacks

   Use of a fixed shared secret of limited entropy (such as a password)
   may allow 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 handshake messages), or an on-line attack
   where the attacker attempts to connect to the server and tries
   different keys.

   For the PSK ciphersuites, an attacker can get the information
   required for an off-line attack by eavesdropping a TLS handshake, or
   by getting a valid client to attempt connection with the attacker (by
   tricking the client to connect to wrong address, or intercepting a
   connection attempt to the correct address, for instance).

   For the DHE_PSK ciphersuites, an attacker can obtain the information
   by getting a valid client to attempt connection with the attacker.
   Passive eavesdropping alone is not sufficient.

   For the RSA_PSK ciphersuites, only the server (authenticated using
   RSA and certificates) can obtain sufficient information for an
   off-line attack.

   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 [4] into account.

6.3  Identity privacy

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

6.4  Implementation notes

   The implementation notes in [10] about correct implementation and use
   of RSA (including Section and Diffie-Hellman (including
   Appendix F.1.1.3) apply to the DHE_PSK and RSA_PSK ciphersuites as

7.  Acknowledgments

   The protocol defined in this document is heavily based on work by Tim
   Dierks and Peter Gutmann, and borrows some text from [6] and [2].
   The DHE_PSK and RSA_PSK ciphersuites are based on earlier work in

   Valuable feedback was also provided by Philip Ginzboorg, Peter
   Gutmann, David Jablon, Nikos Mavroyanopoulos, Bodo Moeller, Eric
   Rescorla, 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
   [12].  However, this draft is not intended for web password
   authentication, but rather for other uses of TLS.

   The DHE_PSK and RSA_PSK ciphersuites borrow heavily from [5].

8.  Open issues

   o  Identity privacy could be provided (in DHE_PSK/RSA_PSK versions)
      by encrypting the psk_identity payload with keys derived from the
      DH value/RSA-encrypted random (but not PSK).  But perhaps this
      would be an unnecessary complication.

   o  The way the PSK is combined with DH value (and is then used to
      calculate the Finished message) is not exactly the traditional
      way.  It should be OK with TLS-PRF, though.

9.  References

9.1  Normative References

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

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

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

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

9.2  Informative References

   [5]   Badra, M., Cherkaoui, O., Hajjeh, I. and A. Serhrouchni,
         "Pre-Shared-Key key Exchange methods for TLS",
         draft-badra-tls-key-exchange-00 (work in progress), August

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

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

   [8]   Zeilenga, K., "SASLprep: Stringprep profile for user names and
         passwords", draft-ietf-sasl-saslprep-10 (work in progress),
         July 2004.

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

   [10]  Dierks, T. and E. Rescorla, "The TLS Protocol Version 1.1",
         draft-ietf-tls-rfc2246-bis-08 (work in progress), August 2004.

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

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

   [13]  Taylor, D., Wu, T., Mavroyanopoulos, N. and T. Perrin, "Using
         SRP for TLS Authentication", draft-ietf-tls-srp-07 draft-ietf-tls-srp-08 (work in
         progress), June August 2004.

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

Authors' and Contributors' Addresses

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


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


   Mohamad Badra
   ENST Telecom
   46 rue Barrault
   75634 Paris

   Omar Cherkaoui
   UQAM University
   Montreal (Quebec)

   Ibrahim Hajjeh
   ENST Telecom
   46 rue Barrault
   75634 Paris

   Ahmed Serhrouchni
   ENST Telecom
   46 rue Barrault
   75634 Paris

Appendix A.  Changelog

   (This section should be removed by the RFC Editor before

   Changes from -01 to -02:

   o  Clarified text about DHE_PSK ciphersuites in Section 1.

   o  Clarified explanation of HMAC-SHA1/MD5 use of PRF in Section 2.

   o  Added note about certificate validation and self-signed
      certificates in Section 4.

   o  Added Mohamad Badra et al. as contributors.

   Changes from draft-ietf-tls-psk-00 to -01:

   o  Added DHE_PSK and RSA_PSK key exchange algorithms, and updated
      other text accordingly

   o  Removed SHA-1 hash from PSK key exchange premaster secret
      construction (since premaster secret doesn't need to be 48 bytes).

   o  Added unknown_psk_identity alert message.

   o  Updated IANA considerations section.

   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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