Network Working Group                                          P. Eronen
Internet-Draft                                                     Nokia
Expires: August 6, 2004                                    H. Tschofenig
                                                        February 6, 2004

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

Status of this Memo

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

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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

Conventions used in this document

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

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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 SRP for shared secret authentication
   [TLS-SRP]. However, SRP requires more computational resources and may
   have some IPR issues. However, it does provide protection against
   dictionary attacks.

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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.

   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

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   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).

   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.

      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

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   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
   tries to connect to the server and tries different keys. Note that
   the protocol requires the client to prove it knows the key first, so
   just attempting to connect to a server does not reveal information
   required 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 [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 same
   identity over time may eventually allow an attacker to use traffic
   analysis to the identify parties.  It should be noted that this is no
   worse than client certificates, since they are also sent in

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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 Peter Gutmann
   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 other uses of TLS.

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.

Informative References

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

   [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

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     mailing list 2004-01-13,

   [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.

Authors' Addresses

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


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


Appendix A. Comparison with draft-ietf-tls-sharedkeys-02 (informative)

   [TLS-SHAREDKEYS] presents another way to use shared keys with TLS.
   Instead of defining new ciphersuites, it re-uses the TLS session
   cache and session resumption functionality.

   The approach presented in this document is, in our opinion, more
   elegant and better in line with the design of TLS. However, it does
   probably require more changes to existing TLS implementations.
   Nevertheless, these changes should be rather straightforward,
   especially for implementations that already support multiple key
   exchange algorithms and have a modular architecture.

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   The changes required are roughly the following:

   1. An API to pass psk_identities and keys around from the application
      to the TLS library. Most likely, both push-style interface (use
      this psk_identity and key) and callbacks (given a psk_identity,
      fetch corresponding shared secret) would be useful.

   2. An API to determine which psk_identity was used for a session.

   3. PSK ciphersuite identifiers must be added to the list of supported

   4. Processing of PSK messages in the handshake code.

   The session-cache based approach probably requires the following
   changes (depending on details of the TLS implementation):

   1. Most TLS implementations do not expose an API that allows detailed
      modification of the session cache, so some modifications are
      required (especially if the implementation is done in some
      reasonably type-safe language, the application cannot just use
      some pointer tricks to access private data structures).

      On the client side, we need an API to communicate session_id, key
      and whatever is used to look up entries from the session cache
      (for instance, some implementations use IP address and port
      number) to the TLS implementation (and initialize other session
      cache fields to some sensible values).

      On the server side, we need to communicate session_id and key.
      Most likely, both push-style interface (use this session_id and
      key) and pull callbacks (given a session_id, fetch corresponding
      shared secret) would be useful (but callbacks may require more

   2. An API to determine which session_id was used (and to determine if
      shared secret or normal RSA was used).

   3. The session resumption code normally checks that resumed sessions
      use the same ciphersuite as the original session. Unless a single
      ciphersuite is hardcoded to the session cache, this code has to be
      modified (and the session cache needs a flag indicating which
      entries were created using ordinary handshake and which using
      shared-secret API--unless the check is omitted for all sessions,
      breaking TLS 1.0 rules).

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   4. If the TLS implementation supports compression, resumed sessions
      must use the same compression method as the original.  Either
      compressions has to be disabled or this code modified.

   5. TLS implementation should also check that the resumed session uses
      the same protocol version; this needs changes as well, unless a
      single version number is hardcoded.

   6. The session cache code may need modifications to ensure the stored
      entries actually stay there long enough to be useful. Currently
      implementations are free to discard entries whenever they want.
      However, probably most implementations would not require any

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