

NETWORK WORKING GROUP                                        N. Williams
Internet-Draft                                                       Sun
Expires: December 30, 2004                                     July 2004


                  A PRF API extension for the GSS-API
                  draft-ietf-kitten-gssapi-prf-02.txt

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Copyright Notice

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

Abstract

   This document defines a Pseudo-Random Function (PRF) extension to the
   Generic Security Service Applicatoin Programming Interface (GSS-API)
   for keying application protocols given an established GSS-API
   security context.  The primary intended use of this function is to
   key secure session layers that don't or cannot use GSS-API
   per-message MIC (message integrity check) and wrap tokens for session
   protection.






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Table of Contents

   1.  Conventions used in this document  . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  GSS_Pseudo_random()  . . . . . . . . . . . . . . . . . . . . .  5
   3.1 C-Bindings . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1 Normative References . . . . . . . . . . . . . . . . . . . . .  8
   5.2 Informative References . . . . . . . . . . . . . . . . . . . .  8
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  8
       Intellectual Property and Copyright Statements . . . . . . . .  9







































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1.  Conventions used in this document

   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 [RFC2119].














































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2.  Introduction

   A need has arisen for users of the GSS-API to key applications'
   cryptographic protocols using established GSS-API security contexts.
   Such applications can use the GSS-API for authentication, but not for
   transport security (for whatever reasons), and since the GSS-API does
   not provide a method for obtaining keying material from established
   security contexts such applications cannot make effective use of the
   GSS-API.

   To address this need we define a pseudo-random function (PRF)
   extension to the GSS-API.







































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3.  GSS_Pseudo_random()

   Inputs:

   o  context CONTEXT handle,
   o  prf_in OCTET STRING,
   o  desired_output_len INTEGER

   Outputs:

   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  prf_out OCTET STRING

   Return major_status codes:
   o  GSS_S_COMPLETE indicates no error.
   o  GSS_S_NO_CONTEXT indicates that a null context has been provided
      as input.
   o  GSS_S_CONTEXT_EXPIRED indicates that an expired context has been
      provided as input.
   o  GSS_S_UNAVAILABLE indicates that the mechanism lacks support for
      this function or, if the security context is not fully
      established, that the context is not ready to compute the PRF.
   o  GSS_S_FAILURE indicates failure or lack of support; the minor
      status code may provide additional information.

   This function applies the established context's mechanism's keyed PRF
   function to the input data (prf_in), keyed with key material
   associated with the given security context and outputs the resulting
   octet string (prf_out) of desired_output_len length.

   The output string of this function MUST be a pseudo-random function
   [GGM1][GGM2] of the input keyed with key material from the
   established security context -- the chances of getting the same
   output given different input parameters should be exponentially
   small.

   This function, applied to the same inputs by an initiator and
   acceptor using the same established context, MUST produce the *same
   results* for both, the initiator and acceptor, even if called
   multiple times for the same context.

   Mechanisms MAY limit the output of the PRF according, possibly in
   ways related to the types of cryptographic keys available for the PRF
   function, thus the prf_out output of GSS_Pseudo_random() MAY be
   smaller than requested.

   Mechanisms may be able to compute PRFs with security contexts that



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   are not fully established, therefore applications MAY call
   GSS_Pseudo_random() with such security contexts.  Such mechanisms
   MUST return GSS_S_UNAVAILABLE when called on to compute a PRF given a
   security context that is not fully established and also not ready for
   PRF computation.  Mechanisms that allow for PRF computation prior to
   full security context establishment MUST use the same PRF and key
   material, for any given security context, both, before and after full
   context establishment, and the PRF and key material negotiation MUT
   be authenticated when the security context is fully established.

3.1  C-Bindings

   OM_uint32 gss_pseudo_random(
     OM_uint32			*minor_status,
     gss_ctx_id_t			context,
     const gss_buffer_t		prf_in,
     ssize_t		desired_output_len,
     gss_buffer_t		prf_out
   );
































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4.  Security Considerations

   Care should be taken in properly designing a mechanism's PRF
   function.

   GSS mechanisms' PRF functions should use a key derived from contexts'
   session keys and should preserve the forward security properties of
   the mechanisms' key exchanges.

   Some mechanisms may support the GSS PRF function with security
   contexts that are not fully established, but applications MUST assume
   that authentication, mutual or otherwise, has not completed until the
   security context is fully established






































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5.  References

5.1  Normative References

   [GGM1]     Goldreich, O., Goldwasser, S. and S. Micali, "How to
              Construct Random Functions", October 1986.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, January 2000.

5.2  Informative References

   [GGM2]     Goldreich, O., Goldwasser, S. and S. Micali, "On the
              Cryptographic Applications of Random Functions", 1985.

   [RFC1750]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.


Author's Address

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US

   EMail: Nicolas.Williams@sun.com

















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Acknowledgment

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




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