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rfc:rfc4401

Network Working Group N. Williams Request for Comments: 4401 Sun Microsystems Category: Standards Track February 2006

       A Pseudo-Random Function (PRF) API Extension for the
  Generic Security Service Application Program Interface (GSS-API)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document defines a Pseudo-Random Function (PRF) extension to the
 Generic Security Service Application Program 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 do not or cannot use GSS-API per-message message
 integrity check (MIC) and wrap tokens for session protection.

Table of Contents

 1. Introduction ....................................................2
    1.1. Conventions Used in This Document ..........................2
 2. GSS_Pseudo_random() .............................................2
    2.1. C-Bindings .................................................5
 3. IANA Considerations .............................................5
 4. Security Considerations .........................................5
 5. References ......................................................7
    5.1. Normative References .......................................7
    5.2. Informative References .....................................7

Williams Standards Track [Page 1] RFC 4401 A PRF Extension for the GSS-API February 2006

1. 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 [RFC2743] 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.
 Though this document specifies an abstract API as an extension to the
 GSS-API version 2, update 1, and though it specifies the bindings of
 this extension for the C programming language, it does not specify a
 revision of the GSS-API and so does not address the matter of how
 portable applications detect support for and ensure access to this
 extension.  We defer this matter to an expected, comprehensive update
 to the GSS-API.

1.1. 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].

2. GSS_Pseudo_random()

 Inputs:
 o  context CONTEXT handle,
 o  prf_key INTEGER,
 o  prf_in OCTET STRING,
 o  desired_output_len INTEGER
 Outputs:
 o  major_status INTEGER,
 o  minor_status INTEGER,
 o  prf_out OCTET STRING

Williams Standards Track [Page 2] RFC 4401 A PRF Extension for the GSS-API February 2006

 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 with
    the given prf_key, or that the given prf_key is not available.
 o  GSS_S_FAILURE indicates general failure, possibly due to the given
    input data being too large or of zero length, or due to the
    desired_output_len being zero; the minor status code may provide
    additional information.
 This function applies the established context's mechanism's keyed
 pseudo-random function (PRF) to the input data ('prf_in'), keyed with
 key material associated with the given security context and
 identified by 'prf_key', and outputs the resulting octet string
 ('prf_out') of desired_output_len length.
 The minimum input data length is one octet.
 Mechanisms MUST be able to consume all the provided prf_in input data
 that is 2^14 or fewer octets.
 If a mechanism cannot consume as much input data as provided by the
 caller, then GSS_Pseudo_random() MUST return GSS_S_FAILURE.
 The minimum desired_output_len is one.
 Mechanisms MUST be able to output at least up to 2^14 octets.
 If the implementation cannot produce the desired output due to lack
 of resources, then it MUST return GSS_S_FAILURE and MUST set a
 suitable minor status code.
 The prf_key can take on the following values: GSS_C_PRF_KEY_FULL,
 GSS_C_PRF_KEY_PARTIAL, or mechanism-specific values, if any.  This
 parameter is intended to distinguish between the best cryptographic
 keys that may be available only after full security context
 establishment and keys that may be available prior to full security
 context establishment.  For some mechanisms, or contexts, those two

Williams Standards Track [Page 3] RFC 4401 A PRF Extension for the GSS-API February 2006

 prf_key values MAY refer to the same cryptographic keys; for
 mechanisms like the Kerberos V GSS-API mechanism [RFC1964] where one
 peer may assert a key that may be considered better than the others
 they MAY be different keys.
 GSS_C_PRF_KEY_PARTIAL corresponds to a key that would have been used
 while the security context was partially established, even if it is
 fully established when GSS_Pseudo_random() is actually called.
 Mechanism-specific prf_key values are intended to refer to any other
 keys that may be available.
 The GSS_C_PRF_KEY_FULL value corresponds to the best key available
 for fully-established security contexts.
 GSS_Pseudo_random() has the following properties:
 o  its output string MUST be a pseudo-random function [GGM1] [GGM2]
    of the input keyed with key material from the given security
    context -- the chances of getting the same output given different
    input parameters should be exponentially small.
 o  when successfully applied to the same inputs by an initiator and
    acceptor using the same security context, it MUST produce the
    _same results_ for both, the initiator and acceptor, even if
    called multiple times (as long as the security context is not
    expired).
 o  upon full establishment of a security context, all cryptographic
    keys and/or negotiations used for computing the PRF with any
    prf_key MUST be authenticated (mutually, if mutual authentication
    is in effect for the given security context).
 o  the outputs of the mechanism's GSS_Pseudo_random() (for different
    inputs) and its per-message tokens for the given security context
    MUST be "cryptographically separate"; in other words, it must not
    be feasible to recover key material for one mechanism operation or
    transform its tokens and PRF outputs from one to the other given
    only said tokens and PRF outputs.  (This is a fancy way of saying
    that key derivation and strong cryptographic operations and
    constructions must be used.)
 o  as implied by the above requirement, it MUST NOT be possible to
    access any raw keys of a security context through
    GSS_Pseudo_random(), no matter what inputs are given.

Williams Standards Track [Page 4] RFC 4401 A PRF Extension for the GSS-API February 2006

2.1. C-Bindings

 #define GSS_C_PRF_KEY_FULL 0
 #define GSS_C_PRF_KEY_PARTIAL 1
 OM_uint32 gss_pseudo_random(
   OM_uint32                     *minor_status,
   gss_ctx_id_t                  context,
   int                           prf_key,
   const gss_buffer_t            prf_in,
   ssize_t                       desired_output_len,
   gss_buffer_t                  prf_out
 );
 Additional major status codes for the C-bindings:
 o  GSS_S_CALL_INACCESSIBLE_READ
 o  GSS_S_CALL_INACCESSIBLE_WRITE
 See [RFC2744].

3. IANA Considerations

 This document has no IANA considerations currently.  If and when a
 relevant IANA registry of GSS-API symbols is created, then the
 generic and language-specific function names, constant names, and
 constant values described above should be added to such a registry.

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'
 authenticated 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.
 Callers of GSS_Pseudo_random() should avoid accidentally calling it
 with the same inputs.  One useful technique is to prepend to the
 prf_in input string, by convention, a string indicating the intended
 purpose of the PRF output in such a way that unique contexts in which
 the function is called yield unique inputs to it.

Williams Standards Track [Page 5] RFC 4401 A PRF Extension for the GSS-API February 2006

 Pseudo-random functions are, by their nature, capable of producing
 only limited amounts of cryptographically secure output.  The exact
 amount of output that one can safely use, unfortunately, varies from
 one PRF to another (which prevents us from recommending specific
 numbers).  Because of this, we recommend that unless you really know
 what you are doing (i.e., you are a cryptographer and are qualified
 to pass judgement on cryptographic functions in areas of period,
 presence of short cycles, etc.), you limit the amount of the PRF
 output used to the necessary minimum.  See [RFC4086] for more
 information about "Randomness Requirements for Security".
 For some mechanisms, the computational cost of computing
 GSS_Pseudo_random() may increase significantly as the length of the
 prf_in data and/or the desired_output_length increase.  This means
 that if an application can be tricked into providing very large input
 octet strings and requesting very long output octet strings, then
 that may constitute a denial of service attack on the application;
 therefore, applications SHOULD place appropriate limits on the size
 of any input octet strings received from their peers without
 integrity protection.

Williams Standards Track [Page 6] RFC 4401 A PRF Extension for the GSS-API February 2006

5. References

5.1. Normative References

 [GGM1]     Goldreich, O., Goldwasser, S., and S. Micali, "How to
            Construct Random Functions", Journal of the ACM, 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",
            Proceedings of CRYPTO 84 on Advances in cryptology, 1985.
 [RFC4086]  Eastlake, D., 3rd, Schiller, J., and S. Crocker,
            "Randomness Requirements for Security", BCP 106, RFC 4086,
            June 2005.
 [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC
            1964, June 1996.

Author's Address

 Nicolas Williams
 Sun Microsystems
 5300 Riata Trace Ct
 Austin, TX  78727
 US
 EMail: Nicolas.Williams@sun.com

Williams Standards Track [Page 7] RFC 4401 A PRF Extension for the GSS-API February 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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 Administrative Support Activity (IASA).

Williams Standards Track [Page 8]

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