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Network Working Group                                        N. WilliamsRequest for Comments: 4401                              Sun MicrosystemsCategory: Standards Track                                  February 2006A Pseudo-Random Function (PRF) API Extension for theGeneric 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 Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................22. GSS_Pseudo_random() .............................................22.1. C-Bindings .................................................53. IANA Considerations .............................................54. Security Considerations .........................................55. References ......................................................75.1. Normative References .......................................75.2. Informative References .....................................7Williams                    Standards Track                     [Page 1]

RFC 4401            A PRF Extension for the GSS-API        February 20061.  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 STRINGWilliams                    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 twoWilliams                    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 20062.1.  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 20065.  References5.1.  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",RFC1964, June 1996.Author's Address   Nicolas Williams   Sun Microsystems   5300 Riata Trace Ct   Austin, TX  78727   US   EMail: Nicolas.Williams@sun.comWilliams                    Standards Track                     [Page 7]

RFC 4401            A PRF Extension for the GSS-API        February 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Williams                    Standards Track                     [Page 8]