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

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:

  • context CONTEXT handle,
  • prf_key INTEGER,
  • prf_in OCTET STRING,
  • desired_output_len INTEGER

Outputs:

  • major_status INTEGER,
  • minor_status INTEGER,
  • prf_out OCTET STRING

Return major_status codes:

  • GSS_S_COMPLETE indicates no error.
  • GSS_S_NO_CONTEXT indicates that a null context has been provided as input.
  • GSS_S_CONTEXT_EXPIRED indicates that an expired context has been provided as input.
  • 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.
  • 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 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.
  • 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).
  • 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).
  • 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.)
  • 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.

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:

  • GSS_S_CALL_INACCESSIBLE_READ
  • 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.

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.

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

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