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IETF RFC 4401
A Pseudo-Random Function (PRF) API Extension for the Generic Security Service Application Program Interface (GSS-API)
Last modified on Thursday, February 9th, 2006
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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 © 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 [RFC 2743] 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 [RFC 2119].
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
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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
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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 [RFC 1964] 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.
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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 [RFC 2744].
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.
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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 [RFC 4086] 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.
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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.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC 2744] 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.
[RFC 4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC 1964] 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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RFC 4401 A PRF Extension for the GSS-API February 2006
Full Copyright Statement
Copyright © 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
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Acknowledgement
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A Pseudo-Random Function (PRF) API Extension for the Generic Security Service Application Program Interface (GSS-API)
RFC TOTAL SIZE: 15272 bytes
PUBLICATION DATE: Thursday, February 9th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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