

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


               A PRF for the Kerberos V GSS-API Mechanism
                draft-ietf-kitten-krb5-gssapi-prf-02.txt

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Copyright Notice

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

Abstract

   This document defines the Pseudo-Random Function (PRF) for the
   Kerberos V mechanism for the Generic Security Service Application
   Programming Interface (GSS-API), based on the PRF defined for the
   Kerberos V cryptographic framework, for keying application protocols
   given an established Kerberos V GSS-API security context.








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Table of Contents

   1. Conventions used in this document  . . . . . . . . . . . . . . . 3
   2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . . 4
   3. Security Considerations  . . . . . . . . . . . . . . . . . . . . 5
   4. Normative References . . . . . . . . . . . . . . . . . . . . . . 5
      Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
      Intellectual Property and Copyright Statements . . . . . . . . . 7











































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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.  Kerberos V GSS Mechanism PRF

   The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism [CFX]
   shall be the output of a PRF+ function based on the enctype's PRF
   function keyed with the negotiated session key of the security
   context and key usage X (TBD).

   The security context MUST be fully established, else the mechanism
   MUST fail with GSS_S_UNAVAILABLE as the major status code and
   GSS_KRB5_S_KG_CTX_INCOMPLETE as the minor status code.

   This PRF+ MUST be keyed with a key derived, with key usage (TBD),
   from the session used by the initiator and acceptor, after the
   security context is fully established, to derive keys for per-message
   tokens.  For the current Kerberos V mechanism [CFX] this means that
   the PRF+ MUST be keyed with the acceptor-asserted subkey, if it did
   assert such a key, or the initiator's sub-session key otherwise.

   The PRF+ function is a simple counter-based extension of the Kerberos
   V pseudo-random function [KRB5-CRYPTO] for the enctype of the
   security context's keys:

   	PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn)

   	Tn = pseudo-random-function(K, n || S)

   	where '||' is the concatenation operator, 'n' is encoded as a
   	network byte order 32-bit unsigned binary number, and where
   	truncate(L, S) truncates the input octet string S to length L.

   The maximum output size of the Kerberos V mechanism's GSS-API PRF
   then is, necessarily, 2^32 octets.

   Implementations MUST support output size of up to 2^14 octets at
   least.

   If the implementation cannot produce the desired output then it MUST
   output what it can.

   The minimum input octet string length that implementations MUST
   support is also 2^14 octets.  If the input octet string is longer
   than the maximum that an implementation can process then the
   implementation MUST fail with GSS_S_FAILURE as the major status code
   and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status code.







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3.  Security Considerations

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

   Legacy Kerberos V enctypes may be weak, particularly the single-DES
   enctypes.

   See also [GSS-PRF] for generic security considerations of
   GSS_Pseudo_random().

   The computational cost of computing this PRF+ may vary depending on
   the Kerberos V enctypes being used, but generally the computation of
   this PRF+ gets more expensive as the input and output octet string
   lengths grow (note that the use of a counter in the PRF+ construction
   allows for parallelization).  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 constitue 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.

4  Normative References

   [CFX]      Zhu, L., Jaganathan, K. and S. Hartman, "The Kerberos
              Version 5 GSS-API Mechanism: Version 2".

   [GSS-PRF]  Williams, N., "A PRF API extension for the GSS-API".

   [KRB5-CRYPTO]
              Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5".

   [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.


Author's Address

   Nicolas Williams
   Sun Microsystems



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   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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