Network Working Group                                        N. Williams
Request for Comments: 4402                                           Sun
Category: Standards Track                                  February 2006


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

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 the Pseudo-Random Function (PRF) for the
   Kerberos V mechanism for the Generic Security Service Application
   Program 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.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................2
   2. Kerberos V GSS Mechanism PRF ....................................2
   3. IANA Considerations .............................................3
   4. Security Considerations .........................................3
   5. Normative References ............................................4

1.  Introduction

   This document specifies the Kerberos V GSS-API mechanism's [RFC4121]
   pseudo-random function corresponding to [RFC4401].  The function is a
   "PRF+" style construction.  For more information see [RFC4401],
   [RFC2743], [RFC2744], and [RFC4121].

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.  Kerberos V GSS Mechanism PRF

   The GSS-API PRF [RFC4401] function for the Kerberos V mechanism
   [RFC4121] shall be the output of a PRF+ function based on the
   encryption type's PRF function keyed with the negotiated session key
   of the security context corresponding to the 'prf_key' input
   parameter of GSS_Pseudo_random().

   This PRF+ MUST be keyed with the key indicated by the 'prf_key' input
   parameter as follows:

   o  GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the
      acceptor, if any, or the sub-session asserted by the initiator, if
      any, or the Ticket's session key

   o  GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the
      initiator, if any, or the Ticket's session key

   The PRF+ function is a simple counter-based extension of the Kerberos
   V pseudo-random function [RFC3961] for the encryption type of the
   security context's keys:

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

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

   where '||' is the concatenation operator, 'n' is encoded as a network
   byte order 32-bit unsigned binary number, truncate(L, S) truncates
   the input octet string S to length L, and pseudo-random() is the
   Kerberos V pseudo-random function [RFC3961].

   The maximum output size of the Kerberos V mechanism's GSS-API PRF
   then is, necessarily, 2^32 times the output size of the pseudo-
   random() function for the encryption type of the given key.

   When the input size is longer than 2^14 octets as per [RFC4401] and
   exceeds an implementation's resources, then the mechanism MUST return
   GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status
   code.

3.  IANA Considerations

   This document has no IANA considerations currently.  If and when a
   relevant IANA registry of GSS-API symbols and constants is created,
   then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be
   added to such a registry.

4.  Security Considerations

   Kerberos V encryption types' 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 encryption types may be weak, particularly the
   single-DES encryption types.

   See also [RFC4401] for generic security considerations of
   GSS_Pseudo_random().

   See also [RFC3961] for generic security considerations of the
   Kerberos V cryptographic framework.

   Use of Ticket session keys, rather than sub-session keys, when
   initiators and acceptors fail to assert sub-session keys, is
   dangerous as ticket reuse can lead to key reuse; therefore,
   initiators should assert sub-session keys always, and acceptors
   should assert sub-session keys at least when initiators fail to do
   so.

   The computational cost of computing this PRF+ may vary depending on
   the Kerberos V encryption types 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
   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.  Normative References

   [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.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC4121]  Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
              Version 5 Generic Security Service Application Program
              Interface (GSS-API) Mechanism: Version 2", RFC 4121,
              July 2005.

   [RFC4401]  Williams, N., "A Pseudo-Random Function (PRF) API
              Extension for the Generic Security Service Application
              Program Interface (GSS-API)", RFC 4401, February 2006.

Author's Address

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US

   EMail: Nicolas.Williams@sun.com

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   Copyright (C) The Internet Society (2006).

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