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















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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.




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  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.







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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: [email protected]



















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Acknowledgement

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  Administrative Support Activity (IASA).







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