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+Network Working Group                                        N. Williams
+Request for Comments: 4402                                           Sun
+Category: Standards Track                                  February 2006
+
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+   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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+Williams                    Standards Track                     [Page 1]
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+RFC 4402           A PRF for the Kerberos V Mechanism      February 2006
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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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+
+
+Williams                    Standards Track                     [Page 2]
+
+RFC 4402           A PRF for the Kerberos V Mechanism      February 2006
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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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+Williams                    Standards Track                     [Page 3]
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+RFC 4402           A PRF for the Kerberos V Mechanism      February 2006
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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: Nicolas.Williams@sun.com
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+Williams                    Standards Track                     [Page 4]
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+RFC 4402           A PRF for the Kerberos V Mechanism      February 2006
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+Full Copyright Statement
+
+   Copyright (C) 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
+   retain all their rights.
+
+   This document and the information contained herein are provided on an
+   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
+   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
+   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
+   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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+
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+Acknowledgement
+
+   Funding for the RFC Editor function is provided by the IETF
+   Administrative Support Activity (IASA).
+
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+Williams                    Standards Track                     [Page 5]
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