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+NETWORK WORKING GROUP N. Williams
+Internet-Draft Sun
+Expires: December 15, 2005 June 13, 2005
+
+
+ A PRF for the Kerberos V GSS-API Mechanism
+ draft-ietf-kitten-krb5-gssapi-prf-04.txt
+
+Status of this Memo
+
+ By submitting this Internet-Draft, each author represents that any
+ applicable patent or other IPR claims of which he or she is aware
+ have been or will be disclosed, and any of which he or she becomes
+ aware will be disclosed, in accordance with Section 6 of BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF), its areas, and its working groups. Note that
+ other groups may also distribute working documents as Internet-
+ Drafts.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ The list of current Internet-Drafts can be accessed at
+ http://www.ietf.org/ietf/1id-abstracts.txt.
+
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html.
+
+ This Internet-Draft will expire on December 15, 2005.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2005).
+
+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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+Williams Expires December 15, 2005 [Page 1]
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+Internet-Draft A PRF for the Kerberos V Mech June 2005
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+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 1.1 Conventions used in this document . . . . . . . . . . . . . . 3
+ 2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . 3
+ 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
+ 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
+ 5. Normative References . . . . . . . . . . . . . . . . . . . . . 4
+ Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5
+ Intellectual Property and Copyright Statements . . . . . . . . 6
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+Williams Expires December 15, 2005 [Page 2]
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+Internet-Draft A PRF for the Kerberos V Mech June 2005
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+1. Introduction
+
+ This document specifies the Kerberos V GSS-API mechanism's pseudo-
+ random function corresponding to [GSS-PRF]. The function is a "PRF+"
+ style construction.
+
+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 [GSS-PRF] function for the Kerberos V mechanism
+ [RFC1964] 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 [GSS-PRF] and
+
+
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+Williams Expires December 15, 2005 [Page 3]
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+Internet-Draft A PRF for the Kerberos V Mech June 2005
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+ 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 [GSS-PRF] 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
+
+ [CFX] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
+ Version 5 GSS-API Mechanism: Version 2".
+
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+Williams Expires December 15, 2005 [Page 4]
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+ [GSS-PRF] Williams, N., "A PRF API extension for the GSS-API".
+
+ [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
+ RFC 1964, June 1996.
+
+ [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.
+
+
+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 Expires December 15, 2005 [Page 5]
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+Internet-Draft A PRF for the Kerberos V Mech June 2005
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+Intellectual Property Statement
+
+ The IETF takes no position regarding the validity or scope of any
+ Intellectual Property Rights or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
+ this document or the extent to which any license under such rights
+ might or might not be available; nor does it represent that it has
+ made any independent effort to identify any such rights. Information
+ on the procedures with respect to rights in RFC documents can be
+ found in BCP 78 and BCP 79.
+
+ Copies of IPR disclosures made to the IETF Secretariat and any
+ assurances of licenses to be made available, or the result of an
+ attempt made to obtain a general license or permission for the use of
+ such proprietary rights by implementers or users of this
+ specification can be obtained from the IETF on-line IPR repository at
+ http://www.ietf.org/ipr.
+
+ The IETF invites any interested party to bring to its attention any
+ copyrights, patents or patent applications, or other proprietary
+ rights that may cover technology that may be required to implement
+ this standard. Please address the information to the IETF at
+ ietf-ipr@ietf.org.
+
+
+Disclaimer of Validity
+
+ 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
+ INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
+ WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+
+Copyright Statement
+
+ Copyright (C) The Internet Society (2005). 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.
+
+
+Acknowledgment
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+Williams Expires December 15, 2005 [Page 6]
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