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. Williams Expires December 15, 2005 [Page 1] Internet-Draft A PRF for the Kerberos V Mech June 2005 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 Williams Expires December 15, 2005 [Page 2] Internet-Draft A PRF for the Kerberos V Mech June 2005 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 Williams Expires December 15, 2005 [Page 3] Internet-Draft A PRF for the Kerberos V Mech June 2005 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". Williams Expires December 15, 2005 [Page 4] Internet-Draft A PRF for the Kerberos V Mech June 2005 [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 Williams Expires December 15, 2005 [Page 5] Internet-Draft A PRF for the Kerberos V Mech June 2005 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. 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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]