NETWORK WORKING GROUP N. Williams Internet-Draft Sun Expires: December 30, 2004 July 2004 A PRF for the Kerberos V GSS-API Mechanism draft-ietf-kitten-krb5-gssapi-prf-02.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 30, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. 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 30, 2004 [Page 1] Internet-Draft A PRF for the Kerberos V Mech July 2004 Table of Contents 1. Conventions used in this document . . . . . . . . . . . . . . . 3 2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . . 4 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 4. Normative References . . . . . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 Intellectual Property and Copyright Statements . . . . . . . . . 7 Williams Expires December 30, 2004 [Page 2] Internet-Draft A PRF for the Kerberos V Mech July 2004 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]. Williams Expires December 30, 2004 [Page 3] Internet-Draft A PRF for the Kerberos V Mech July 2004 2. Kerberos V GSS Mechanism PRF The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism [CFX] shall be the output of a PRF+ function based on the enctype's PRF function keyed with the negotiated session key of the security context and key usage X (TBD). The security context MUST be fully established, else the mechanism MUST fail with GSS_S_UNAVAILABLE as the major status code and GSS_KRB5_S_KG_CTX_INCOMPLETE as the minor status code. This PRF+ MUST be keyed with a key derived, with key usage (TBD), from the session used by the initiator and acceptor, after the security context is fully established, to derive keys for per-message tokens. For the current Kerberos V mechanism [CFX] this means that the PRF+ MUST be keyed with the acceptor-asserted subkey, if it did assert such a key, or the initiator's sub-session key otherwise. The PRF+ function is a simple counter-based extension of the Kerberos V pseudo-random function [KRB5-CRYPTO] for the enctype of the security context's keys: PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn) Tn = pseudo-random-function(K, n || S) where '||' is the concatenation operator, 'n' is encoded as a network byte order 32-bit unsigned binary number, and where truncate(L, S) truncates the input octet string S to length L. The maximum output size of the Kerberos V mechanism's GSS-API PRF then is, necessarily, 2^32 octets. Implementations MUST support output size of up to 2^14 octets at least. If the implementation cannot produce the desired output then it MUST output what it can. The minimum input octet string length that implementations MUST support is also 2^14 octets. If the input octet string is longer than the maximum that an implementation can process then the implementation MUST fail with GSS_S_FAILURE as the major status code and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status code. Williams Expires December 30, 2004 [Page 4] Internet-Draft A PRF for the Kerberos V Mech July 2004 3. Security Considerations Kerberos V enctypes' 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 enctypes may be weak, particularly the single-DES enctypes. See also [GSS-PRF] for generic security considerations of GSS_Pseudo_random(). The computational cost of computing this PRF+ may vary depending on the Kerberos V enctypes 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 constitue 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. 4 Normative References [CFX] Zhu, L., Jaganathan, K. and S. Hartman, "The Kerberos Version 5 GSS-API Mechanism: Version 2". [GSS-PRF] Williams, N., "A PRF API extension for the GSS-API". [KRB5-CRYPTO] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5". [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. Author's Address Nicolas Williams Sun Microsystems Williams Expires December 30, 2004 [Page 5] Internet-Draft A PRF for the Kerberos V Mech July 2004 5300 Riata Trace Ct Austin, TX 78727 US EMail: Nicolas.Williams@sun.com Williams Expires December 30, 2004 [Page 6] Internet-Draft A PRF for the Kerberos V Mech July 2004 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 (2004). 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 30, 2004 [Page 7]