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Williams 3 Internet-Draft Sun 4 Expires: December 30, 2004 July 2004 6 A PRF for the Kerberos V GSS-API Mechanism 7 draft-ietf-kitten-krb5-gssapi-prf-02.txt 9 Status of this Memo 11 By submitting this Internet-Draft, I certify that any applicable 12 patent or other IPR claims of which I am aware have been disclosed, 13 and any of which I become aware will be disclosed, in accordance with 14 RFC 3668. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that 18 other groups may also distribute working documents as 19 Internet-Drafts. 21 Internet-Drafts are draft documents valid for a maximum of six months 22 and may be updated, replaced, or obsoleted by other documents at any 23 time. It is inappropriate to use Internet-Drafts as reference 24 material or to cite them other than as "work in progress." 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 32 This Internet-Draft will expire on December 30, 2004. 34 Copyright Notice 36 Copyright (C) The Internet Society (2004). All Rights Reserved. 38 Abstract 40 This document defines the Pseudo-Random Function (PRF) for the 41 Kerberos V mechanism for the Generic Security Service Application 42 Programming Interface (GSS-API), based on the PRF defined for the 43 Kerberos V cryptographic framework, for keying application protocols 44 given an established Kerberos V GSS-API security context. 46 Table of Contents 48 1. Conventions used in this document . . . . . . . . . . . . . . . 3 49 2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . . 4 50 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 51 4. Normative References . . . . . . . . . . . . . . . . . . . . . . 5 52 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5 53 Intellectual Property and Copyright Statements . . . . . . . . . 7 55 1. Conventions used in this document 57 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 58 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 59 document are to be interpreted as described in [RFC2119]. 61 2. Kerberos V GSS Mechanism PRF 63 The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism [CFX] 64 shall be the output of a PRF+ function based on the enctype's PRF 65 function keyed with the negotiated session key of the security 66 context and key usage X (TBD). 68 The security context MUST be fully established, else the mechanism 69 MUST fail with GSS_S_UNAVAILABLE as the major status code and 70 GSS_KRB5_S_KG_CTX_INCOMPLETE as the minor status code. 72 This PRF+ MUST be keyed with a key derived, with key usage (TBD), 73 from the session used by the initiator and acceptor, after the 74 security context is fully established, to derive keys for per-message 75 tokens. For the current Kerberos V mechanism [CFX] this means that 76 the PRF+ MUST be keyed with the acceptor-asserted subkey, if it did 77 assert such a key, or the initiator's sub-session key otherwise. 79 The PRF+ function is a simple counter-based extension of the Kerberos 80 V pseudo-random function [KRB5-CRYPTO] for the enctype of the 81 security context's keys: 83 PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn) 85 Tn = pseudo-random-function(K, n || S) 87 where '||' is the concatenation operator, 'n' is encoded as a 88 network byte order 32-bit unsigned binary number, and where 89 truncate(L, S) truncates the input octet string S to length L. 91 The maximum output size of the Kerberos V mechanism's GSS-API PRF 92 then is, necessarily, 2^32 octets. 94 Implementations MUST support output size of up to 2^14 octets at 95 least. 97 If the implementation cannot produce the desired output then it MUST 98 output what it can. 100 The minimum input octet string length that implementations MUST 101 support is also 2^14 octets. If the input octet string is longer 102 than the maximum that an implementation can process then the 103 implementation MUST fail with GSS_S_FAILURE as the major status code 104 and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status code. 106 3. Security Considerations 108 Kerberos V enctypes' PRF functions use a key derived from contexts' 109 session keys and should preserve the forward security properties of 110 the mechanisms' key exchanges. 112 Legacy Kerberos V enctypes may be weak, particularly the single-DES 113 enctypes. 115 See also [GSS-PRF] for generic security considerations of 116 GSS_Pseudo_random(). 118 The computational cost of computing this PRF+ may vary depending on 119 the Kerberos V enctypes being used, but generally the computation of 120 this PRF+ gets more expensive as the input and output octet string 121 lengths grow (note that the use of a counter in the PRF+ construction 122 allows for parallelization). This means that if an application can 123 be tricked into providing very large input octet strings and 124 requesting very long output octet strings then that may constitue a 125 denial of service attack on the application; therefore applications 126 SHOULD place appropriate limits on the size of any input octet 127 strings received from their peers without integrity protection. 129 4 Normative References 131 [CFX] Zhu, L., Jaganathan, K. and S. Hartman, "The Kerberos 132 Version 5 GSS-API Mechanism: Version 2". 134 [GSS-PRF] Williams, N., "A PRF API extension for the GSS-API". 136 [KRB5-CRYPTO] 137 Raeburn, K., "Encryption and Checksum Specifications for 138 Kerberos 5". 140 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 141 Requirement Levels", BCP 14, RFC 2119, March 1997. 143 [RFC2743] Linn, J., "Generic Security Service Application Program 144 Interface Version 2, Update 1", RFC 2743, January 2000. 146 [RFC2744] Wray, J., "Generic Security Service API Version 2 : 147 C-bindings", RFC 2744, January 2000. 149 Author's Address 151 Nicolas Williams 152 Sun Microsystems 153 5300 Riata Trace Ct 154 Austin, TX 78727 155 US 157 EMail: Nicolas.Williams@sun.com 159 Intellectual Property Statement 161 The IETF takes no position regarding the validity or scope of any 162 Intellectual Property Rights or other rights that might be claimed to 163 pertain to the implementation or use of the technology described in 164 this document or the extent to which any license under such rights 165 might or might not be available; nor does it represent that it has 166 made any independent effort to identify any such rights. Information 167 on the procedures with respect to rights in RFC documents can be 168 found in BCP 78 and BCP 79. 170 Copies of IPR disclosures made to the IETF Secretariat and any 171 assurances of licenses to be made available, or the result of an 172 attempt made to obtain a general license or permission for the use of 173 such proprietary rights by implementers or users of this 174 specification can be obtained from the IETF on-line IPR repository at 175 http://www.ietf.org/ipr. 177 The IETF invites any interested party to bring to its attention any 178 copyrights, patents or patent applications, or other proprietary 179 rights that may cover technology that may be required to implement 180 this standard. Please address the information to the IETF at 181 ietf-ipr@ietf.org. 183 Disclaimer of Validity 185 This document and the information contained herein are provided on an 186 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 187 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 188 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 189 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 190 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 191 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 193 Copyright Statement 195 Copyright (C) The Internet Society (2004). This document is subject 196 to the rights, licenses and restrictions contained in BCP 78, and 197 except as set forth therein, the authors retain all their rights. 199 Acknowledgment 201 Funding for the RFC Editor function is currently provided by the 202 Internet Society.