1 2 3 4 5 6 7Network Working Group N. Williams 8Request for Comments: 4402 Sun 9Category: Standards Track February 2006 10 11 12 A Pseudo-Random Function (PRF) for the Kerberos V Generic Security 13 Service Application Program Interface (GSS-API) Mechanism 14 15Status of This Memo 16 17 This document specifies an Internet standards track protocol for the 18 Internet community, and requests discussion and suggestions for 19 improvements. Please refer to the current edition of the "Internet 20 Official Protocol Standards" (STD 1) for the standardization state 21 and status of this protocol. Distribution of this memo is unlimited. 22 23Copyright Notice 24 25 Copyright (C) The Internet Society (2006). 26 27Abstract 28 29 This document defines the Pseudo-Random Function (PRF) for the 30 Kerberos V mechanism for the Generic Security Service Application 31 Program Interface (GSS-API), based on the PRF defined for the 32 Kerberos V cryptographic framework, for keying application protocols 33 given an established Kerberos V GSS-API security context. 34 35Table of Contents 36 37 1. Introduction ....................................................2 38 1.1. Conventions Used in This Document ..........................2 39 2. Kerberos V GSS Mechanism PRF ....................................2 40 3. IANA Considerations .............................................3 41 4. Security Considerations .........................................3 42 5. Normative References ............................................4 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58Williams Standards Track [Page 1] 59 60RFC 4402 A PRF for the Kerberos V Mechanism February 2006 61 62 631. Introduction 64 65 This document specifies the Kerberos V GSS-API mechanism's [RFC4121] 66 pseudo-random function corresponding to [RFC4401]. The function is a 67 "PRF+" style construction. For more information see [RFC4401], 68 [RFC2743], [RFC2744], and [RFC4121]. 69 701.1. Conventions Used in This Document 71 72 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 73 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 74 document are to be interpreted as described in [RFC2119]. 75 762. Kerberos V GSS Mechanism PRF 77 78 The GSS-API PRF [RFC4401] function for the Kerberos V mechanism 79 [RFC4121] shall be the output of a PRF+ function based on the 80 encryption type's PRF function keyed with the negotiated session key 81 of the security context corresponding to the 'prf_key' input 82 parameter of GSS_Pseudo_random(). 83 84 This PRF+ MUST be keyed with the key indicated by the 'prf_key' input 85 parameter as follows: 86 87 o GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the 88 acceptor, if any, or the sub-session asserted by the initiator, if 89 any, or the Ticket's session key 90 91 o GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the 92 initiator, if any, or the Ticket's session key 93 94 The PRF+ function is a simple counter-based extension of the Kerberos 95 V pseudo-random function [RFC3961] for the encryption type of the 96 security context's keys: 97 98 PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn) 99 100 Tn = pseudo-random(K, n || S) 101 102 where '||' is the concatenation operator, 'n' is encoded as a network 103 byte order 32-bit unsigned binary number, truncate(L, S) truncates 104 the input octet string S to length L, and pseudo-random() is the 105 Kerberos V pseudo-random function [RFC3961]. 106 107 The maximum output size of the Kerberos V mechanism's GSS-API PRF 108 then is, necessarily, 2^32 times the output size of the pseudo- 109 random() function for the encryption type of the given key. 110 111 112 113 114Williams Standards Track [Page 2] 115 116RFC 4402 A PRF for the Kerberos V Mechanism February 2006 117 118 119 When the input size is longer than 2^14 octets as per [RFC4401] and 120 exceeds an implementation's resources, then the mechanism MUST return 121 GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status 122 code. 123 1243. IANA Considerations 125 126 This document has no IANA considerations currently. If and when a 127 relevant IANA registry of GSS-API symbols and constants is created, 128 then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be 129 added to such a registry. 130 1314. Security Considerations 132 133 Kerberos V encryption types' PRF functions use a key derived from 134 contexts' session keys and should preserve the forward security 135 properties of the mechanisms' key exchanges. 136 137 Legacy Kerberos V encryption types may be weak, particularly the 138 single-DES encryption types. 139 140 See also [RFC4401] for generic security considerations of 141 GSS_Pseudo_random(). 142 143 See also [RFC3961] for generic security considerations of the 144 Kerberos V cryptographic framework. 145 146 Use of Ticket session keys, rather than sub-session keys, when 147 initiators and acceptors fail to assert sub-session keys, is 148 dangerous as ticket reuse can lead to key reuse; therefore, 149 initiators should assert sub-session keys always, and acceptors 150 should assert sub-session keys at least when initiators fail to do 151 so. 152 153 The computational cost of computing this PRF+ may vary depending on 154 the Kerberos V encryption types being used, but generally the 155 computation of this PRF+ gets more expensive as the input and output 156 octet string lengths grow (note that the use of a counter in the PRF+ 157 construction allows for parallelization). This means that if an 158 application can be tricked into providing very large input octet 159 strings and requesting very long output octet strings, then that may 160 constitute a denial of service attack on the application; therefore, 161 applications SHOULD place appropriate limits on the size of any input 162 octet strings received from their peers without integrity protection. 163 164 165 166 167 168 169 170Williams Standards Track [Page 3] 171 172RFC 4402 A PRF for the Kerberos V Mechanism February 2006 173 174 1755. Normative References 176 177 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 178 Requirement Levels", BCP 14, RFC 2119, March 1997. 179 180 [RFC2743] Linn, J., "Generic Security Service Application Program 181 Interface Version 2, Update 1", RFC 2743, January 2000. 182 183 [RFC2744] Wray, J., "Generic Security Service API Version 2 : 184 C-bindings", RFC 2744, January 2000. 185 186 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 187 Kerberos 5", RFC 3961, February 2005. 188 189 [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 190 Version 5 Generic Security Service Application Program 191 Interface (GSS-API) Mechanism: Version 2", RFC 4121, 192 July 2005. 193 194 [RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API 195 Extension for the Generic Security Service Application 196 Program Interface (GSS-API)", RFC 4401, February 2006. 197 198Author's Address 199 200 Nicolas Williams 201 Sun Microsystems 202 5300 Riata Trace Ct 203 Austin, TX 78727 204 US 205 206 EMail: Nicolas.Williams@sun.com 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226Williams Standards Track [Page 4] 227 228RFC 4402 A PRF for the Kerberos V Mechanism February 2006 229 230 231Full Copyright Statement 232 233 Copyright (C) The Internet Society (2006). 234 235 This document is subject to the rights, licenses and restrictions 236 contained in BCP 78, and except as set forth therein, the authors 237 retain all their rights. 238 239 This document and the information contained herein are provided on an 240 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 241 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 242 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 243 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 244 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 245 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 246 247Intellectual Property 248 249 The IETF takes no position regarding the validity or scope of any 250 Intellectual Property Rights or other rights that might be claimed to 251 pertain to the implementation or use of the technology described in 252 this document or the extent to which any license under such rights 253 might or might not be available; nor does it represent that it has 254 made any independent effort to identify any such rights. Information 255 on the procedures with respect to rights in RFC documents can be 256 found in BCP 78 and BCP 79. 257 258 Copies of IPR disclosures made to the IETF Secretariat and any 259 assurances of licenses to be made available, or the result of an 260 attempt made to obtain a general license or permission for the use of 261 such proprietary rights by implementers or users of this 262 specification can be obtained from the IETF on-line IPR repository at 263 http://www.ietf.org/ipr. 264 265 The IETF invites any interested party to bring to its attention any 266 copyrights, patents or patent applications, or other proprietary 267 rights that may cover technology that may be required to implement 268 this standard. Please address the information to the IETF at 269 ietf-ipr@ietf.org. 270 271Acknowledgement 272 273 Funding for the RFC Editor function is provided by the IETF 274 Administrative Support Activity (IASA). 275 276 277 278 279 280 281 282Williams Standards Track [Page 5] 283 284