1NETWORK WORKING GROUP N. Williams 2Internet-Draft Sun 3Expires: November 13, 2005 May 12, 2005 4 5 6 A PRF for the Kerberos V GSS-API Mechanism 7 draft-ietf-kitten-krb5-gssapi-prf-03.txt 8 9Status of this Memo 10 11 By submitting this Internet-Draft, each author represents that any 12 applicable patent or other IPR claims of which he or she is aware 13 have been or will be disclosed, and any of which he or she becomes 14 aware will be disclosed, in accordance with Section 6 of BCP 79. 15 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 Internet- 19 Drafts. 20 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." 25 26 The list of current Internet-Drafts can be accessed at 27 http://www.ietf.org/ietf/1id-abstracts.txt. 28 29 The list of Internet-Draft Shadow Directories can be accessed at 30 http://www.ietf.org/shadow.html. 31 32 This Internet-Draft will expire on November 13, 2005. 33 34Copyright Notice 35 36 Copyright (C) The Internet Society (2005). 37 38Abstract 39 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. 45 46 47 48 49 50 51 52Williams Expires November 13, 2005 [Page 1] 53 54Internet-Draft A PRF for the Kerberos V Mech May 2005 55 56 57Table of Contents 58 59 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 1.1 Conventions used in this document . . . . . . . . . . . . . . 3 61 2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . 3 62 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 63 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 64 5. Normative References . . . . . . . . . . . . . . . . . . . . . 5 65 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5 66 Intellectual Property and Copyright Statements . . . . . . . . 6 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108Williams Expires November 13, 2005 [Page 2] 109 110Internet-Draft A PRF for the Kerberos V Mech May 2005 111 112 1131. Introduction 114 115 This document specifies the Kerberos V GSS-API mechanism's pseudo- 116 random function corresponding to [GSS-PRF]. The function is a "PRF+" 117 style construction. 118 1191.1 Conventions used in this document 120 121 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 122 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 123 document are to be interpreted as described in [RFC2119]. 124 1252. Kerberos V GSS Mechanism PRF 126 127 The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism 128 [RFC1964] shall be the output of a PRF+ function based on the 129 encryption type's PRF function keyed with the negotiated session key 130 of the security context corresponding to the 'prf_key' input 131 parameter of GSS_Pseudo_random(). 132 133 This PRF+ MUST be keyed with the key indicated by the 'prf_key' input 134 parameter as follows: 135 136 o GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the 137 acceptor, if any, or the sub-session asserted by the initiator, if 138 any, or the Ticket's session key 139 140 o GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the 141 initiator, if any, or the Ticket's session key 142 143 The PRF+ function is a simple counter-based extension of the Kerberos 144 V pseudo-random function [RFC3961] for the encryption type of the 145 security context's keys: 146 147 PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn) 148 149 Tn = pseudo-random(K, n || S) 150 151 where '||' is the concatenation operator, 'n' is encoded as a network 152 byte order 32-bit unsigned binary number, truncate(L, S) truncates 153 the input octet string S to length L, and pseudo-random() is the 154 Kerberos V pseudo-random function [RFC3961]. 155 156 The maximum output size of the Kerberos V mechanism's GSS-API PRF 157 then is, necessarily, 2^32 times the output size of the pseudo- 158 random() function for the encryption type of the given key. 159 160 When the input size is longer than 2^14 octets as per [GSS-PRF] and 161 162 163 164Williams Expires November 13, 2005 [Page 3] 165 166Internet-Draft A PRF for the Kerberos V Mech May 2005 167 168 169 exceeds an implementation's resources then the mechanism MUST return 170 GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status 171 code. 172 1733. IANA Considerations 174 175 This document has no IANA considerations currently. If and when a 176 relevant IANA registry of GSS-API symbols and constants is created 177 then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be 178 added to such a registry. 179 1804. Security Considerations 181 182 Kerberos V encryption types' PRF functions use a key derived from 183 contexts' session keys and should preserve the forward security 184 properties of the mechanisms' key exchanges. 185 186 Legacy Kerberos V encryption types may be weak, particularly the 187 single-DES encryption types. 188 189 See also [GSS-PRF] for generic security considerations of 190 GSS_Pseudo_random(). 191 192 See also [RFC3961] for generic security considerations of the 193 Kerberos V cryptographic framework. 194 195 Care should be taken not to exceed the useful lifetime of an 196 established security context's session key's useful lifetime as 197 implementations are not required to prevent overuse of the 198 GSS_Pseudo_random() function. This can effectively be achieved by 199 limiting the number of GSS_Pseudo_random() calls to, say, a handful 200 of calls per-security context. 201 202 Use of Ticket session keys, rather than sub-session keys, when 203 initiators and acceptors fail to assert sub-session keys, is 204 dangerous as ticket reuse can lead to key reuse, therefore initiators 205 should assert sub-session keys always, and acceptors should assert 206 sub-session keys at least when initiators fail to do so.. 207 208 The computational cost of computing this PRF+ may vary depending on 209 the Kerberos V encryption types being used, but generally the 210 computation of this PRF+ gets more expensive as the input and output 211 octet string lengths grow (note that the use of a counter in the PRF+ 212 construction allows for parallelization). This means that if an 213 application can be tricked into providing very large input octet 214 strings and requesting very long output octet strings then that may 215 constitute a denial of service attack on the application; therefore 216 applications SHOULD place appropriate limits on the size of any input 217 218 219 220Williams Expires November 13, 2005 [Page 4] 221 222Internet-Draft A PRF for the Kerberos V Mech May 2005 223 224 225 octet strings received from their peers without integrity protection. 226 2275. Normative References 228 229 [CFX] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 230 Version 5 GSS-API Mechanism: Version 2". 231 232 [GSS-PRF] Williams, N., "A PRF API extension for the GSS-API". 233 234 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 235 RFC 1964, June 1996. 236 237 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 238 Requirement Levels", BCP 14, RFC 2119, March 1997. 239 240 [RFC2743] Linn, J., "Generic Security Service Application Program 241 Interface Version 2, Update 1", RFC 2743, January 2000. 242 243 [RFC2744] Wray, J., "Generic Security Service API Version 2 : 244 C-bindings", RFC 2744, January 2000. 245 246 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 247 Kerberos 5", RFC 3961, February 2005. 248 249 250Author's Address 251 252 Nicolas Williams 253 Sun Microsystems 254 5300 Riata Trace Ct 255 Austin, TX 78727 256 US 257 258 Email: Nicolas.Williams@sun.com 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276Williams Expires November 13, 2005 [Page 5] 277 278Internet-Draft A PRF for the Kerberos V Mech May 2005 279 280 281Intellectual Property Statement 282 283 The IETF takes no position regarding the validity or scope of any 284 Intellectual Property Rights or other rights that might be claimed to 285 pertain to the implementation or use of the technology described in 286 this document or the extent to which any license under such rights 287 might or might not be available; nor does it represent that it has 288 made any independent effort to identify any such rights. Information 289 on the procedures with respect to rights in RFC documents can be 290 found in BCP 78 and BCP 79. 291 292 Copies of IPR disclosures made to the IETF Secretariat and any 293 assurances of licenses to be made available, or the result of an 294 attempt made to obtain a general license or permission for the use of 295 such proprietary rights by implementers or users of this 296 specification can be obtained from the IETF on-line IPR repository at 297 http://www.ietf.org/ipr. 298 299 The IETF invites any interested party to bring to its attention any 300 copyrights, patents or patent applications, or other proprietary 301 rights that may cover technology that may be required to implement 302 this standard. Please address the information to the IETF at 303 ietf-ipr@ietf.org. 304 305 306Disclaimer of Validity 307 308 This document and the information contained herein are provided on an 309 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 310 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 311 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 312 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 313 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 314 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 315 316 317Copyright Statement 318 319 Copyright (C) The Internet Society (2005). This document is subject 320 to the rights, licenses and restrictions contained in BCP 78, and 321 except as set forth therein, the authors retain all their rights. 322 323 324Acknowledgment 325 326 Funding for the RFC Editor function is currently provided by the 327 Internet Society. 328 329 330 331 332Williams Expires November 13, 2005 [Page 6] 333 334 335