1 2 3 4NETWORK WORKING GROUP N. Williams 5Internet-Draft Sun 6Expires: December 15, 2005 June 13, 2005 7 8 9 A PRF for the Kerberos V GSS-API Mechanism 10 draft-ietf-kitten-krb5-gssapi-prf-04.txt 11 12Status of this Memo 13 14 By submitting this Internet-Draft, each author represents that any 15 applicable patent or other IPR claims of which he or she is aware 16 have been or will be disclosed, and any of which he or she becomes 17 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 19 Internet-Drafts are working documents of the Internet Engineering 20 Task Force (IETF), its areas, and its working groups. Note that 21 other groups may also distribute working documents as Internet- 22 Drafts. 23 24 Internet-Drafts are draft documents valid for a maximum of six months 25 and may be updated, replaced, or obsoleted by other documents at any 26 time. It is inappropriate to use Internet-Drafts as reference 27 material or to cite them other than as "work in progress." 28 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt. 31 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html. 34 35 This Internet-Draft will expire on December 15, 2005. 36 37Copyright Notice 38 39 Copyright (C) The Internet Society (2005). 40 41Abstract 42 43 This document defines the Pseudo-Random Function (PRF) for the 44 Kerberos V mechanism for the Generic Security Service Application 45 Programming Interface (GSS-API), based on the PRF defined for the 46 Kerberos V cryptographic framework, for keying application protocols 47 given an established Kerberos V GSS-API security context. 48 49 50 51 52 53 54 55Williams Expires December 15, 2005 [Page 1] 56 57Internet-Draft A PRF for the Kerberos V Mech June 2005 58 59 60Table of Contents 61 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 1.1 Conventions used in this document . . . . . . . . . . . . . . 3 64 2. Kerberos V GSS Mechanism PRF . . . . . . . . . . . . . . . . . 3 65 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 66 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 67 5. Normative References . . . . . . . . . . . . . . . . . . . . . 4 68 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5 69 Intellectual Property and Copyright Statements . . . . . . . . 6 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 99 100 101 102 103 104 105 106 107 108 109 110 111Williams Expires December 15, 2005 [Page 2] 112 113Internet-Draft A PRF for the Kerberos V Mech June 2005 114 115 1161. Introduction 117 118 This document specifies the Kerberos V GSS-API mechanism's pseudo- 119 random function corresponding to [GSS-PRF]. The function is a "PRF+" 120 style construction. 121 1221.1 Conventions used in this document 123 124 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 125 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 126 document are to be interpreted as described in [RFC2119]. 127 1282. Kerberos V GSS Mechanism PRF 129 130 The GSS-API PRF [GSS-PRF] function for the Kerberos V mechanism 131 [RFC1964] shall be the output of a PRF+ function based on the 132 encryption type's PRF function keyed with the negotiated session key 133 of the security context corresponding to the 'prf_key' input 134 parameter of GSS_Pseudo_random(). 135 136 This PRF+ MUST be keyed with the key indicated by the 'prf_key' input 137 parameter as follows: 138 139 o GSS_C_PRF_KEY_FULL -- use the sub-session key asserted by the 140 acceptor, if any, or the sub-session asserted by the initiator, if 141 any, or the Ticket's session key 142 143 o GSS_C_PRF_KEY_PARTIAL -- use the sub-session key asserted by the 144 initiator, if any, or the Ticket's session key 145 146 The PRF+ function is a simple counter-based extension of the Kerberos 147 V pseudo-random function [RFC3961] for the encryption type of the 148 security context's keys: 149 150 PRF+(K, L, S) = truncate(L, T1 || T2 || .. || Tn) 151 152 Tn = pseudo-random(K, n || S) 153 154 where '||' is the concatenation operator, 'n' is encoded as a network 155 byte order 32-bit unsigned binary number, truncate(L, S) truncates 156 the input octet string S to length L, and pseudo-random() is the 157 Kerberos V pseudo-random function [RFC3961]. 158 159 The maximum output size of the Kerberos V mechanism's GSS-API PRF 160 then is, necessarily, 2^32 times the output size of the pseudo- 161 random() function for the encryption type of the given key. 162 163 When the input size is longer than 2^14 octets as per [GSS-PRF] and 164 165 166 167Williams Expires December 15, 2005 [Page 3] 168 169Internet-Draft A PRF for the Kerberos V Mech June 2005 170 171 172 exceeds an implementation's resources then the mechanism MUST return 173 GSS_S_FAILURE and GSS_KRB5_S_KG_INPUT_TOO_LONG as the minor status 174 code. 175 1763. IANA Considerations 177 178 This document has no IANA considerations currently. If and when a 179 relevant IANA registry of GSS-API symbols and constants is created 180 then the GSS_KRB5_S_KG_INPUT_TOO_LONG minor status code should be 181 added to such a registry. 182 1834. Security Considerations 184 185 Kerberos V encryption types' PRF functions use a key derived from 186 contexts' session keys and should preserve the forward security 187 properties of the mechanisms' key exchanges. 188 189 Legacy Kerberos V encryption types may be weak, particularly the 190 single-DES encryption types. 191 192 See also [GSS-PRF] for generic security considerations of 193 GSS_Pseudo_random(). 194 195 See also [RFC3961] for generic security considerations of the 196 Kerberos V cryptographic framework. 197 198 Use of Ticket session keys, rather than sub-session keys, when 199 initiators and acceptors fail to assert sub-session keys, is 200 dangerous as ticket reuse can lead to key reuse, therefore initiators 201 should assert sub-session keys always, and acceptors should assert 202 sub-session keys at least when initiators fail to do so.. 203 204 The computational cost of computing this PRF+ may vary depending on 205 the Kerberos V encryption types being used, but generally the 206 computation of this PRF+ gets more expensive as the input and output 207 octet string lengths grow (note that the use of a counter in the PRF+ 208 construction allows for parallelization). This means that if an 209 application can be tricked into providing very large input octet 210 strings and requesting very long output octet strings then that may 211 constitute a denial of service attack on the application; therefore 212 applications SHOULD place appropriate limits on the size of any input 213 octet strings received from their peers without integrity protection. 214 2155. Normative References 216 217 [CFX] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos 218 Version 5 GSS-API Mechanism: Version 2". 219 220 221 222 223Williams Expires December 15, 2005 [Page 4] 224 225Internet-Draft A PRF for the Kerberos V Mech June 2005 226 227 228 [GSS-PRF] Williams, N., "A PRF API extension for the GSS-API". 229 230 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 231 RFC 1964, June 1996. 232 233 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 234 Requirement Levels", BCP 14, RFC 2119, March 1997. 235 236 [RFC2743] Linn, J., "Generic Security Service Application Program 237 Interface Version 2, Update 1", RFC 2743, January 2000. 238 239 [RFC2744] Wray, J., "Generic Security Service API Version 2 : 240 C-bindings", RFC 2744, January 2000. 241 242 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 243 Kerberos 5", RFC 3961, February 2005. 244 245 246Author's Address 247 248 Nicolas Williams 249 Sun Microsystems 250 5300 Riata Trace Ct 251 Austin, TX 78727 252 US 253 254 Email: Nicolas.Williams@sun.com 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279Williams Expires December 15, 2005 [Page 5] 280 281Internet-Draft A PRF for the Kerberos V Mech June 2005 282 283 284Intellectual Property Statement 285 286 The IETF takes no position regarding the validity or scope of any 287 Intellectual Property Rights or other rights that might be claimed to 288 pertain to the implementation or use of the technology described in 289 this document or the extent to which any license under such rights 290 might or might not be available; nor does it represent that it has 291 made any independent effort to identify any such rights. Information 292 on the procedures with respect to rights in RFC documents can be 293 found in BCP 78 and BCP 79. 294 295 Copies of IPR disclosures made to the IETF Secretariat and any 296 assurances of licenses to be made available, or the result of an 297 attempt made to obtain a general license or permission for the use of 298 such proprietary rights by implementers or users of this 299 specification can be obtained from the IETF on-line IPR repository at 300 http://www.ietf.org/ipr. 301 302 The IETF invites any interested party to bring to its attention any 303 copyrights, patents or patent applications, or other proprietary 304 rights that may cover technology that may be required to implement 305 this standard. Please address the information to the IETF at 306 ietf-ipr@ietf.org. 307 308 309Disclaimer of Validity 310 311 This document and the information contained herein are provided on an 312 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 313 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 314 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 315 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 316 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 317 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 318 319 320Copyright Statement 321 322 Copyright (C) The Internet Society (2005). This document is subject 323 to the rights, licenses and restrictions contained in BCP 78, and 324 except as set forth therein, the authors retain all their rights. 325 326 327Acknowledgment 328 329 Funding for the RFC Editor function is currently provided by the 330 Internet Society. 331 332 333 334 335Williams Expires December 15, 2005 [Page 6] 336 337 338