1 2 3NETWORK WORKING GROUP L. Zhu 4Internet-Draft P. Leach 5Updates: 4120 (if approved) Microsoft Corporation 6Intended status: Standards Track July 7, 2007 7Expires: January 8, 2008 8 9 10 Anonymity Support for Kerberos 11 draft-ietf-krb-wg-anon-04 12 13Status of this Memo 14 15 By submitting this Internet-Draft, each author represents that any 16 applicable patent or other IPR claims of which he or she is aware 17 have been or will be disclosed, and any of which he or she becomes 18 aware will be disclosed, in accordance with Section 6 of BCP 79. 19 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 24 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 29 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 32 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 35 36 This Internet-Draft will expire on January 8, 2008. 37 38Copyright Notice 39 40 Copyright (C) The IETF Trust (2007). 41 42Abstract 43 44 This document defines extensions to the Kerberos protocol for the 45 Kerberos client to authenticate the Kerberos Key Distribution Center 46 and the Kerberos server, without revealing the client's identity. 47 These extensions can be used to secure communication between the 48 anonymous client and the server. 49 50 51 52 53 54Zhu & Leach Expires January 8, 2008 [Page 1] 55 56Internet-Draft Kerberos Anonymity Support July 2007 57 58 59Table of Contents 60 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 62 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 63 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 64 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 4 65 5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 8 66 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 67 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 68 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 69 9. Normative References . . . . . . . . . . . . . . . . . . . . . 10 70 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 71 Intellectual Property and Copyright Statements . . . . . . . . . . 11 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 110Zhu & Leach Expires January 8, 2008 [Page 2] 111 112Internet-Draft Kerberos Anonymity Support July 2007 113 114 1151. Introduction 116 117 In certain situations, the Kerberos [RFC4120] client may wish to 118 authenticate a server and/or protect communications without revealing 119 its own identity. For example, consider an application which 120 provides read access to a research database, and which permits 121 queries by arbitrary requestors. A client of such a service might 122 wish to authenticate the service, to establish trust in the 123 information received from it, but might not wish to disclose its 124 identity to the service for privacy reasons. 125 126 Extensions to [RFC4120] are specified in this document by which a 127 client can authenticate the Key Distribution Center (KDC) and request 128 an anonymous ticket. The client can use the anonymous ticket to 129 authenticate the server and protect subsequent client-server 130 communications. These extensions provide Kerberos with functional 131 equivalence to Transport Layer Security (TLS) [RFC4346]. 132 133 By using the extensions defined in this specification, the client may 134 reveal its identity in its initial request to its own KDC, but it can 135 remain anonymous thereafter to KDCs on the cross-realm authentication 136 path, and to the server with which it communicates. 137 138 1392. Conventions Used in This Document 140 141 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 142 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 143 document are to be interpreted as described in [RFC2119]. 144 145 1463. Definitions 147 148 The anonymous Kerberos realm name is defined as a well-known realm 149 name based on [KRBNAM]. The value is the literal "WELLKNOWN: 150 ANONYMOUS". An anonymous Kerberos realm name MUST NOT be present in 151 the transited field [RFC4120] of a ticket. 152 153 The anonymous Kerberos principal name is defined as a well-known 154 Kerberos principal name based on [KRBNAM]. The value of the name- 155 type field [RFC4120] is KRB_NT_WELLKNOWN [KRBNAM], and the value of 156 the name-string field [RFC4120] is a sequence of two KerberosString 157 components: "WELLKNOWN", "ANONYMOUS". 158 159 Note that in this specification, the anonymous principal name and 160 realm are only applicable to the client in Kerberos messages, the 161 server MUST NOT be anonymous in any Kerberos message. 162 163 164 165 166Zhu & Leach Expires January 8, 2008 [Page 3] 167 168Internet-Draft Kerberos Anonymity Support July 2007 169 170 171 The anonymous ticket flag is defined as bit 14 (with the first bit 172 being bit 0) in the TicketFlags: 173 174 TicketFlags ::= KerberosFlags 175 -- anonymous(14) 176 -- TicketFlags and KerberosFlags are defined in [RFC4120] 177 178 An anonymous ticket is a ticket that has all of the following 179 properties: 180 181 o The cname field [RFC4120] contains the anonymous Kerberos 182 principal name. 183 184 o The crealm field [RFC4120] contains the client's realm name, or 185 the name of the realm that issued the initial ticket for the 186 client principal, or the anonymous realm name. 187 188 o The anonymous ticket contains no information that can reveal the 189 client's identity. However the ticket may contain the client 190 realm, intermediate realms on the client's authentication path, 191 and authorization data that may provide information related to the 192 client's identity. For example, an anonymous principal that is 193 identifiable only within a particular group of users can be 194 implemented using authorization data and such authorization data, 195 if included in the anonymous ticket, shall disclose the client's 196 membership of that group. 197 198 o The anonymous ticket flag is set. 199 200 The anonymous KDC option is defined as bit 14 (with the first bit 201 being bit 0) in the KDCOptions: 202 203 KDCOptions ::= KerberosFlags 204 -- anonymous(14) 205 -- KDCOptions and KerberosFlags are defined in [RFC4120] 206 207 As described in Section 4, the anonymous KDC option is set to request 208 an anonymous ticket. 209 210 2114. Protocol Description 212 213 In order to request an anonymous ticket, the client sets the 214 anonymous KDC option in an Authentication Exchange (AS) or Ticket 215 Granting Service (TGS) request [RFC4120]. The client can request an 216 anonymous Ticket Granting Ticket (TGT) based on a normal TGT. Unless 217 otherwise specified, the client can obtain an anonymous ticket with 218 the anonymous realm name only by requesting an anonymous ticket in an 219 220 221 222Zhu & Leach Expires January 8, 2008 [Page 4] 223 224Internet-Draft Kerberos Anonymity Support July 2007 225 226 227 AS exchange with the client realm set as anonymous in the request. 228 229 If the client wishes to authenticate the KDC anonymously, it sets the 230 client name as anonymous in the AS exchange and provides a 231 PA_PK_AS_REQ pre-authentication data [RFC4556] where both the 232 signerInfos field and the certificates field of the SignedData 233 [RFC3852] of the PA_PK_AS_REQ are empty. Because the anonymous 234 client does not have an associated asymmetric key pair, the client 235 MUST choose the Diffie-Hellman key agreement method by filling in the 236 Diffie-Hellman domain parameters in the clientPublicValue [RFC4556]. 237 238 If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is 239 anonymous, or if the client in the AS request is anonymous, the 240 anonymous KDC option MUST be set in the request. Otherwise, the KDC 241 MUST return a KRB-ERROR message with the code KDC_ERR_BADOPTION 242 [RFC4120], and there is no accompanying e-data defined in this 243 document. 244 245 Upon receiving the AS request with a PA_PK_AS_REQ [RFC4556] from the 246 anonymous client, the KDC processes the request according to Section 247 3.1.2 of [RFC4120]. The KDC skips the checks for the client's 248 signature and the client's public key (such as the verification of 249 the binding between the client's public key and the client name), but 250 performs otherwise-applicable checks, and proceeds as normal 251 according to [RFC4556]. For example, the AS MUST check if the 252 client's Diffie-Hellman domain parameters are acceptable. The 253 Diffie-Hellman key agreement method MUST be used and the reply key is 254 derived according to Section 3.2.3.1 of [RFC4556]. If the 255 clientPublicValue is not present in the request, the KDC MUST return 256 a KRB-ERROR [RFC4120] with the code 257 KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556] and there is no 258 accompanying e-data. If all goes well, an anonymous ticket is 259 generated according to Section 3.1.3 of [RFC4120] and a PA_PK_AS_REP 260 [RFC4556] pre-authentication data is included in the KDC reply 261 according to [RFC4556]. If the KDC does not have an asymmetric key 262 pair, it MAY reply anonymously or reject the authentication attempt. 263 If the KDC replies anonymously, both the signerInfos field and the 264 certificates field of the SignedData [RFC3852] of PA_PK_AS_REP in the 265 reply are empty. The server name in the anonymous KDC reply contains 266 the name of the TGS. 267 268 Upon receipt of the KDC reply that contains an anonymous ticket and a 269 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then 270 authenticate the KDC based on the KDC's signature in the 271 PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply 272 (the reply is anonymous), the client MUST reject the returned ticket 273 if it cannot authenticate the KDC otherwise. 274 275 276 277 278Zhu & Leach Expires January 8, 2008 [Page 5] 279 280Internet-Draft Kerberos Anonymity Support July 2007 281 282 283 The client can use the client keys to mutually authenticate with the 284 KDC, request an anonymous TGT in the AS request. And in that case, 285 the reply key is selected as normal according to Section 3.1.3 of 286 [RFC4120]. 287 288 For the TGS exchange, the reply key is selected as normal according 289 to Section 3.3.3 of [RFC4120]. 290 291 When policy allows, the KDC issues an anonymous ticket. Based on 292 local policy, the client realm in the anonymous ticket can be the 293 anonymous realm name or the realm of the KDC. However, in all cases, 294 the client name and the client realm in the EncKDCRepPart of the 295 reply [RFC4120] MUST match with the corresponding client name and the 296 client realm of the anonymous ticket in the reply. The client MUST 297 use the client name and the client realm returned in the 298 EncKDCRepPart in subsequent message exchanges when using the obtained 299 anonymous ticket. 300 301 When propagating authorization data in the ticket or in the enc- 302 authorization-data field [RFC4120] of the request, the TGS MUST 303 ensure that the client confidentiality is not violated in the 304 returned anonymous ticket. The TGS MUST process the authorization 305 data recursively according to Section 5.2.6 of [RFC4120] beyond the 306 container levels such that all embedded authorization elements are 307 interpreted. Identity-based authorization data SHOULD NOT be present 308 in an anonymous ticket in that it typically reveals the client's 309 identity. The specification of a new authorization data type MUST 310 specify the processing rules of the authorization data when an 311 anonymous ticket is returned. If there is no processing rule defined 312 for an authorization data element or the authorization data element 313 is unknown, the TGS MUST process it when an anonymous ticket is 314 returned as follows: 315 316 o If the authorization data element may reveal the client's 317 identity, it MUST be removed unless otherwise specified. 318 319 o If the authorization data element is intended to restrict the use 320 of the ticket or limit the rights otherwise conveyed in the 321 ticket, it cannot be removed in order to hide the client's 322 identity. In this case, the authentication attempt MUST be 323 rejected, and the KDC MUST return an error message with the code 324 KDC_ERR_POLICY [RFC4120]. There is no accompanying e-data defined 325 in this document. Note this is applicable to both critical and 326 optional authorization data. 327 328 o If the authorization data element is unknown, the TGS MAY remove 329 it, or transfer it into the returned anonymous ticket, or reject 330 the authentication attempt, based on local policy for that 331 332 333 334Zhu & Leach Expires January 8, 2008 [Page 6] 335 336Internet-Draft Kerberos Anonymity Support July 2007 337 338 339 authorization data type unless otherwise specified. If there is 340 no policy defined for a given unknown authorization data type, the 341 authentication MUST be rejected. The error code is KDC_ERR_POLICY 342 when the authentication is rejected. 343 344 The AD-INITIAL-VERIFIED-CAS authorization data [RFC4556] MAY be 345 removed from an anonymous ticket based on local policy of the TGS. 346 347 The TGS MUST add the name of the previous realm according to Section 348 3.3.3.2 of [RFC4120]. If the client's realm is the anonymous realm, 349 the abbreviation forms [RFC4120] that build on the preceding name 350 cannot be used at the start of the transited encoding. The null- 351 subfield form (e.g., encoding ending with ",") [RFC4120] could not be 352 used next to the anonymous realm that can potentially be at the 353 beginning where the client realm is filled in. 354 355 The KDC fills out the authtime field of the anonymous ticket in the 356 reply as follows: If the anonymous ticket is returned in an AS 357 exchange, the authtime field of the ticket contains the request time. 358 If the anonymous ticket is returned in a TGS exchange, the authtime 359 field contains the authtime of the ticket in the PA-TGS-REQ pre- 360 authentication data [RFC4120]. An anonymous ticket can be renewed, 361 and the authtime field of a renewed ticket is the authtime in the 362 anonymous ticket on which the renewed ticket was based. 363 364 If the client is anonymous and the KDC does not have a key to encrypt 365 the reply (this can happen when, for example, the KDC does not 366 support PKINIT [RFC4556]), the KDC MUST return an error message with 367 the code KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying 368 e-data defined in this document. 369 370 If a client requires anonymous communication then the client MUST 371 check to make sure that the ticket in the reply is actually anonymous 372 by checking the presence of the anonymous ticket flag. This is 373 because KDCs ignore unknown KDC options. A KDC that does not 374 understand the anonymous KDC option will not return an error, but 375 will instead return a normal ticket. 376 377 The subsequent client and server communications then proceed as 378 described in [RFC4120]. 379 380 A server accepting an anonymous service ticket may assume that 381 subsequent requests using the same ticket originate from the same 382 client. Requests with different tickets are likely to originate from 383 different clients. 384 385 386 387 388 389 390Zhu & Leach Expires January 8, 2008 [Page 7] 391 392Internet-Draft Kerberos Anonymity Support July 2007 393 394 3955. GSS-API Implementation Notes 396 397 At the GSS-API [RFC2743] level, the use of an anonymous principal by 398 the initiator/client requires the initiator/client to assert the 399 "anonymous" flag when calling GSS_Init_Sec_Context(). 400 401 GSS-API does not know or define "anonymous credentials", so the 402 (printable) name of the anonymous principal will rarely be used by or 403 relevant for the initiator/client. The printable name is relevant 404 for the acceptor/server when performing an authorization decision 405 based on the initiator name that is returned from the acceptor side 406 upon the successful security context establishment. 407 408 A GSS-API initiator MUST carefully check the resulting context 409 attributes from the initial call to GSS_Init_Sec_Context() when 410 requesting anonymity, because (as in the GSS-API tradition and for 411 backwards compatibility) anonymity is just another optional context 412 attribute. It could be that the mechanism doesn't recognize the 413 attribute at all or that anonymity is not available for some other 414 reasons -- and in that case the initiator must NOT send the initial 415 security context token to the acceptor, because it will likely reveal 416 the initiators identity to the acceptor, something that can rarely be 417 "un-done". 418 419 GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to 420 represent the anonymous identity. In addition, Section 2.1.1 of 421 [RFC1964] defines the single string representation of a Kerberos 422 principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For 423 the anonymous principals, the name component within the exportable 424 name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm 425 name according to Section 2.1.1 of [RFC1964]. Note that in this 426 specification only the client/initiator can be anonymous. 427 428 Portable initiators are RECOMMENDED to use default credentials 429 whenever possible, and request anonymity only through the input 430 anon_req_flag [RFC2743] to GSS_Init_Sec_Context(). 431 432 4336. Security Considerations 434 435 Since KDCs ignore unknown options [RFC4120], a client requiring 436 anonymous communication needs to make sure that the ticket is 437 actually anonymous. This is because a KDC that that does not 438 understand the anonymous option would not return an anonymous ticket. 439 440 By using the mechanism defined in this specification, the client does 441 not reveal its identity to the server but its identity may be 442 revealed to the KDC of the server principal (when the server 443 444 445 446Zhu & Leach Expires January 8, 2008 [Page 8] 447 448Internet-Draft Kerberos Anonymity Support July 2007 449 450 451 principal is in a different realm than that of the client), and any 452 KDC on the cross-realm authentication path. The Kerberos client MUST 453 verify the ticket being used is indeed anonymous before communicating 454 with the server, otherwise the client's identity may be revealed 455 unintentionally. 456 457 In cases where specific server principals must not have access to the 458 client's identity (for example, an anonymous poll service), the KDC 459 can define server principal specific policy that insure any normal 460 service ticket can NEVER be issued to any of these server principals. 461 462 If the KDC that issued an anonymous ticket were to maintain records 463 of the association of identities to an anonymous ticket, then someone 464 obtaining such records could breach the anonymity. Additionally, the 465 implementations of most (for now all) KDC's respond to requests at 466 the time that they are received. Traffic analysis on the connection 467 to the KDC will allow an attacker to match client identities to 468 anonymous tickets issued. Because there are plaintext parts of the 469 tickets that are exposed on the wire, such matching by a third party 470 observer is relatively straightforward. 471 472 4737. Acknowledgements 474 475 JK Jaganathan helped editing early revisions of this document. 476 477 Clifford Neuman contributed the core notions of this document. 478 479 Ken Raeburn reviewed the document and provided suggestions for 480 improvements. 481 482 Martin Rex wrote the text for GSS-API considerations. 483 484 Nicolas Williams reviewed the GSS-API considerations section and 485 suggested ideas for improvements. 486 487 Sam Hartman and Nicolas Williams were great champions of this work. 488 489 In addition, the following individuals made significant 490 contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love 491 Hoernquist Aestrand, and Jeffery Hutzelman. 492 493 4948. IANA Considerations 495 496 Section 3 defines the anonymous Kerberos name and the anonymous 497 Kerberos realm based on [KRBNAM]. The IANA registry for [KRBNAM] 498 need to be updated to add references to this document. 499 500 501 502Zhu & Leach Expires January 8, 2008 [Page 9] 503 504Internet-Draft Kerberos Anonymity Support July 2007 505 506 5079. Normative References 508 509 [KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints", 510 draft-ietf-krb-wg-naming, work in progress. 511 512 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 513 RFC 1964, June 1996. 514 515 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 516 Requirement Levels", BCP 14, RFC 2119, March 1997. 517 518 [RFC2743] Linn, J., "Generic Security Service Application Program 519 Interface Version 2, Update 1", RFC 2743, January 2000. 520 521 [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", 522 RFC 3852, July 2004. 523 524 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 525 Kerberos Network Authentication Service (V5)", RFC 4120, 526 July 2005. 527 528 [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security 529 (TLS) Protocol Version 1.1", RFC 4346, April 2006. 530 531 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial 532 Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. 533 534 535Authors' Addresses 536 537 Larry Zhu 538 Microsoft Corporation 539 One Microsoft Way 540 Redmond, WA 98052 541 US 542 543 Email: lzhu@microsoft.com 544 545 546 Paul Leach 547 Microsoft Corporation 548 One Microsoft Way 549 Redmond, WA 98052 550 US 551 552 Email: paulle@microsoft.com 553 554 555 556 557 558 559Zhu & Leach Expires January 8, 2008 [Page 10] 560 561Internet-Draft Kerberos Anonymity Support July 2007 562 563 564Full Copyright Statement 565 566 Copyright (C) The IETF Trust (2007). 567 568 This document is subject to the rights, licenses and restrictions 569 contained in BCP 78, and except as set forth therein, the authors 570 retain all their rights. 571 572 This document and the information contained herein are provided on an 573 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 574 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 575 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 576 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 577 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 578 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 579 580 581Intellectual Property 582 583 The IETF takes no position regarding the validity or scope of any 584 Intellectual Property Rights or other rights that might be claimed to 585 pertain to the implementation or use of the technology described in 586 this document or the extent to which any license under such rights 587 might or might not be available; nor does it represent that it has 588 made any independent effort to identify any such rights. 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Please address the information to the IETF at 603 ietf-ipr@ietf.org. 604 605 606Acknowledgment 607 608 Funding for the RFC Editor function is provided by the IETF 609 Administrative Support Activity (IASA). 610 611 612 613 614 615Zhu & Leach Expires January 8, 2008 [Page 11] 616 617 618