1 2 3 4 5 6 7Internet Engineering Task Force (IETF) L. Zhu 8Request for Comments: 6112 P. Leach 9Updates: 4120, 4121, 4556 Microsoft Corporation 10Category: Standards Track S. Hartman 11ISSN: 2070-1721 Painless Security 12 April 2011 13 14 15 Anonymity Support for Kerberos 16 17Abstract 18 19 This document defines extensions to the Kerberos protocol to allow a 20 Kerberos client to securely communicate with a Kerberos application 21 service without revealing its identity, or without revealing more 22 than its Kerberos realm. It also defines extensions that allow a 23 Kerberos client to obtain anonymous credentials without revealing its 24 identity to the Kerberos Key Distribution Center (KDC). This 25 document updates RFCs 4120, 4121, and 4556. 26 27Status of This Memo 28 29 This is an Internet Standards Track document. 30 31 This document is a product of the Internet Engineering Task Force 32 (IETF). It represents the consensus of the IETF community. It has 33 received public review and has been approved for publication by the 34 Internet Engineering Steering Group (IESG). Further information on 35 Internet Standards is available in Section 2 of RFC 5741. 36 37 Information about the current status of this document, any errata, 38 and how to provide feedback on it may be obtained at 39 http://www.rfc-editor.org/info/rfc6112. 40 41Copyright Notice 42 43 Copyright (c) 2011 IETF Trust and the persons identified as the 44 document authors. All rights reserved. 45 46 This document is subject to BCP 78 and the IETF Trust's Legal 47 Provisions Relating to IETF Documents 48 (http://trustee.ietf.org/license-info) in effect on the date of 49 publication of this document. Please review these documents 50 carefully, as they describe your rights and restrictions with respect 51 to this document. Code Components extracted from this document must 52 include Simplified BSD License text as described in Section 4.e of 53 the Trust Legal Provisions and are provided without warranty as 54 described in the Simplified BSD License. 55 56 57 58Zhu, et al. Standards Track [Page 1] 59 60RFC 6112 Kerberos Anonymity Support April 2011 61 62 63 This document may contain material from IETF Documents or IETF 64 Contributions published or made publicly available before November 65 10, 2008. The person(s) controlling the copyright in some of this 66 material may not have granted the IETF Trust the right to allow 67 modifications of such material outside the IETF Standards Process. 68 Without obtaining an adequate license from the person(s) controlling 69 the copyright in such materials, this document may not be modified 70 outside the IETF Standards Process, and derivative works of it may 71 not be created outside the IETF Standards Process, except to format 72 it for publication as an RFC or to translate it into languages other 73 than English. 74 75Table of Contents 76 77 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 78 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 79 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 80 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5 81 4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . . 5 82 4.1.1. Anonymous PKINIT . . . . . . . . . . . . . . . . . . . 6 83 4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 7 84 4.3. Subsequent Exchanges and Protocol Actions Common to AS 85 and TGS for Anonymity Support . . . . . . . . . . . . . . 9 86 5. Interoperability Requirements . . . . . . . . . . . . . . . . 10 87 6. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 10 88 7. PKINIT Client Contribution to the Ticket Session Key . . . . . 11 89 7.1. Combining Two Protocol Keys . . . . . . . . . . . . . . . 12 90 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 91 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 92 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 93 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 94 11.1. Normative References . . . . . . . . . . . . . . . . . . . 15 95 11.2. Informative References . . . . . . . . . . . . . . . . . . 16 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114Zhu, et al. Standards Track [Page 2] 115 116RFC 6112 Kerberos Anonymity Support April 2011 117 118 1191. Introduction 120 121 In certain situations, the Kerberos [RFC4120] client may wish to 122 authenticate a server and/or protect communications without revealing 123 the client's own identity. For example, consider an application that 124 provides read access to a research database and that permits queries 125 by arbitrary requesters. A client of such a service might wish to 126 authenticate the service, to establish trust in the information 127 received from it, but might not wish to disclose the client's 128 identity to the service for privacy reasons. 129 130 Extensions to Kerberos are specified in this document by which a 131 client can authenticate the Key Distribution Center (KDC) and request 132 an anonymous ticket. The client can use the anonymous ticket to 133 authenticate the server and protect subsequent client-server 134 communications. 135 136 By using the extensions defined in this specification, the client can 137 request an anonymous ticket where the client may reveal the client's 138 identity to the client's own KDC, or the client can hide the client's 139 identity completely by using anonymous Public Key Cryptography for 140 Initial Authentication in Kerberos (PKINIT) as defined in 141 Section 4.1. Using the returned anonymous ticket, the client remains 142 anonymous in subsequent Kerberos exchanges thereafter to KDCs on the 143 cross-realm authentication path and to the server with which it 144 communicates. 145 146 In this specification, the client realm in the anonymous ticket is 147 the anonymous realm name when anonymous PKINIT is used to obtain the 148 ticket. The client realm is the client's real realm name if the 149 client is authenticated using the client's long-term keys. Note that 150 the membership of a realm can imply a member of the community 151 represented by the realm. 152 153 The interaction with Generic Security Service Application Program 154 Interface (GSS-API) is described after the protocol description. 155 1562. Conventions Used in This Document 157 158 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 159 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 160 document are to be interpreted as described in [RFC2119]. 161 1623. Definitions 163 164 The anonymous Kerberos realm name is defined as a well-known realm 165 name based on [RFC6111], and the value of this well-known realm name 166 is the literal "WELLKNOWN:ANONYMOUS". 167 168 169 170Zhu, et al. Standards Track [Page 3] 171 172RFC 6112 Kerberos Anonymity Support April 2011 173 174 175 The anonymous Kerberos principal name is defined as a well-known 176 Kerberos principal name based on [RFC6111]. The value of the name- 177 type field is KRB_NT_WELLKNOWN [RFC6111], and the value of the name- 178 string field is a sequence of two KerberosString components: 179 "WELLKNOWN", "ANONYMOUS". 180 181 The anonymous ticket flag is defined as bit 16 (with the first bit 182 being bit 0) in the TicketFlags: 183 184 TicketFlags ::= KerberosFlags 185 -- anonymous(16) 186 -- TicketFlags and KerberosFlags are defined in [RFC4120] 187 188 This is a new ticket flag that is used to indicate that a ticket is 189 an anonymous one. 190 191 An anonymous ticket is a ticket that has all of the following 192 properties: 193 194 o The cname field contains the anonymous Kerberos principal name. 195 196 o The crealm field contains the client's realm name or the anonymous 197 realm name. 198 199 o The anonymous ticket contains no information that can reveal the 200 client's identity. However, the ticket may contain the client 201 realm, intermediate realms on the client's authentication path, 202 and authorization data that may provide information related to the 203 client's identity. For example, an anonymous principal that is 204 identifiable only within a particular group of users can be 205 implemented using authorization data and such authorization data, 206 if included in the anonymous ticket, would disclose the client's 207 membership of that group. 208 209 o The anonymous ticket flag is set. 210 211 The anonymous KDC option is defined as bit 16 (with the first bit 212 being bit 0) in the KDCOptions: 213 214 KDCOptions ::= KerberosFlags 215 -- anonymous(16) 216 -- KDCOptions and KerberosFlags are defined in [RFC4120] 217 218 219 220 221 222 223 224 225 226Zhu, et al. Standards Track [Page 4] 227 228RFC 6112 Kerberos Anonymity Support April 2011 229 230 231 As described in Section 4, the anonymous KDC option is set to request 232 an anonymous ticket in an Authentication Service (AS) request or a 233 Ticket Granting Service (TGS) request. 234 2354. Protocol Description 236 237 In order to request an anonymous ticket, the client sets the 238 anonymous KDC option in an AS request or a TGS request. 239 240 The rest of this section is organized as follows: it first describes 241 protocol actions specific to AS exchanges, then it describes those of 242 TGS exchanges. These are then followed by the description of 243 protocol actions common to both AS and TGS and those in subsequent 244 exchanges. 245 2464.1. Anonymity Support in AS Exchange 247 248 The client requests an anonymous ticket by setting the anonymous KDC 249 option in an AS exchange. 250 251 The Kerberos client can use the client's long-term keys, the client's 252 X.509 certificates [RFC4556], or any other pre-authentication data, 253 to authenticate to the KDC and requests an anonymous ticket in an AS 254 exchange where the client's identity is known to the KDC. 255 256 If the client in the AS request is anonymous, the anonymous KDC 257 option MUST be set in the request. Otherwise, the KDC MUST return a 258 KRB-ERROR message with the code KDC_ERR_BADOPTION. 259 260 If the client is anonymous and the KDC does not have a key to encrypt 261 the reply (this can happen when, for example, the KDC does not 262 support PKINIT [RFC4556]), the KDC MUST return an error message with 263 the code KDC_ERR_NULL_KEY [RFC4120]. 264 265 When policy allows, the KDC issues an anonymous ticket. If the 266 client name in the request is the anonymous principal, the client 267 realm (crealm) in the reply is the anonymous realm, otherwise, the 268 client realm is the realm of the AS. According to [RFC4120], the 269 client name and the client realm in the EncTicketPart of the reply 270 MUST match with the corresponding client name and the client realm of 271 the KDC reply; the client MUST use the client name and the client 272 realm returned in the KDC-REP in subsequent message exchanges when 273 using the obtained anonymous ticket. 274 275 Care MUST be taken by the KDC not to reveal the client's identity in 276 the authorization data of the returned ticket when populating the 277 authorization data in a returned anonymous ticket. 278 279 280 281 282Zhu, et al. Standards Track [Page 5] 283 284RFC 6112 Kerberos Anonymity Support April 2011 285 286 287 The AD-INITIAL-VERIFIED-CAS authorization data, as defined in 288 [RFC4556], contains the issuer name of the client certificate. This 289 authorization is not applicable and MUST NOT be present in the 290 returned anonymous ticket when anonymous PKINIT is used. When the 291 client is authenticated (i.e., anonymous PKINIT is not used), if it 292 is undesirable to disclose such information about the client's 293 identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be 294 removed from the returned anonymous ticket. 295 296 The client can use the client keys to mutually authenticate with the 297 KDC and request an anonymous Ticket Granting Ticket (TGT) in the AS 298 request. In that case, the reply key is selected as normal, 299 according to Section 3.1.3 of [RFC4120]. 300 3014.1.1. Anonymous PKINIT 302 303 This sub-section defines anonymous PKINIT. 304 305 As described earlier in this section, the client can request an 306 anonymous ticket by authenticating to the KDC using the client's 307 identity; alternatively, without revealing the client's identity to 308 the KDC, the Kerberos client can request an anonymous ticket as 309 follows: the client sets the client name as the anonymous principal 310 in the AS exchange and provides PA_PK_AS_REQ pre-authentication data 311 [RFC4556] where the signerInfos field of the SignedData [RFC5652] of 312 the PA_PK_AS_REQ is empty, and the certificates field is absent. 313 Because the anonymous client does not have an associated asymmetric 314 key pair, the client MUST choose the Diffie-Hellman key agreement 315 method by filling in the Diffie-Hellman domain parameters in the 316 clientPublicValue [RFC4556]. This use of the anonymous client name 317 in conjunction with PKINIT is referred to as anonymous PKINIT. If 318 anonymous PKINIT is used, the realm name in the returned anonymous 319 ticket MUST be the anonymous realm. 320 321 Upon receiving the anonymous PKINIT request from the client, the KDC 322 processes the request, according to Section 3.1.2 of [RFC4120]. The 323 KDC skips the checks for the client's signature and the client's 324 public key (such as the verification of the binding between the 325 client's public key and the client name), but performs otherwise 326 applicable checks, and proceeds as normal, according to [RFC4556]. 327 For example, the AS MUST check if the client's Diffie-Hellman domain 328 parameters are acceptable. The Diffie-Hellman key agreement method 329 MUST be used and the reply key is derived according to Section 330 3.2.3.1 of [RFC4556]. If the clientPublicValue is not present in the 331 request, the KDC MUST return a KRB-ERROR with the code 332 KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556]. If all goes 333 well, an anonymous ticket is generated, according to Section 3.1.3 of 334 [RFC4120], and PA_PK_AS_REP [RFC4556] pre-authentication data is 335 336 337 338Zhu, et al. Standards Track [Page 6] 339 340RFC 6112 Kerberos Anonymity Support April 2011 341 342 343 included in the KDC reply, according to [RFC4556]. If the KDC does 344 not have an asymmetric key pair, it MAY reply anonymously or reject 345 the authentication attempt. If the KDC replies anonymously, the 346 signerInfos field of the SignedData [RFC5652] of PA_PK_AS_REP in the 347 reply is empty, and the certificates field is absent. The server 348 name in the anonymous KDC reply contains the name of the TGS. 349 350 Upon receipt of the KDC reply that contains an anonymous ticket and 351 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then 352 authenticate the KDC based on the KDC's signature in the 353 PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply 354 (the reply is anonymous), the client MUST reject the returned ticket 355 if it cannot authenticate the KDC otherwise. 356 357 A KDC that supports anonymous PKINIT MUST indicate the support of 358 PKINIT, according to Section 3.4 of [RFC4556]. In addition, such a 359 KDC MUST indicate support for anonymous PKINIT by including a padata 360 element of padata-type PA_PKINIT_KX and empty padata-value when 361 including PA-PK-AS-REQ in an error reply. 362 363 When included in a KDC error, PA_PKINIT_KX indicates support for 364 anonymous PKINIT. As discussed in Section 7, when included in an AS- 365 REP, PA_PKINIT_KX proves that the KDC and client both contributed to 366 the session key for any use of Diffie-Hellman key agreement with 367 PKINIT. 368 369 Note that in order to obtain an anonymous ticket with the anonymous 370 realm name, the client MUST set the client name as the anonymous 371 principal in the request when requesting an anonymous ticket in an AS 372 exchange. Anonymity PKINIT is the only way via which an anonymous 373 ticket with the anonymous realm as the client realm can be generated 374 in this specification. 375 3764.2. Anonymity Support in TGS Exchange 377 378 The client requests an anonymous ticket by setting the anonymous KDC 379 option in a TGS exchange, and in that request the client can use a 380 normal Ticket Granting Ticket (TGT) with the client's identity, or an 381 anonymous TGT, or an anonymous cross-realm TGT. If the client uses a 382 normal TGT, the client's identity is known to the TGS. 383 384 Note that the client can completely hide the client's identity in an 385 AS exchange using anonymous PKINIT, as described in the previous 386 section. 387 388 389 390 391 392 393 394Zhu, et al. Standards Track [Page 7] 395 396RFC 6112 Kerberos Anonymity Support April 2011 397 398 399 If the ticket in the PA-TGS-REQ of the TGS request is an anonymous 400 one, the anonymous KDC option MUST be set in the request. Otherwise, 401 the KDC MUST return a KRB-ERROR message with the code 402 KDC_ERR_BADOPTION. 403 404 When policy allows, the KDC issues an anonymous ticket. If the 405 ticket in the TGS request is an anonymous one, the client name and 406 the client realm are copied from that ticket; otherwise, the ticket 407 in the TGS request is a normal ticket, the returned anonymous ticket 408 contains the client name as the anonymous principal and the client 409 realm as the true realm of the client. In all cases, according to 410 [RFC4120] the client name and the client realm in the EncTicketPart 411 of the reply MUST match with the corresponding client name and the 412 client realm of the anonymous ticket in the reply; the client MUST 413 use the client name and the client realm returned in the KDC-REP in 414 subsequent message exchanges when using the obtained anonymous 415 ticket. 416 417 Care MUST be taken by the TGS not to reveal the client's identity in 418 the authorization data of the returned ticket. When propagating 419 authorization data in the ticket or in the enc-authorization-data 420 field of the request, the TGS MUST ensure that the client 421 confidentiality is not violated in the returned anonymous ticket. 422 The TGS MUST process the authorization data recursively, according to 423 Section 5.2.6 of [RFC4120], beyond the container levels such that all 424 embedded authorization elements are interpreted. The TGS SHOULD NOT 425 populate identity-based authorization data into an anonymous ticket 426 in that such authorization data typically reveals the client's 427 identity. The specification of a new authorization data type MUST 428 specify the processing rules of the authorization data when an 429 anonymous ticket is returned. If there is no processing rule defined 430 for an authorization data element or the authorization data element 431 is unknown, the TGS MUST process it when an anonymous ticket is 432 returned as follows: 433 434 o If the authorization data element may reveal the client's 435 identity, it MUST be removed unless otherwise specified. 436 437 o If the authorization data element, that could reveal the client's 438 identity, is intended to restrict the use of the ticket or limit 439 the rights otherwise conveyed in the ticket, it cannot be removed 440 in order to hide the client's identity. In this case, the 441 authentication attempt MUST be rejected, and the TGS MUST return 442 an error message with the code KDC_ERR_POLICY. Note this is 443 applicable to both critical and optional authorization data. 444 445 446 447 448 449 450Zhu, et al. Standards Track [Page 8] 451 452RFC 6112 Kerberos Anonymity Support April 2011 453 454 455 o If the authorization data element is unknown, the TGS MAY remove 456 it, or transfer it into the returned anonymous ticket, or reject 457 the authentication attempt, based on local policy for that 458 authorization data type unless otherwise specified. If there is 459 no policy defined for a given unknown authorization data type, the 460 authentication MUST be rejected. The error code is KDC_ERR_POLICY 461 when the authentication is rejected. 462 463 The AD-INITIAL-VERIFIED-CAS authorization data, as defined in 464 [RFC4556], contains the issuer name of the client certificate. If it 465 is undesirable to disclose such information about the client's 466 identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be 467 removed from an anonymous ticket. 468 469 The TGS encodes the name of the previous realm into the transited 470 field, according to Section 3.3.3.2 of [RFC4120]. Based on local 471 policy, the TGS MAY omit the previous realm, if the cross realm TGT 472 is an anonymous one, in order to hide the authentication path of the 473 client. The unordered set of realms in the transited field, if 474 present, can reveal which realm may potentially be the realm of the 475 client or the realm that issued the anonymous TGT. The anonymous 476 Kerberos realm name MUST NOT be present in the transited field of a 477 ticket. The true name of the realm that issued the anonymous ticket 478 MAY be present in the transited field of a ticket. 479 4804.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for 481 Anonymity Support 482 483 In both AS and TGS exchanges, the realm field in the KDC request is 484 always the realm of the target KDC, not the anonymous realm when the 485 client requests an anonymous ticket. 486 487 Absent other information, the KDC MUST NOT include any identifier in 488 the returned anonymous ticket that could reveal the client's identity 489 to the server. 490 491 Unless anonymous PKINIT is used, if a client requires anonymous 492 communication, then the client MUST check to make sure that the 493 ticket in the reply is actually anonymous by checking the presence of 494 the anonymous ticket flag in the flags field of the EncKDCRepPart. 495 This is because KDCs ignore unknown KDC options. A KDC that does not 496 understand the anonymous KDC option will not return an error, but 497 will instead return a normal ticket. 498 499 The subsequent client and server communications then proceed as 500 described in [RFC4120]. 501 502 503 504 505 506Zhu, et al. Standards Track [Page 9] 507 508RFC 6112 Kerberos Anonymity Support April 2011 509 510 511 Note that the anonymous principal name and realm are only applicable 512 to the client in Kerberos messages, the server cannot be anonymous in 513 any Kerberos message per this specification. 514 515 A server accepting an anonymous service ticket may assume that 516 subsequent requests using the same ticket originate from the same 517 client. Requests with different tickets are likely to originate from 518 different clients. 519 520 Upon receipt of an anonymous ticket, the transited policy check is 521 performed in the same way as that of a normal ticket if the client's 522 realm is not the anonymous realm; if the client realm is the 523 anonymous realm, absent other information any realm in the 524 authentication path is allowed by the cross-realm policy check. 525 5265. Interoperability Requirements 527 528 Conforming implementations MUST support the anonymous principal with 529 a non-anonymous realm, and they MAY support the anonymous principal 530 with the anonymous realm using anonymous PKINIT. 531 5326. GSS-API Implementation Notes 533 534 GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to 535 represent the anonymous identity. In addition, Section 2.1.1 of 536 [RFC1964] defines the single string representation of a Kerberos 537 principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The 538 anonymous principal with the anonymous realm corresponds to the GSS- 539 API anonymous principal. A principal with the anonymous principal 540 name and a non-anonymous realm is an authenticated principal; hence, 541 such a principal does not correspond to the anonymous principal in 542 GSS-API with the GSS_C_NT_ANONYMOUS name type. The [RFC1964] name 543 syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the 544 anonymous principal name with a non-anonymous realm name and for 545 displaying and exporting these names. In addition, this syntax must 546 be used along with the name type GSS_C_NT_ANONYMOUS for displaying 547 and exporting the anonymous principal with the anonymous realm. 548 549 At the GSS-API [RFC2743] level, an initiator/client requests the use 550 of an anonymous principal with the anonymous realm by asserting the 551 "anonymous" flag when calling GSS_Init_Sec_Context(). The GSS-API 552 implementation MAY provide implementation-specific means for 553 requesting the use of an anonymous principal with a non-anonymous 554 realm. 555 556 GSS-API does not know or define "anonymous credentials", so the 557 (printable) name of the anonymous principal will rarely be used by or 558 relevant for the initiator/client. The printable name is relevant 559 560 561 562Zhu, et al. Standards Track [Page 10] 563 564RFC 6112 Kerberos Anonymity Support April 2011 565 566 567 for the acceptor/server when performing an authorization decision 568 based on the initiator name that is returned from the acceptor side 569 upon the successful security context establishment. 570 571 A GSS-API initiator MUST carefully check the resulting context 572 attributes from the initial call to GSS_Init_Sec_Context() when 573 requesting anonymity, because (as in the GSS-API tradition and for 574 backwards compatibility) anonymity is just another optional context 575 attribute. It could be that the mechanism doesn't recognize the 576 attribute at all or that anonymity is not available for some other 577 reasons -- and in that case the initiator MUST NOT send the initial 578 security context token to the acceptor, because it will likely reveal 579 the initiators identity to the acceptor, something that can rarely be 580 "un-done". 581 582 Portable initiators are RECOMMENDED to use default credentials 583 whenever possible, and request anonymity only through the input 584 anon_req_flag [RFC2743] to GSS_Init_Sec_Context(). 585 5867. PKINIT Client Contribution to the Ticket Session Key 587 588 The definition in this section was motivated by protocol analysis of 589 anonymous PKINIT (defined in this document) in building tunneling 590 channels [RFC6113] and subsequent channel bindings. In order to 591 enable applications of anonymous PKINIT to form channels, all 592 implementations of anonymous PKINIT need to meet the requirements of 593 this section. There is otherwise no connection to the rest of this 594 document. 595 596 PKINIT is useful for constructing tunneling channels. To ensure that 597 an attacker cannot create a channel with a given name, it is 598 desirable that neither the KDC nor the client unilaterally determine 599 the ticket session key. To achieve that end, a KDC conforming to 600 this definition MUST encrypt a randomly generated key, called the KDC 601 contribution key, in the PA_PKINIT_KX padata (defined next in this 602 section). The KDC contribution key is then combined with the reply 603 key to form the ticket session key of the returned ticket. These two 604 keys are then combined using the KRB-FX-CF2 operation defined in 605 Section 7.1, where K1 is the KDC contribution key, K2 is the reply 606 key, the input pepper1 is American Standard Code for Information 607 Interchange (ASCII) [ASAX34] string "PKINIT", and the input pepper2 608 is ASCII string "KeyExchange". 609 610 611 612 613 614 615 616 617 618Zhu, et al. Standards Track [Page 11] 619 620RFC 6112 Kerberos Anonymity Support April 2011 621 622 623 PA_PKINIT_KX 147 624 -- padata for PKINIT that contains an encrypted 625 -- KDC contribution key. 626 627 PA-PKINIT-KX ::= EncryptedData -- EncryptionKey 628 -- Contains an encrypted key randomly 629 -- generated by the KDC (known as the KDC contribution key). 630 -- Both EncryptedData and EncryptionKey are defined in [RFC4120] 631 632 The PA_PKINIT_KX padata MUST be included in the KDC reply when 633 anonymous PKINIT is used; it SHOULD be included if PKINIT is used 634 with the Diffie-Hellman key exchange but the client is not anonymous; 635 it MUST NOT be included otherwise (e.g., when PKINIT is used with the 636 public key encryption as the key exchange). 637 638 The padata-value field of the PA-PKINIT-KX type padata contains the 639 DER [X.680] [X.690] encoding of the Abstract Syntax Notation One 640 (ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is an 641 EncryptedData. The cleartext data being encrypted is the DER-encoded 642 KDC contribution key randomly generated by the KDC. The encryption 643 key is the reply key and the key usage number is 644 KEY_USAGE_PA_PKINIT_KX (44). 645 646 The client then decrypts the KDC contribution key and verifies the 647 ticket session key in the returned ticket is the combined key of the 648 KDC contribution key and the reply key as described above. A 649 conforming client MUST reject anonymous PKINIT authentication if the 650 PA_PKINIT_KX padata is not present in the KDC reply or if the ticket 651 session key of the returned ticket is not the combined key of the KDC 652 contribution key and the reply key when PA-PKINIT-KX is present in 653 the KDC reply. 654 6557.1. Combining Two Protocol Keys 656 657 KRB-FX-CF2() combines two protocol keys based on the pseudo-random() 658 function defined in [RFC3961]. 659 660 Given two input keys, K1 and K2, where K1 and K2 can be of two 661 different enctypes, the output key of KRB-FX-CF2(), K3, is derived as 662 follows: 663 664 KRB-FX-CF2(protocol key, protocol key, octet string, 665 octet string) -> (protocol key) 666 667 PRF+(K1, pepper1) -> octet-string-1 668 PRF+(K2, pepper2) -> octet-string-2 669 KRB-FX-CF2(K1, K2, pepper1, pepper2) -> 670 random-to-key(octet-string-1 ^ octet-string-2) 671 672 673 674Zhu, et al. Standards Track [Page 12] 675 676RFC 6112 Kerberos Anonymity Support April 2011 677 678 679 Where ^ denotes the exclusive-OR operation. PRF+() is defined as 680 follows: 681 682 PRF+(protocol key, octet string) -> (octet string) 683 684 PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) || 685 pseudo-random( key, 2 || shared-info ) || 686 pseudo-random( key, 3 || shared-info ) || ... 687 688 Here the counter value 1, 2, 3, and so on are encoded as a one-octet 689 integer. The pseudo-random() operation is specified by the enctype 690 of the protocol key. PRF+() uses the counter to generate enough bits 691 as needed by the random-to-key() [RFC3961] function for the 692 encryption type specified for the resulting key; unneeded bits are 693 removed from the tail. 694 6958. Security Considerations 696 697 Since KDCs ignore unknown options, a client requiring anonymous 698 communication needs to make sure that the returned ticket is actually 699 anonymous. This is because a KDC that does not understand the 700 anonymous option would not return an anonymous ticket. 701 702 By using the mechanism defined in this specification, the client does 703 not reveal the client's identity to the server but the client 704 identity may be revealed to the KDC of the server principal (when the 705 server principal is in a different realm than that of the client), 706 and any KDC on the cross-realm authentication path. The Kerberos 707 client MUST verify the ticket being used is indeed anonymous before 708 communicating with the server, otherwise, the client's identity may 709 be revealed unintentionally. 710 711 In cases where specific server principals must not have access to the 712 client's identity (for example, an anonymous poll service), the KDC 713 can define server-principal-specific policy that ensures any normal 714 service ticket can NEVER be issued to any of these server principals. 715 716 If the KDC that issued an anonymous ticket were to maintain records 717 of the association of identities to an anonymous ticket, then someone 718 obtaining such records could breach the anonymity. Additionally, the 719 implementations of most (for now all) KDC's respond to requests at 720 the time that they are received. Traffic analysis on the connection 721 to the KDC will allow an attacker to match client identities to 722 anonymous tickets issued. Because there are plaintext parts of the 723 tickets that are exposed on the wire, such matching by a third-party 724 observer is relatively straightforward. A service that is 725 authenticated by the anonymous principals may be able to infer the 726 727 728 729 730Zhu, et al. Standards Track [Page 13] 731 732RFC 6112 Kerberos Anonymity Support April 2011 733 734 735 identity of the client by examining and linking quasi-static protocol 736 information such as the IP address from which a request is received, 737 or by linking multiple uses of the same anonymous ticket. 738 739 Two mechanisms, the FAST facility with the hide-client-names option 740 in [RFC6113] and the Kerberos5 starttls option [STARTTLS], protect 741 the client identity so that an attacker would never be able to 742 observe the client identity sent to the KDC. Transport or network 743 layer security between the client and the server will help prevent 744 tracking of a particular ticket to link a ticket to a user. In 745 addition, clients can limit how often a ticket is reused to minimize 746 ticket linking. 747 748 The client's real identity is not revealed when the client is 749 authenticated as the anonymous principal. Application servers MAY 750 reject the authentication in order to, for example, prevent 751 information disclosure or as part of Denial of Service (DoS) 752 prevention. Application servers MUST avoid accepting anonymous 753 credentials in situations where they must record the client's 754 identity; for example, when there must be an audit trail. 755 7569. Acknowledgements 757 758 JK Jaganathan helped editing early revisions of this document. 759 760 Clifford Neuman contributed the core notions of this document. 761 762 Ken Raeburn reviewed the document and provided suggestions for 763 improvements. 764 765 Martin Rex wrote the text for GSS-API considerations. 766 767 Nicolas Williams reviewed the GSS-API considerations section and 768 suggested ideas for improvements. 769 770 Sam Hartman and Nicolas Williams were great champions of this work. 771 772 Miguel Garcia and Phillip Hallam-Baker reviewed the document and 773 provided helpful suggestions. 774 775 In addition, the following individuals made significant 776 contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love 777 Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia. 778 779 780 781 782 783 784 785 786Zhu, et al. Standards Track [Page 14] 787 788RFC 6112 Kerberos Anonymity Support April 2011 789 790 79110. IANA Considerations 792 793 This document defines a new 'anonymous' Kerberos well-known name and 794 a new 'anonymous' Kerberos well-known realm based on [RFC6111]. IANA 795 has added these two values to the Kerberos naming registries that are 796 created in [RFC6111]. 797 79811. References 799 80011.1. Normative References 801 802 [ASAX34] American Standards Institute, "American Standard Code for 803 Information Interchange", ASA X3.4-1963, June 1963. 804 805 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 806 RFC 1964, June 1996. 807 808 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 809 Requirement Levels", BCP 14, RFC 2119, March 1997. 810 811 [RFC2743] Linn, J., "Generic Security Service Application Program 812 Interface Version 2, Update 1", RFC 2743, January 2000. 813 814 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 815 Kerberos 5", RFC 3961, February 2005. 816 817 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 818 Kerberos Network Authentication Service (V5)", RFC 4120, 819 July 2005. 820 821 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial 822 Authentication in Kerberos (PKINIT)", RFC 4556, 823 June 2006. 824 825 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", 826 STD 70, RFC 5652, September 2009. 827 828 [RFC6111] Zhu, L., "Additional Kerberos Naming Constraints", 829 RFC 6111, April 2011. 830 831 [X.680] "Abstract Syntax Notation One (ASN.1): Specification of 832 Basic Notation", ITU-T Recommendation X.680: ISO/IEC 833 International Standard 8824-1:1998, 1997. 834 835 [X.690] "ASN.1 encoding rules: Specification of Basic Encoding 836 Rules (BER), Canonical Encoding Rules (CER) and 837 Distinguished Encoding Rules (DER)", ITU-T Recommendation 838 X.690 ISO/IEC International Standard 8825-1:1998, 1997. 839 840 841 842Zhu, et al. Standards Track [Page 15] 843 844RFC 6112 Kerberos Anonymity Support April 2011 845 846 84711.2. Informative References 848 849 [RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for 850 Kerberos Pre-Authentication", RFC 6113, April 2011. 851 852 [STARTTLS] Josefsson, S., "Using Kerberos V5 over the Transport 853 Layer Security (TLS) protocol", Work in Progress, 854 August 2010. 855 856Authors' Addresses 857 858 Larry Zhu 859 Microsoft Corporation 860 One Microsoft Way 861 Redmond, WA 98052 862 US 863 864 EMail: larry.zhu@microsoft.com 865 866 867 Paul Leach 868 Microsoft Corporation 869 One Microsoft Way 870 Redmond, WA 98052 871 US 872 873 EMail: paulle@microsoft.com 874 875 876 Sam Hartman 877 Painless Security 878 879 EMail: hartmans-ietf@mit.edu 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898Zhu, et al. Standards Track [Page 16] 899 900