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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (July 26, 2016) is 2124 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. 'ASAX34' Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group L. Zhu 3 Internet-Draft P. Leach 4 Obsoletes: 6112 (if approved) Microsoft Corporation 5 Updates: 4120, 4121, 4556 (if approved) S. Hartman, Ed. 6 Intended status: Standards Track Painless Security 7 Expires: January 27, 2017 S. Emery, Ed. 8 Oracle 9 July 26, 2016 11 Anonymity Support for Kerberos 12 draft-ietf-kitten-rfc6112bis-01 14 Abstract 16 This document defines extensions to the Kerberos protocol to allow a 17 Kerberos client to securely communicate with a Kerberos application 18 service without revealing its identity, or without revealing more 19 than its Kerberos realm. It also defines extensions that allow a 20 Kerberos client to obtain anonymous credentials without revealing its 21 identity to the Kerberos Key Distribution Center (KDC). This 22 document updates RFCs 4120, 4121, and 4556. This document obsoletes 23 RFC 6112 and reclassifies that document as historic. RFC 6112 24 contained errors and the protocol described in that specification is 25 not interoperable with any known implementation. This specification 26 describes a protocol that interoperates with multiple 27 implementations. 29 Status of This Memo 31 This Internet-Draft is submitted in full conformance with the 32 provisions of BCP 78 and BCP 79. 34 Internet-Drafts are working documents of the Internet Engineering 35 Task Force (IETF). Note that other groups may also distribute 36 working documents as Internet-Drafts. The list of current Internet- 37 Drafts is at http://datatracker.ietf.org/drafts/current/. 39 Internet-Drafts are draft documents valid for a maximum of six months 40 and may be updated, replaced, or obsoleted by other documents at any 41 time. It is inappropriate to use Internet-Drafts as reference 42 material or to cite them other than as "work in progress." 44 This Internet-Draft will expire on January 27, 2017. 46 Copyright Notice 48 Copyright (c) 2016 IETF Trust and the persons identified as the 49 document authors. All rights reserved. 51 This document is subject to BCP 78 and the IETF Trust's Legal 52 Provisions Relating to IETF Documents 53 (http://trustee.ietf.org/license-info) in effect on the date of 54 publication of this document. Please review these documents 55 carefully, as they describe your rights and restrictions with respect 56 to this document. Code Components extracted from this document must 57 include Simplified BSD License text as described in Section 4.e of 58 the Trust Legal Provisions and are provided without warranty as 59 described in the Simplified BSD License. 61 This document may contain material from IETF Documents or IETF 62 Contributions published or made publicly available before November 63 10, 2008. The person(s) controlling the copyright in some of this 64 material may not have granted the IETF Trust the right to allow 65 modifications of such material outside the IETF Standards Process. 66 Without obtaining an adequate license from the person(s) controlling 67 the copyright in such materials, this document may not be modified 68 outside the IETF Standards Process, and derivative works of it may 69 not be created outside the IETF Standards Process, except to format 70 it for publication as an RFC or to translate it into languages other 71 than English. 73 Table of Contents 75 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 76 1.1. Changes Since RFC 6112 . . . . . . . . . . . . . . . . . 4 77 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 78 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 79 4. Protocol Description . . . . . . . . . . . . . . . . . . . . 5 80 4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . 5 81 4.1.1. Anonymous PKINIT . . . . . . . . . . . . . . . . . . 6 82 4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 8 83 4.3. Subsequent Exchanges and Protocol Actions Common to AS 84 and TGS for Anonymity Support . . . . . . . . . . . . . . 9 85 5. Interoperability Requirements . . . . . . . . . . . . . . . . 10 86 6. GSS-API Implementation Notes . . . . . . . . . . . . . . . . 10 87 7. PKINIT Client Contribution to the Ticket Session Key . . . 11 88 7.1. Combining Two Protocol Keys . . . . . . . . . . . . . . . 13 89 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13 90 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 91 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 92 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 93 11.1. Normative References . . . . . . . . . . . . . . . . . . 15 94 11.2. Informative References . . . . . . . . . . . . . . . . . 16 95 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 97 1. Introduction 99 In certain situations, the Kerberos [RFC4120] client may wish to 100 authenticate a server and/or protect communications without revealing 101 the client's own identity. For example, consider an application that 102 provides read access to a research database and that permits queries 103 by arbitrary requesters. A client of such a service might wish to 104 authenticate the service, to establish trust in the information 105 received from it, but might not wish to disclose the client's 106 identity to the service for privacy reasons. 108 Extensions to Kerberos are specified in this document by which a 109 client can authenticate the Key Distribution Center (KDC) and request 110 an anonymous ticket. The client can use the anonymous ticket to 111 authenticate the server and protect subsequent client-server 112 communications. 114 By using the extensions defined in this specification, the client can 115 request an anonymous ticket where the client may reveal the client's 116 identity to the client's own KDC, or the client can hide the client's 117 identity completely by using anonymous Public Key Cryptography for 118 Initial Authentication in Kerberos (PKINIT) as defined in 119 Section 4.1. Using the returned anonymous ticket, the client remains 120 anonymous in subsequent Kerberos exchanges thereafter to KDCs on the 121 cross-realm authentication path and to the server with which it 122 communicates. 124 In this specification, the client realm in the anonymous ticket is 125 the anonymous realm name when anonymous PKINIT is used to obtain the 126 ticket. The client realm is the client's real realm name if the 127 client is authenticated using the client's long-term keys. Note that 128 a membership in a realm can imply a member of the community 129 represented by the realm. 131 The interaction with Generic Security Service Application Program 132 Interface (GSS-API) is described after the protocol description. 134 This specification replaces RFC 6112 to correct technical errors in 135 that specification. RFC 6112 is classified is historic; 136 implementation of RFC 6112 is NOT RECOMMENDED: existing 137 implementations comply with this specification and not RFC 6112. 139 1.1. Changes Since RFC 6112 141 In Section 7, the pepper2 string is corrected to comply with the 142 string actually used by implementations. 144 The requirement for the anonymous option to be used when an anonymous 145 ticket is used in a TGS request is reduced from a MUST to a SHOULD. 147 2. Conventions Used in This Document 149 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 150 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 151 document are to be interpreted as described in [RFC2119]. 153 3. Definitions 155 The anonymous Kerberos realm name is defined as a well-known realm 156 name based on [RFC6111], and the value of this well-known realm name 157 is the literal "WELLKNOWN:ANONYMOUS". 159 The anonymous Kerberos principal name is defined as a well-known 160 Kerberos principal name based on [RFC6111]. The value of the name- 161 type field is KRB_NT_WELLKNOWN [RFC6111], and the value of the name- 162 string field is a sequence of two KerberosString components: 163 "WELLKNOWN", "ANONYMOUS". 165 The anonymous ticket flag is defined as bit 16 (with the first bit 166 being bit 0) in the TicketFlags: 168 TicketFlags ::= KerberosFlags 169 -- anonymous(16) 170 -- TicketFlags and KerberosFlags are defined in [RFC4120] 172 This is a new ticket flag that is used to indicate that a ticket is 173 an anonymous one. 175 An anonymous ticket is a ticket that has all of the following 176 properties: 178 o The cname field contains the anonymous Kerberos principal name. 180 o The crealm field contains the client's realm name or the anonymous 181 realm name. 183 o The anonymous ticket contains no information that can reveal the 184 client's identity. However, the ticket may contain the client 185 realm, intermediate realms on the client's authentication path, 186 and authorization data that may provide information related to the 187 client's identity. For example, an anonymous principal that is 188 identifiable only as being in a particular group of users can be 189 implemented using authorization data. Such authorization data, if 190 included in the anonymous ticket, would disclose the that the 191 client is a member of the group observed. 193 o The anonymous ticket flag is set. 195 The anonymous KDC option is defined as bit 16 (with the first bit 196 being bit 0) in the KDCOptions: 198 KDCOptions ::= KerberosFlags 199 -- anonymous(16) 200 -- KDCOptions and KerberosFlags are defined in [RFC4120] 202 As described in Section 4, the anonymous KDC option is set to request 203 an anonymous ticket in an Authentication Service (AS) request or a 204 Ticket Granting Service (TGS) request. 206 4. Protocol Description 208 In order to request an anonymous ticket, the client sets the 209 anonymous KDC option in an AS request or a TGS request. 211 The rest of this section is organized as follows: it first describes 212 protocol actions specific to AS exchanges, then it describes those of 213 TGS exchanges. These are then followed by the description of 214 protocol actions common to both AS and TGS and those in subsequent 215 exchanges. 217 4.1. Anonymity Support in AS Exchange 219 The client requests an anonymous ticket by setting the anonymous KDC 220 option in an AS exchange. 222 The Kerberos client can use the client's long-term keys, the client's 223 X.509 certificates [RFC4556], or any other pre-authentication data, 224 to authenticate to the KDC and request an anonymous ticket in an AS 225 exchange where the client's identity is known to the KDC. 227 If the client in the AS request is anonymous, the anonymous KDC 228 option MUST be set in the request. Otherwise, the KDC MUST return a 229 KRB-ERROR message with the code KDC_ERR_BADOPTION. 231 If the client is anonymous and the KDC does not have a key to encrypt 232 the reply (this can happen when, for example, the KDC does not 233 support PKINIT [RFC4556]), the KDC MUST return an error message with 234 the code KDC_ERR_NULL_KEY [RFC4120]. 236 When policy allows, the KDC issues an anonymous ticket. If the 237 client name in the request is the anonymous principal, the client 238 realm (crealm) in the reply is the anonymous realm, otherwise, the 239 client realm is the realm of the AS. As specified by [RFC4120], the 240 client name and the client realm in the EncTicketPart of the reply 241 MUST match with the corresponding client name and the client realm of 242 the KDC reply; the client MUST use the client name and the client 243 realm returned in the KDC-REP in subsequent message exchanges when 244 using the obtained anonymous ticket. 246 Care MUST be taken by the KDC not to reveal the client's identity in 247 the authorization data of the returned ticket when populating the 248 authorization data in a returned anonymous ticket. 250 The AD_INITIAL_VERIFIED_CAS authorization data, as defined in 251 [RFC4556], contains the issuer name of the client certificate. This 252 authorization is not applicable and MUST NOT be present in the 253 returned anonymous ticket when anonymous PKINIT is used. When the 254 client is authenticated (i.e., anonymous PKINIT is not used), if it 255 is undesirable to disclose such information about the client's 256 identity, the AD_INITIAL_VERIFIED_CAS authorization data SHOULD be 257 removed from the returned anonymous ticket. 259 The client can use the client's key to mutually authenticate with the 260 KDC and request an anonymous Ticket Granting Ticket (TGT) in the AS 261 request. In that case, the reply key is selected as normal, 262 according to Section 3.1.3 of [RFC4120]. 264 4.1.1. Anonymous PKINIT 266 This sub-section defines anonymous PKINIT. 268 As described earlier in this section, the client can request an 269 anonymous ticket by authenticating to the KDC using the client's 270 identity; alternatively, without revealing the client's identity to 271 the KDC, the Kerberos client can request an anonymous ticket as 272 follows: the client sets the client name as the anonymous principal 273 in the AS exchange and provides PA_PK_AS_REQ pre-authentication data 274 [RFC4556] where the signerInfos field of the SignedData [RFC5652] of 275 the PA_PK_AS_REQ is empty, and the certificates field is absent. 276 Because the anonymous client does not have an associated asymmetric 277 key pair, the client MUST choose the Diffie-Hellman key agreement 278 method by filling in the Diffie-Hellman domain parameters in the 279 clientPublicValue [RFC4556]. This use of the anonymous client name 280 in conjunction with PKINIT is referred to as anonymous PKINIT. If 281 anonymous PKINIT is used, the realm name in the returned anonymous 282 ticket MUST be the anonymous realm. 284 Upon receiving the anonymous PKINIT request from the client, the KDC 285 processes the request, according to Section 3.1.2 of [RFC4120]. The 286 KDC skips the checks for the client's signature and the client's 287 public key (such as the verification of the binding between the 288 client's public key and the client name), but performs otherwise 289 applicable checks, and proceeds as normal, according to [RFC4556]. 290 For example, the AS MUST check if the client's Diffie-Hellman domain 291 parameters are acceptable. The Diffie-Hellman key agreement method 292 MUST be used and the reply key is derived according to 293 Section 3.2.3.1 of [RFC4556]. If the clientPublicValue is not 294 present in the request, the KDC MUST return a KRB-ERROR with the code 295 KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556]. If all goes 296 well, an anonymous ticket is generated, according to Section 3.1.3 of 297 [RFC4120], and PA_PK_AS_REP [RFC4556] pre-authentication data is 298 included in the KDC reply, according to [RFC4556]. If the KDC does 299 not have an asymmetric key pair, it MAY reply anonymously or reject 300 the authentication attempt. If the KDC replies anonymously, the 301 signerInfos field of the SignedData [RFC5652] of PA_PK_AS_REP in the 302 reply is empty, and the certificates field is absent. The server 303 name in the anonymous KDC reply contains the name of the TGS. 305 Upon receipt of the KDC reply that contains an anonymous ticket and 306 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then 307 authenticate the KDC based on the KDC's signature in the 308 PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply 309 (the reply is anonymous), the client MUST reject the returned ticket 310 if it cannot authenticate the KDC otherwise. 312 A KDC that supports anonymous PKINIT MUST indicate the support of 313 PKINIT, according to Section 3.4 of [RFC4556]. In addition, such a 314 KDC MUST indicate support for anonymous PKINIT by including a padata 315 element of padata-type PA_PKINIT_KX and empty padata-value when 316 including PA-PK-AS-REQ in an error reply. 318 When included in a KDC error, PA_PKINIT_KX indicates support for 319 anonymous PKINIT. As discussed in Section 7, when included in an AS- 320 REP, PA_PKINIT_KX proves that the KDC and client both contributed to 321 the session key for any use of Diffie-Hellman key agreement with 322 PKINIT. 324 Note that in order to obtain an anonymous ticket with the anonymous 325 realm name, the client MUST set the client name as the anonymous 326 principal in the request when requesting an anonymous ticket in an AS 327 exchange. Anonymous PKINIT is the only way via which an anonymous 328 ticket with the anonymous realm as the client realm can be generated 329 in this specification. 331 4.2. Anonymity Support in TGS Exchange 333 The client requests an anonymous ticket by setting the anonymous KDC 334 option in a TGS exchange, and in that request the client can use a 335 normal Ticket Granting Ticket (TGT) with the client's identity, or an 336 anonymous TGT, or an anonymous cross-realm TGT. If the client uses a 337 normal TGT, the client's identity is known to the TGS. 339 Note that the client can completely hide the client's identity in an 340 AS exchange using anonymous PKINIT, as described in the previous 341 section. 343 If the ticket in the PA-TGS-REQ of the TGS request is an anonymous 344 one, the anonymous KDC option SHOULD be set in the request. 346 When policy allows, the KDC issues an anonymous ticket. If the 347 ticket in the TGS request is an anonymous one, the client name and 348 the client realm are copied from that ticket; otherwise, the ticket 349 in the TGS request is a normal ticket, the returned anonymous ticket 350 contains the client name as the anonymous principal and the client 351 realm as the true realm of the client. In all cases, according to 352 [RFC4120] the client name and the client realm in the EncTicketPart 353 of the reply MUST match with the corresponding client name and the 354 client realm of the anonymous ticket in the reply; the client MUST 355 use the client name and the client realm returned in the KDC-REP in 356 subsequent message exchanges when using the obtained anonymous 357 ticket. 359 Care MUST be taken by the TGS not to reveal the client's identity in 360 the authorization data of the returned ticket. When propagating 361 authorization data in the ticket or in the enc-authorization-data 362 field of the request, the TGS MUST ensure that the client 363 confidentiality is not violated in the returned anonymous ticket. 364 The TGS MUST process the authorization data recursively, according to 365 Section 5.2.6 of [RFC4120], beyond the container levels such that all 366 embedded authorization elements are interpreted. The TGS SHOULD NOT 367 populate identity-based authorization data into an anonymous ticket 368 in that such authorization data typically reveals the client's 369 identity. The specification of a new authorization data type MUST 370 specify the processing rules of the authorization data when an 371 anonymous ticket is returned. If there is no processing rule defined 372 for an authorization data element or the authorization data element 373 is unknown, the TGS MUST process it when an anonymous ticket is 374 returned as follows: 376 o If the authorization data element may reveal the client's 377 identity, it MUST be removed unless otherwise specified. 379 o If the authorization data element, that could reveal the client's 380 identity, is intended to restrict the use of the ticket or limit 381 the rights otherwise conveyed in the ticket, it cannot be removed 382 in order to hide the client's identity. In this case, the 383 authentication attempt MUST be rejected, and the TGS MUST return 384 an error message with the code KDC_ERR_POLICY. Note this is 385 applicable to both critical and optional authorization data. 387 o If the authorization data element is unknown, the TGS MAY remove 388 it, or transfer it into the returned anonymous ticket, or reject 389 the authentication attempt, based on local policy for that 390 authorization data type unless otherwise specified. If there is 391 no policy defined for a given unknown authorization data type, the 392 authentication MUST be rejected. The error code is KDC_ERR_POLICY 393 when the authentication is rejected. 395 The AD_INITIAL_VERIFIED_CAS authorization data, as defined in 396 [RFC4556], contains the issuer name of the client certificate. If it 397 is undesirable to disclose such information about the client's 398 identity, the AD_INITIAL_VERIFIED_CAS authorization data SHOULD be 399 removed from an anonymous ticket. 401 The TGS encodes the name of the previous realm into the transited 402 field, according to Section 3.3.3.2 of [RFC4120]. Based on local 403 policy, the TGS MAY omit the previous realm, if the cross realm TGT 404 is an anonymous one, in order to hide the authentication path of the 405 client. The unordered set of realms in the transited field, if 406 present, can reveal which realm may potentially be the realm of the 407 client or the realm that issued the anonymous TGT. The anonymous 408 Kerberos realm name MUST NOT be present in the transited field of a 409 ticket. The true name of the realm that issued the anonymous ticket 410 MAY be present in the transited field of a ticket. 412 4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for 413 Anonymity Support 415 In both AS and TGS exchanges, the realm field in the KDC request is 416 always the realm of the target KDC, not the anonymous realm when the 417 client requests an anonymous ticket. 419 Absent other information, the KDC MUST NOT include any identifier in 420 the returned anonymous ticket that could reveal the client's identity 421 to the server. 423 Unless anonymous PKINIT is used, if a client requires anonymous 424 communication, then the client MUST check to make sure that the 425 ticket in the reply is actually anonymous by checking the presence of 426 the anonymous ticket flag in the flags field of the EncKDCRepPart. 427 This is because KDCs ignore unknown KDC options. A KDC that does not 428 understand the anonymous KDC option will not return an error, but 429 will instead return a normal ticket. 431 The subsequent client and server communications then proceed as 432 described in [RFC4120]. 434 Note that the anonymous principal name and realm are only applicable 435 to the client in Kerberos messages, the server cannot be anonymous in 436 any Kerberos message per this specification. 438 A server accepting an anonymous service ticket may assume that 439 subsequent requests using the same ticket originate from the same 440 client. Requests with different tickets are likely to originate from 441 different clients. 443 Upon receipt of an anonymous ticket, the transited policy check is 444 performed in the same way as that of a normal ticket if the client's 445 realm is not the anonymous realm; if the client realm is the 446 anonymous realm, absent other information any realm in the 447 authentication path is allowed by the cross-realm policy check. 449 5. Interoperability Requirements 451 Conforming implementations MUST support the anonymous principal with 452 a non-anonymous realm, and they MAY support the anonymous principal 453 with the anonymous realm using anonymous PKINIT. 455 6. GSS-API Implementation Notes 457 GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to 458 represent the anonymous identity. In addition, Section 2.1.1 of 459 [RFC1964] defines the single string representation of a Kerberos 460 principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The 461 anonymous principal with the anonymous realm corresponds to the GSS- 462 API anonymous principal. A principal with the anonymous principal 463 name and a non-anonymous realm is an authenticated principal; hence, 464 such a principal does not correspond to the anonymous principal in 465 GSS-API with the GSS_C_NT_ANONYMOUS name type. The [RFC1964] name 466 syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the 467 anonymous principal name with a non-anonymous realm name and for 468 displaying and exporting these names. In addition, this syntax must 469 be used along with the name type GSS_C_NT_ANONYMOUS for displaying 470 and exporting the anonymous principal with the anonymous realm. 472 At the GSS-API [RFC2743] level, an initiator/client requests the use 473 of an anonymous principal with the anonymous realm by asserting the 474 "anonymous" flag when calling GSS_Init_Sec_Context(). The GSS-API 475 implementation MAY provide implementation-specific means for 476 requesting the use of an anonymous principal with a non-anonymous 477 realm. 479 GSS-API does not know or define "anonymous credentials", so the 480 (printable) name of the anonymous principal will rarely be used by or 481 relevant for the initiator/client. The printable name is relevant 482 for the acceptor/server when performing an authorization decision 483 based on the initiator name that is returned from the acceptor side 484 upon the successful security context establishment. 486 A GSS-API initiator MUST carefully check the resulting context 487 attributes from the initial call to GSS_Init_Sec_Context() when 488 requesting anonymity, because (as in the GSS-API tradition and for 489 backwards compatibility) anonymity is just another optional context 490 attribute. It could be that the mechanism doesn't recognize the 491 attribute at all or that anonymity is not available for some other 492 reasons -- and in that case the initiator MUST NOT send the initial 493 security context token to the acceptor, because it will likely reveal 494 the initiators identity to the acceptor, something that can rarely be 495 "un-done". 497 Portable initiators are RECOMMENDED to use default credentials 498 whenever possible, and request anonymity only through the input 499 anon_req_flag [RFC2743] to GSS_Init_Sec_Context(). 501 7. PKINIT Client Contribution to the Ticket Session Key 503 The definition in this section was motivated by protocol analysis of 504 anonymous PKINIT (defined in this document) in building secure 505 channels [RFC6113] and subsequent channel bindings [RFC5056]. In 506 order to enable applications of anonymous PKINIT to form secure 507 channels, all implementations of anonymous PKINIT need to meet the 508 requirements of this section. There is otherwise no connection to 509 the rest of this document. 511 PKINIT is useful for constructing secure channels. To ensure that an 512 attacker cannot create a channel by observing exchanges, it is 513 desirable that neither the KDC nor the client unilaterally determine 514 the ticket session key. The specific reason why the ticket session 515 key is derived jointly is discussed at the end of this section. To 516 achieve that end, a KDC conforming to this definition MUST encrypt a 517 randomly generated key, called the KDC contribution key, in the 518 PA_PKINIT_KX padata (defined next in this section). The KDC 519 contribution key is then combined with the reply key to form the 520 ticket session key of the returned ticket. These two keys are then 521 combined using the KRB-FX-CF2 operation defined in Section 7.1, where 522 K1 is the KDC contribution key, K2 is the reply key, the input 523 pepper1 is American Standard Code for Information Interchange (ASCII) 524 [ASAX34] string "PKINIT", and the input pepper2 is ASCII string 525 "KEYEXCHANGE". 527 PA_PKINIT_KX 147 528 -- padata for PKINIT that contains an encrypted 529 -- KDC contribution key. 531 PA-PKINIT-KX ::= EncryptedData -- EncryptionKey 532 -- Contains an encrypted key randomly 533 -- generated by the KDC (known as the KDC contribution key). 534 -- Both EncryptedData and EncryptionKey are defined in [RFC4120] 536 The PA_PKINIT_KX padata MUST be included in the KDC reply when 537 anonymous PKINIT is used; it SHOULD be included if PKINIT is used 538 with the Diffie-Hellman key exchange but the client is not anonymous; 539 it MUST NOT be included otherwise (e.g., when PKINIT is used with the 540 public key encryption as the key exchange). 542 The padata-value field of the PA-PKINIT-KX type padata contains the 543 DER [X.680] [X.690] encoding of the Abstract Syntax Notation One 544 (ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is an 545 EncryptedData. The cleartext data being encrypted is the DER-encoded 546 KDC contribution key randomly generated by the KDC. The encryption 547 key is the reply key and the key usage number is 548 KEY_USAGE_PA_PKINIT_KX (44). 550 The client then decrypts the KDC contribution key and verifies the 551 ticket session key in the returned ticket is the combined key of the 552 KDC contribution key and the reply key as described above. A 553 conforming client MUST reject anonymous PKINIT authentication if the 554 PA_PKINIT_KX padata is not present in the KDC reply or if the ticket 555 session key of the returned ticket is not the combined key of the KDC 556 contribution key and the reply key when PA-PKINIT-KX is present in 557 the KDC reply. 559 This protocol provides a binding between the party which generated 560 the session key and the DH exchange used to generate they reply key. 561 Hypothetically, if the KDC did not use PA-PKINIT-KX, the client and 562 KDC would perfrom a DH key exchange to determine a shared key, and 563 that key would be used as a reply key. The KDC would then generate a 564 ticket with a session key encrypting the reply with the DH agreement. 565 A MITM attacker would just decrypt the session key + ticket using the 566 DH key from the attacker and KDC DH exchange, and re-encrypt it using 567 the key from the attacker and client DH exchange, while keeping a 568 copy of the session key and ticket. By requiring the session key in 569 a way that can be verified by the client, this protocol binds the 570 ticket to the DH exchange and prevents the MITM attack. 572 7.1. Combining Two Protocol Keys 574 KRB-FX-CF2() combines two protocol keys based on the pseudo-random() 575 function defined in [RFC3961]. 577 Given two input keys, K1 and K2, where K1 and K2 can be of two 578 different enctypes, the output key of KRB-FX-CF2(), K3, is derived as 579 follows: 581 KRB-FX-CF2(protocol key, protocol key, octet string, 582 octet string) -> (protocol key) 584 PRF+(K1, pepper1) -> octet-string-1 585 PRF+(K2, pepper2) -> octet-string-2 586 KRB-FX-CF2(K1, K2, pepper1, pepper2) -> 587 random-to-key(octet-string-1 ^ octet-string-2) 589 Where ^ denotes the exclusive-OR operation. PRF+() is defined as 590 follows: 592 PRF+(protocol key, octet string) -> (octet string) 594 PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) || 595 pseudo-random( key, 2 || shared-info ) || 596 pseudo-random( key, 3 || shared-info ) || ... 598 Here the counter value 1, 2, 3, and so on are encoded as a one-octet 599 integer. The pseudo-random() operation is specified by the enctype 600 of the protocol key. PRF+() uses the counter to generate enough bits 601 as needed by the random-to-key() [RFC3961] function for the 602 encryption type specified for the resulting key; unneeded bits are 603 removed from the tail. 605 8. Security Considerations 607 Since KDCs ignore unknown options, a client requiring anonymous 608 communication needs to make sure that the returned ticket is actually 609 anonymous. This is because a KDC that does not understand the 610 anonymous option would not return an anonymous ticket. 612 By using the mechanism defined in this specification, the client does 613 not reveal the client's identity to the server but the client 614 identity may be revealed to the KDC of the server principal (when the 615 server principal is in a different realm than that of the client), 616 and any KDC on the cross-realm authentication path. The Kerberos 617 client MUST verify the ticket being used is indeed anonymous before 618 communicating with the server, otherwise, the client's identity may 619 be revealed unintentionally. 621 In cases where specific server principals must not have access to the 622 client's identity (for example, an anonymous poll service), the KDC 623 can define server-principal-specific policy that ensures any normal 624 service ticket can NEVER be issued to any of these server principals. 626 If the KDC that issued an anonymous ticket were to maintain records 627 of the association of identities to an anonymous ticket, then someone 628 obtaining such records could breach the anonymity. Additionally, the 629 implementations of most (for now all) KDC's respond to requests at 630 the time that they are received. Traffic analysis on the connection 631 to the KDC will allow an attacker to match client identities to 632 anonymous tickets issued. Because there are plaintext parts of the 633 tickets that are exposed on the wire, such matching by a third-party 634 observer is relatively straightforward. A service that is 635 authenticated by the anonymous principals may be able to infer the 636 identity of the client by examining and linking quasi-static protocol 637 information such as the IP address from which a request is received, 638 or by linking multiple uses of the same anonymous ticket. 640 Two mechanisms, the FAST facility with the hide-client-names option 641 in [RFC6113] and the Kerberos5 starttls option [STARTTLS], protect 642 the client identity so that an attacker would never be able to 643 observe the client identity sent to the KDC. Transport or network 644 layer security between the client and the server will help prevent 645 tracking of a particular ticket to link a ticket to a user. In 646 addition, clients can limit how often a ticket is reused to minimize 647 ticket linking. 649 The client's real identity is not revealed when the client is 650 authenticated as the anonymous principal. Application servers MAY 651 reject the authentication in order to, for example, prevent 652 information disclosure or as part of Denial of Service (DoS) 653 prevention. Application servers MUST avoid accepting anonymous 654 credentials in situations where they must record the client's 655 identity; for example, when there must be an audit trail. 657 9. Acknowledgements 659 JK Jaganathan helped editing early revisions of this document. 661 Clifford Neuman contributed the core notions of this document. 663 Ken Raeburn reviewed the document and provided suggestions for 664 improvements. 666 Martin Rex wrote the text for GSS-API considerations. 668 Nicolas Williams reviewed the GSS-API considerations section and 669 suggested ideas for improvements. 671 Sam Hartman and Nicolas Williams were great champions of this work. 673 Miguel Garcia and Phillip Hallam-Baker reviewed the document and 674 provided helpful suggestions. 676 In addition, the following individuals made significant 677 contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love 678 Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia. 680 10. IANA Considerations 682 This document defines a new 'anonymous' Kerberos well-known name and 683 a new 'anonymous' Kerberos well-known realm based on [RFC6111]. IANA 684 has added these two values to the Kerberos naming registries that are 685 created in [RFC6111]. 687 11. References 689 11.1. Normative References 691 [ASAX34] American Standards Institute, "American Standard Code for 692 Information Interchange", ASA X3.4-1963, June 1963. 694 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 695 RFC 1964, DOI 10.17487/RFC1964, June 1996, 696 . 698 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 699 Requirement Levels", BCP 14, RFC 2119, 700 DOI 10.17487/RFC2119, March 1997, 701 . 703 [RFC2743] Linn, J., "Generic Security Service Application Program 704 Interface Version 2, Update 1", RFC 2743, 705 DOI 10.17487/RFC2743, January 2000, 706 . 708 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for 709 Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February 710 2005, . 712 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 713 Kerberos Network Authentication Service (V5)", RFC 4120, 714 DOI 10.17487/RFC4120, July 2005, 715 . 717 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial 718 Authentication in Kerberos (PKINIT)", RFC 4556, 719 DOI 10.17487/RFC4556, June 2006, 720 . 722 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 723 RFC 5652, DOI 10.17487/RFC5652, September 2009, 724 . 726 [RFC6111] Zhu, L., "Additional Kerberos Naming Constraints", 727 RFC 6111, April 2011. 729 [X.680] "Abstract Syntax Notation One (ASN.1): Specification of 730 Basic Notation", ITU-T Recommendation X.680: ISO/IEC 731 International Standard 8824-1:1998, 1997. 733 [X.690] "ASN.1 encoding rules: Specification of Basic Encoding 734 Rules (BER), Canonical Encoding Rules (CER) and 735 Distinguished Encoding Rules (DER)", ITU-T Recommendation 736 X.690 ISO/IEC International Standard 8825-1:1998, 1997. 738 11.2. Informative References 740 [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure 741 Channels", RFC 5056, November 2007. 743 [RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for 744 Kerberos Pre-Authentication", RFC 6113, April 2011. 746 [STARTTLS] 747 Josefsson, S., "Using Kerberos V5 over the Transport Layer 748 Security (TLS) protocol", Work in Progress, August 2010. 750 Authors' Addresses 752 Larry Zhu 753 Microsoft Corporation 754 One Microsoft Way 755 Redmond, WA 98052 756 US 758 EMail: larry.zhu@microsoft.com 759 Paul Leach 760 Microsoft Corporation 761 One Microsoft Way 762 Redmond, WA 98052 763 US 765 EMail: paulle@microsoft.com 767 Sam Hartman (editor) 768 Painless Security 770 EMail: hartmans-ietf@mit.edu 772 Shawn Emery (editor) 773 Oracle 775 EMail: shawn.emery@oracle.com