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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 Updates: 4120 (if approved) Microsoft Corporation 5 Intended status: Standards Track July 7, 2007 6 Expires: January 8, 2008 8 Anonymity Support for Kerberos 9 draft-ietf-krb-wg-anon-04 11 Status of this Memo 13 By submitting this Internet-Draft, each author represents that any 14 applicable patent or other IPR claims of which he or she is aware 15 have been or will be disclosed, and any of which he or she becomes 16 aware will be disclosed, in accordance with Section 6 of BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six months 24 and may be updated, replaced, or obsoleted by other documents at any 25 time. It is inappropriate to use Internet-Drafts as reference 26 material or to cite them other than as "work in progress." 28 The list of current Internet-Drafts can be accessed at 29 http://www.ietf.org/ietf/1id-abstracts.txt. 31 The list of Internet-Draft Shadow Directories can be accessed at 32 http://www.ietf.org/shadow.html. 34 This Internet-Draft will expire on January 8, 2008. 36 Copyright Notice 38 Copyright (C) The IETF Trust (2007). 40 Abstract 42 This document defines extensions to the Kerberos protocol for the 43 Kerberos client to authenticate the Kerberos Key Distribution Center 44 and the Kerberos server, without revealing the client's identity. 45 These extensions can be used to secure communication between the 46 anonymous client and the server. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 51 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 52 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 4 54 5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 8 55 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 56 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 57 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 58 9. Normative References . . . . . . . . . . . . . . . . . . . . . 10 59 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 60 Intellectual Property and Copyright Statements . . . . . . . . . . 11 62 1. Introduction 64 In certain situations, the Kerberos [RFC4120] client may wish to 65 authenticate a server and/or protect communications without revealing 66 its own identity. For example, consider an application which 67 provides read access to a research database, and which permits 68 queries by arbitrary requestors. A client of such a service might 69 wish to authenticate the service, to establish trust in the 70 information received from it, but might not wish to disclose its 71 identity to the service for privacy reasons. 73 Extensions to [RFC4120] are specified in this document by which a 74 client can authenticate the Key Distribution Center (KDC) and request 75 an anonymous ticket. The client can use the anonymous ticket to 76 authenticate the server and protect subsequent client-server 77 communications. These extensions provide Kerberos with functional 78 equivalence to Transport Layer Security (TLS) [RFC4346]. 80 By using the extensions defined in this specification, the client may 81 reveal its identity in its initial request to its own KDC, but it can 82 remain anonymous thereafter to KDCs on the cross-realm authentication 83 path, and to the server with which it communicates. 85 2. Conventions Used in This Document 87 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 88 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 89 document are to be interpreted as described in [RFC2119]. 91 3. Definitions 93 The anonymous Kerberos realm name is defined as a well-known realm 94 name based on [KRBNAM]. The value is the literal "WELLKNOWN: 95 ANONYMOUS". An anonymous Kerberos realm name MUST NOT be present in 96 the transited field [RFC4120] of a ticket. 98 The anonymous Kerberos principal name is defined as a well-known 99 Kerberos principal name based on [KRBNAM]. The value of the name- 100 type field [RFC4120] is KRB_NT_WELLKNOWN [KRBNAM], and the value of 101 the name-string field [RFC4120] is a sequence of two KerberosString 102 components: "WELLKNOWN", "ANONYMOUS". 104 Note that in this specification, the anonymous principal name and 105 realm are only applicable to the client in Kerberos messages, the 106 server MUST NOT be anonymous in any Kerberos message. 108 The anonymous ticket flag is defined as bit 14 (with the first bit 109 being bit 0) in the TicketFlags: 111 TicketFlags ::= KerberosFlags 112 -- anonymous(14) 113 -- TicketFlags and KerberosFlags are defined in [RFC4120] 115 An anonymous ticket is a ticket that has all of the following 116 properties: 118 o The cname field [RFC4120] contains the anonymous Kerberos 119 principal name. 121 o The crealm field [RFC4120] contains the client's realm name, or 122 the name of the realm that issued the initial ticket for the 123 client principal, or the anonymous realm name. 125 o The anonymous ticket contains no information that can reveal the 126 client's identity. However the ticket may contain the client 127 realm, intermediate realms on the client's authentication path, 128 and authorization data that may provide information related to the 129 client's identity. For example, an anonymous principal that is 130 identifiable only within a particular group of users can be 131 implemented using authorization data and such authorization data, 132 if included in the anonymous ticket, shall disclose the client's 133 membership of that group. 135 o The anonymous ticket flag is set. 137 The anonymous KDC option is defined as bit 14 (with the first bit 138 being bit 0) in the KDCOptions: 140 KDCOptions ::= KerberosFlags 141 -- anonymous(14) 142 -- KDCOptions and KerberosFlags are defined in [RFC4120] 144 As described in Section 4, the anonymous KDC option is set to request 145 an anonymous ticket. 147 4. Protocol Description 149 In order to request an anonymous ticket, the client sets the 150 anonymous KDC option in an Authentication Exchange (AS) or Ticket 151 Granting Service (TGS) request [RFC4120]. The client can request an 152 anonymous Ticket Granting Ticket (TGT) based on a normal TGT. Unless 153 otherwise specified, the client can obtain an anonymous ticket with 154 the anonymous realm name only by requesting an anonymous ticket in an 155 AS exchange with the client realm set as anonymous in the request. 157 If the client wishes to authenticate the KDC anonymously, it sets the 158 client name as anonymous in the AS exchange and provides a 159 PA_PK_AS_REQ pre-authentication data [RFC4556] where both the 160 signerInfos field and the certificates field of the SignedData 161 [RFC3852] of the PA_PK_AS_REQ are empty. Because the anonymous 162 client does not have an associated asymmetric key pair, the client 163 MUST choose the Diffie-Hellman key agreement method by filling in the 164 Diffie-Hellman domain parameters in the clientPublicValue [RFC4556]. 166 If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is 167 anonymous, or if the client in the AS request is anonymous, the 168 anonymous KDC option MUST be set in the request. Otherwise, the KDC 169 MUST return a KRB-ERROR message with the code KDC_ERR_BADOPTION 170 [RFC4120], and there is no accompanying e-data defined in this 171 document. 173 Upon receiving the AS request with a PA_PK_AS_REQ [RFC4556] from the 174 anonymous client, the KDC processes the request according to Section 175 3.1.2 of [RFC4120]. The KDC skips the checks for the client's 176 signature and the client's public key (such as the verification of 177 the binding between the client's public key and the client name), but 178 performs otherwise-applicable checks, and proceeds as normal 179 according to [RFC4556]. For example, the AS MUST check if the 180 client's Diffie-Hellman domain parameters are acceptable. The 181 Diffie-Hellman key agreement method MUST be used and the reply key is 182 derived according to Section 3.2.3.1 of [RFC4556]. If the 183 clientPublicValue is not present in the request, the KDC MUST return 184 a KRB-ERROR [RFC4120] with the code 185 KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556] and there is no 186 accompanying e-data. If all goes well, an anonymous ticket is 187 generated according to Section 3.1.3 of [RFC4120] and a PA_PK_AS_REP 188 [RFC4556] pre-authentication data is included in the KDC reply 189 according to [RFC4556]. If the KDC does not have an asymmetric key 190 pair, it MAY reply anonymously or reject the authentication attempt. 191 If the KDC replies anonymously, both the signerInfos field and the 192 certificates field of the SignedData [RFC3852] of PA_PK_AS_REP in the 193 reply are empty. The server name in the anonymous KDC reply contains 194 the name of the TGS. 196 Upon receipt of the KDC reply that contains an anonymous ticket and a 197 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then 198 authenticate the KDC based on the KDC's signature in the 199 PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply 200 (the reply is anonymous), the client MUST reject the returned ticket 201 if it cannot authenticate the KDC otherwise. 203 The client can use the client keys to mutually authenticate with the 204 KDC, request an anonymous TGT in the AS request. And in that case, 205 the reply key is selected as normal according to Section 3.1.3 of 206 [RFC4120]. 208 For the TGS exchange, the reply key is selected as normal according 209 to Section 3.3.3 of [RFC4120]. 211 When policy allows, the KDC issues an anonymous ticket. Based on 212 local policy, the client realm in the anonymous ticket can be the 213 anonymous realm name or the realm of the KDC. However, in all cases, 214 the client name and the client realm in the EncKDCRepPart of the 215 reply [RFC4120] MUST match with the corresponding client name and the 216 client realm of the anonymous ticket in the reply. The client MUST 217 use the client name and the client realm returned in the 218 EncKDCRepPart in subsequent message exchanges when using the obtained 219 anonymous ticket. 221 When propagating authorization data in the ticket or in the enc- 222 authorization-data field [RFC4120] of the request, the TGS MUST 223 ensure that the client confidentiality is not violated in the 224 returned anonymous ticket. The TGS MUST process the authorization 225 data recursively according to Section 5.2.6 of [RFC4120] beyond the 226 container levels such that all embedded authorization elements are 227 interpreted. Identity-based authorization data SHOULD NOT be present 228 in an anonymous ticket in that it typically reveals the client's 229 identity. The specification of a new authorization data type MUST 230 specify the processing rules of the authorization data when an 231 anonymous ticket is returned. If there is no processing rule defined 232 for an authorization data element or the authorization data element 233 is unknown, the TGS MUST process it when an anonymous ticket is 234 returned as follows: 236 o If the authorization data element may reveal the client's 237 identity, it MUST be removed unless otherwise specified. 239 o If the authorization data element is intended to restrict the use 240 of the ticket or limit the rights otherwise conveyed in the 241 ticket, it cannot be removed in order to hide the client's 242 identity. In this case, the authentication attempt MUST be 243 rejected, and the KDC MUST return an error message with the code 244 KDC_ERR_POLICY [RFC4120]. There is no accompanying e-data defined 245 in this document. Note this is applicable to both critical and 246 optional authorization data. 248 o If the authorization data element is unknown, the TGS MAY remove 249 it, or transfer it into the returned anonymous ticket, or reject 250 the authentication attempt, based on local policy for that 251 authorization data type unless otherwise specified. If there is 252 no policy defined for a given unknown authorization data type, the 253 authentication MUST be rejected. The error code is KDC_ERR_POLICY 254 when the authentication is rejected. 256 The AD-INITIAL-VERIFIED-CAS authorization data [RFC4556] MAY be 257 removed from an anonymous ticket based on local policy of the TGS. 259 The TGS MUST add the name of the previous realm according to Section 260 3.3.3.2 of [RFC4120]. If the client's realm is the anonymous realm, 261 the abbreviation forms [RFC4120] that build on the preceding name 262 cannot be used at the start of the transited encoding. The null- 263 subfield form (e.g., encoding ending with ",") [RFC4120] could not be 264 used next to the anonymous realm that can potentially be at the 265 beginning where the client realm is filled in. 267 The KDC fills out the authtime field of the anonymous ticket in the 268 reply as follows: If the anonymous ticket is returned in an AS 269 exchange, the authtime field of the ticket contains the request time. 270 If the anonymous ticket is returned in a TGS exchange, the authtime 271 field contains the authtime of the ticket in the PA-TGS-REQ pre- 272 authentication data [RFC4120]. An anonymous ticket can be renewed, 273 and the authtime field of a renewed ticket is the authtime in the 274 anonymous ticket on which the renewed ticket was based. 276 If the client is anonymous and the KDC does not have a key to encrypt 277 the reply (this can happen when, for example, the KDC does not 278 support PKINIT [RFC4556]), the KDC MUST return an error message with 279 the code KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying 280 e-data defined in this document. 282 If a client requires anonymous communication then the client MUST 283 check to make sure that the ticket in the reply is actually anonymous 284 by checking the presence of the anonymous ticket flag. This is 285 because KDCs ignore unknown KDC options. A KDC that does not 286 understand the anonymous KDC option will not return an error, but 287 will instead return a normal ticket. 289 The subsequent client and server communications then proceed as 290 described in [RFC4120]. 292 A server accepting an anonymous service ticket may assume that 293 subsequent requests using the same ticket originate from the same 294 client. Requests with different tickets are likely to originate from 295 different clients. 297 5. GSS-API Implementation Notes 299 At the GSS-API [RFC2743] level, the use of an anonymous principal by 300 the initiator/client requires the initiator/client to assert the 301 "anonymous" flag when calling GSS_Init_Sec_Context(). 303 GSS-API does not know or define "anonymous credentials", so the 304 (printable) name of the anonymous principal will rarely be used by or 305 relevant for the initiator/client. The printable name is relevant 306 for the acceptor/server when performing an authorization decision 307 based on the initiator name that is returned from the acceptor side 308 upon the successful security context establishment. 310 A GSS-API initiator MUST carefully check the resulting context 311 attributes from the initial call to GSS_Init_Sec_Context() when 312 requesting anonymity, because (as in the GSS-API tradition and for 313 backwards compatibility) anonymity is just another optional context 314 attribute. It could be that the mechanism doesn't recognize the 315 attribute at all or that anonymity is not available for some other 316 reasons -- and in that case the initiator must NOT send the initial 317 security context token to the acceptor, because it will likely reveal 318 the initiators identity to the acceptor, something that can rarely be 319 "un-done". 321 GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to 322 represent the anonymous identity. In addition, Section 2.1.1 of 323 [RFC1964] defines the single string representation of a Kerberos 324 principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For 325 the anonymous principals, the name component within the exportable 326 name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm 327 name according to Section 2.1.1 of [RFC1964]. Note that in this 328 specification only the client/initiator can be anonymous. 330 Portable initiators are RECOMMENDED to use default credentials 331 whenever possible, and request anonymity only through the input 332 anon_req_flag [RFC2743] to GSS_Init_Sec_Context(). 334 6. Security Considerations 336 Since KDCs ignore unknown options [RFC4120], a client requiring 337 anonymous communication needs to make sure that the ticket is 338 actually anonymous. This is because a KDC that that does not 339 understand the anonymous option would not return an anonymous ticket. 341 By using the mechanism defined in this specification, the client does 342 not reveal its identity to the server but its identity may be 343 revealed to the KDC of the server principal (when the server 344 principal is in a different realm than that of the client), and any 345 KDC on the cross-realm authentication path. The Kerberos client MUST 346 verify the ticket being used is indeed anonymous before communicating 347 with the server, otherwise the client's identity may be revealed 348 unintentionally. 350 In cases where specific server principals must not have access to the 351 client's identity (for example, an anonymous poll service), the KDC 352 can define server principal specific policy that insure any normal 353 service ticket can NEVER be issued to any of these server principals. 355 If the KDC that issued an anonymous ticket were to maintain records 356 of the association of identities to an anonymous ticket, then someone 357 obtaining such records could breach the anonymity. Additionally, the 358 implementations of most (for now all) KDC's respond to requests at 359 the time that they are received. Traffic analysis on the connection 360 to the KDC will allow an attacker to match client identities to 361 anonymous tickets issued. Because there are plaintext parts of the 362 tickets that are exposed on the wire, such matching by a third party 363 observer is relatively straightforward. 365 7. Acknowledgements 367 JK Jaganathan helped editing early revisions of this document. 369 Clifford Neuman contributed the core notions of this document. 371 Ken Raeburn reviewed the document and provided suggestions for 372 improvements. 374 Martin Rex wrote the text for GSS-API considerations. 376 Nicolas Williams reviewed the GSS-API considerations section and 377 suggested ideas for improvements. 379 Sam Hartman and Nicolas Williams were great champions of this work. 381 In addition, the following individuals made significant 382 contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love 383 Hoernquist Aestrand, and Jeffery Hutzelman. 385 8. IANA Considerations 387 Section 3 defines the anonymous Kerberos name and the anonymous 388 Kerberos realm based on [KRBNAM]. The IANA registry for [KRBNAM] 389 need to be updated to add references to this document. 391 9. Normative References 393 [KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints", 394 draft-ietf-krb-wg-naming, work in progress. 396 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", 397 RFC 1964, June 1996. 399 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 400 Requirement Levels", BCP 14, RFC 2119, March 1997. 402 [RFC2743] Linn, J., "Generic Security Service Application Program 403 Interface Version 2, Update 1", RFC 2743, January 2000. 405 [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", 406 RFC 3852, July 2004. 408 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 409 Kerberos Network Authentication Service (V5)", RFC 4120, 410 July 2005. 412 [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security 413 (TLS) Protocol Version 1.1", RFC 4346, April 2006. 415 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial 416 Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. 418 Authors' Addresses 420 Larry Zhu 421 Microsoft Corporation 422 One Microsoft Way 423 Redmond, WA 98052 424 US 426 Email: lzhu@microsoft.com 428 Paul Leach 429 Microsoft Corporation 430 One Microsoft Way 431 Redmond, WA 98052 432 US 434 Email: paulle@microsoft.com 436 Full Copyright Statement 438 Copyright (C) The IETF Trust (2007). 440 This document is subject to the rights, licenses and restrictions 441 contained in BCP 78, and except as set forth therein, the authors 442 retain all their rights. 444 This document and the information contained herein are provided on an 445 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 446 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 447 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 448 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 449 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 450 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 452 Intellectual Property 454 The IETF takes no position regarding the validity or scope of any 455 Intellectual Property Rights or other rights that might be claimed to 456 pertain to the implementation or use of the technology described in 457 this document or the extent to which any license under such rights 458 might or might not be available; nor does it represent that it has 459 made any independent effort to identify any such rights. 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