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1 \r
2 \r
3 NETWORK WORKING GROUP L. Zhu\r
4 Internet-Draft P. Leach\r
5 Updates: 4120, 4121 and 4556 Microsoft Corporation\r
6 (if approved) October 8, 2008\r
7 Intended status: Standards Track\r
8 Expires: April 11, 2009\r
9 \r
10 \r
11 Anonymity Support for Kerberos\r
12 draft-ietf-krb-wg-anon-10\r
13 \r
14 Status of this Memo\r
15 \r
16 By submitting this Internet-Draft, each author represents that any\r
17 applicable patent or other IPR claims of which he or she is aware\r
18 have been or will be disclosed, and any of which he or she becomes\r
19 aware will be disclosed, in accordance with Section 6 of BCP 79.\r
20 \r
21 Internet-Drafts are working documents of the Internet Engineering\r
22 Task Force (IETF), its areas, and its working groups. Note that\r
23 other groups may also distribute working documents as Internet-\r
24 Drafts.\r
25 \r
26 Internet-Drafts are draft documents valid for a maximum of six months\r
27 and may be updated, replaced, or obsoleted by other documents at any\r
28 time. It is inappropriate to use Internet-Drafts as reference\r
29 material or to cite them other than as "work in progress."\r
30 \r
31 The list of current Internet-Drafts can be accessed at\r
32 http://www.ietf.org/ietf/1id-abstracts.txt.\r
33 \r
34 The list of Internet-Draft Shadow Directories can be accessed at\r
35 http://www.ietf.org/shadow.html.\r
36 \r
37 This Internet-Draft will expire on April 11, 2009.\r
38 \r
39 Abstract\r
40 \r
41 This document defines extensions to the Kerberos protocol to allow a\r
42 Kerberos client to securely communicate with a Kerberos application\r
43 service without revealing its identity, or without revealing more\r
44 than its Kerberos realm. It also defines extensions which allow a\r
45 Kerberos client to obtain anonymous credentials without revealing its\r
46 identity to the Kerberos Key Distribution Center (KDC). This\r
47 document updates RFC 4120, RFC 4121, and RFC 4556.\r
48 \r
49 \r
50 \r
51 \r
52 \r
53 \r
54 Zhu & Leach Expires April 11, 2009 [Page 1]\r
56 Internet-Draft Kerberos Anonymity Support October 2008\r
57 \r
58 \r
59 Table of Contents\r
60 \r
61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3\r
62 2. Conventions Used in This Document . . . . . . . . . . . . . . 3\r
63 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3\r
64 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5\r
65 4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . . 5\r
66 4.1.1. Anonymous PKINIT . . . . . . . . . . . . . . . . . . . 6\r
67 4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 7\r
68 4.3. Subsequent Exchanges and Protocol Actions Common to AS\r
69 and TGS for Anonymity Support . . . . . . . . . . . . . . 9\r
70 5. Interoperability Requirements . . . . . . . . . . . . . . . . 10\r
71 6. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 10\r
72 7. PKINIT Client Contribution to the Ticket Session Key . . . . . 11\r
73 7.1. Combinging Two protocol Keys . . . . . . . . . . . . . . . 12\r
74 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13\r
75 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13\r
76 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14\r
77 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14\r
78 11.1. Normative References . . . . . . . . . . . . . . . . . . . 14\r
79 11.2. Informative References . . . . . . . . . . . . . . . . . . 15\r
80 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15\r
81 Intellectual Property and Copyright Statements . . . . . . . . . . 16\r
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110 Zhu & Leach Expires April 11, 2009 [Page 2]\r
112 Internet-Draft Kerberos Anonymity Support October 2008\r
113 \r
114 \r
115 1. Introduction\r
116 \r
117 In certain situations, the Kerberos [RFC4120] client may wish to\r
118 authenticate a server and/or protect communications without revealing\r
119 the client's own identity. For example, consider an application\r
120 which provides read access to a research database, and which permits\r
121 queries by arbitrary requestors. A client of such a service might\r
122 wish to authenticate the service, to establish trust in the\r
123 information received from it, but might not wish to disclose the\r
124 client's identity to the service for privacy reasons.\r
125 \r
126 Extensions to Kerberos are specified in this document by which a\r
127 client can authenticate the Key Distribution Center (KDC) and request\r
128 an anonymous ticket. The client can use the anonymous ticket to\r
129 authenticate the server and protect subsequent client-server\r
130 communications.\r
131 \r
132 By using the extensions defined in this specification, the client can\r
133 request an anonymous ticket where the client may reveal the client's\r
134 identity to the client's own KDC, or the client can hide the client's\r
135 identity completely by using anonymous Public Key Cryptography for\r
136 Initial Authentication in Kerberos (PKINIT) as defined in\r
137 Section 4.1. Using the returned anonymous ticket, the client remains\r
138 anonymous in subsequent Kerberos exchanges thereafter to KDCs on the\r
139 cross-realm authentication path, and to the server with which it\r
140 communicates.\r
141 \r
142 In this specification, the client realm in the anonymous ticket is\r
143 the anonymous realm name when anonymous PKINIT is used to obtain the\r
144 ticket. The client realm is the client's real realm name if the\r
145 client is authenticated using the client's long term keys. Note that\r
146 the membership of a realm can imply a member of the community\r
147 represented by the realm.\r
148 \r
149 The interaction with Generic Security Service Application Program\r
150 Interface (GSS-API) is described after the protocol description.\r
151 \r
152 \r
153 2. Conventions Used in This Document\r
154 \r
155 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",\r
156 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this\r
157 document are to be interpreted as described in [RFC2119].\r
158 \r
159 \r
160 3. Definitions\r
161 \r
162 The anonymous Kerberos realm name is defined as a well-known realm\r
163 \r
164 \r
165 \r
166 Zhu & Leach Expires April 11, 2009 [Page 3]\r
168 Internet-Draft Kerberos Anonymity Support October 2008\r
169 \r
170 \r
171 name based on [KRBNAM], and the value of this well-known realm name\r
172 is the literal "WELLKNOWN:ANONYMOUS".\r
173 \r
174 The anonymous Kerberos principal name is defined as a well-known\r
175 Kerberos principal name based on [KRBNAM]. The value of the name-\r
176 type field is KRB_NT_WELLKNOWN [KRBNAM], and the value of the name-\r
177 string field is a sequence of two KerberosString components:\r
178 "WELLKNOWN", "ANONYMOUS".\r
179 \r
180 The anonymous ticket flag is defined as bit 14 (with the first bit\r
181 being bit 0) in the TicketFlags:\r
182 \r
183 TicketFlags ::= KerberosFlags\r
184 -- anonymous(14)\r
185 -- TicketFlags and KerberosFlags are defined in [RFC4120]\r
186 \r
187 This is a new ticket flag that is used to indicate a ticket is an\r
188 anonymous one.\r
189 \r
190 An anonymous ticket is a ticket that has all of the following\r
191 properties:\r
192 \r
193 o The cname field contains the anonymous Kerberos principal name.\r
194 \r
195 o The crealm field contains the client's realm name or the anonymous\r
196 realm name.\r
197 \r
198 o The anonymous ticket contains no information that can reveal the\r
199 client's identity. However the ticket may contain the client\r
200 realm, intermediate realms on the client's authentication path,\r
201 and authorization data that may provide information related to the\r
202 client's identity. For example, an anonymous principal that is\r
203 identifiable only within a particular group of users can be\r
204 implemented using authorization data and such authorization data,\r
205 if included in the anonymous ticket, would disclose the client's\r
206 membership of that group.\r
207 \r
208 o The anonymous ticket flag is set.\r
209 \r
210 The anonymous KDC option is defined as bit 14 (with the first bit\r
211 being bit 0) in the KDCOptions:\r
212 \r
213 KDCOptions ::= KerberosFlags\r
214 -- anonymous(14)\r
215 -- KDCOptions and KerberosFlags are defined in [RFC4120]\r
216 \r
217 As described in Section 4, the anonymous KDC option is set to request\r
218 an anonymous ticket in an Authentication Service (AS) request or an\r
219 \r
220 \r
221 \r
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224 Internet-Draft Kerberos Anonymity Support October 2008\r
225 \r
226 \r
227 Ticket Granting Service (TGS) request.\r
228 \r
229 \r
230 4. Protocol Description\r
231 \r
232 In order to request an anonymous ticket, the client sets the\r
233 anonymous KDC option in an AS request or an TGS request.\r
234 \r
235 The rest of this section is organized as follows: it first describes\r
236 protocol actions specific to AS exchanges, then it describes those of\r
237 TGS exchange. These are then followed by the decription of protocol\r
238 actions common to both AS and TGS and those in subsequent exchanges.\r
239 \r
240 4.1. Anonymity Support in AS Exchange\r
241 \r
242 The client requests an anonymous ticket by setting the anonymous KDC\r
243 option in an AS exchange.\r
244 \r
245 The Kerberos client can use the client's long term keys, or the\r
246 client's X.509 certificates [RFC4556], or any other preauthenication\r
247 data, to authenticate to the KDC and requests an anonymous ticket in\r
248 an AS exchange where the client's identity is known to the KDC.\r
249 \r
250 If the client in the AS request is anonymous, the anonymous KDC\r
251 option MUST be set in the request. Otherwise, the KDC MUST return a\r
252 KRB-ERROR message with the code KDC_ERR_BADOPTION.\r
253 \r
254 If the client is anonymous and the KDC does not have a key to encrypt\r
255 the reply (this can happen when, for example, the KDC does not\r
256 support PKINIT [RFC4556]), the KDC MUST return an error message with\r
257 the code KDC_ERR_NULL_KEY [RFC4120].\r
258 \r
259 When policy allows, the KDC issues an anonymous ticket. If the\r
260 client name in the request is the anonymous principal, the client\r
261 realm (crealm) in the reply is the anonymous realm, otherwise the\r
262 client realm is the realm of the AS. According to [RFC4120] the\r
263 client name and the client realm in the EncTicketPart of the reply\r
264 MUST match with the corresponding client name and the client realm of\r
265 the anonymous ticket in the reply; the client MUST use the client\r
266 name and the client realm returned in the KDC-REP in subsequent\r
267 message exchanges when using the obtained anonymous ticket.\r
268 \r
269 Care MUST be taken by the KDC not to reveal the client's identity in\r
270 the authorization data of the returned ticket when populating the\r
271 authorization data in a returned anonymous ticket.\r
272 \r
273 The AD-INITIAL-VERIFIED-CAS authorization data as defined in\r
274 [RFC4556] contains the issuer name of the client certificate. This\r
275 \r
276 \r
277 \r
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281 \r
282 \r
283 authorization is not applicable and MUST NOT be present in the\r
284 returned anonymous ticket when anonymous PKINIT is used. When the\r
285 client is authenticated (i.e. anonymous PKINIT is not used), if it is\r
286 undesirable to disclose such information about the client's identity,\r
287 the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be removed from\r
288 the returned anonymous ticket.\r
289 \r
290 The client can use the client keys to mutually authenticate with the\r
291 KDC, request an anonymous TGT in the AS request. And in that case,\r
292 the reply key is selected as normal according to Section 3.1.3 of\r
293 [RFC4120].\r
294 \r
295 4.1.1. Anonymous PKINIT\r
296 \r
297 This sub-section defines anonymity PKINIT.\r
298 \r
299 As described earlier in this section, the client can request an\r
300 anonymous ticket by authenticating to the KDC using the client's\r
301 identity; alternatively without revealing the client's identity to\r
302 the KDC, the Kerberos client can request an anonymous ticket as\r
303 follows: the client sets the client name as the anonymous principal\r
304 in the AS exchange and provides a PA_PK_AS_REQ pre-authentication\r
305 data [RFC4556] where both the signerInfos field and the certificates\r
306 field of the SignedData [RFC3852] of the PA_PK_AS_REQ are empty.\r
307 Because the anonymous client does not have an associated asymmetric\r
308 key pair, the client MUST choose the Diffie-Hellman key agreement\r
309 method by filling in the Diffie-Hellman domain parameters in the\r
310 clientPublicValue [RFC4556]. This use of the anonymous client name\r
311 in conjunction with PKINIT is referred to as anonymous PKINIT. If\r
312 anonymous PKINIT is used, the realm name in the returned anonymous\r
313 ticket MUST be the anonymous realm.\r
314 \r
315 Upon receiving the anonymous PKINIT request from the client, the KDC\r
316 processes the request according to Section 3.1.2 of [RFC4120]. The\r
317 KDC skips the checks for the client's signature and the client's\r
318 public key (such as the verification of the binding between the\r
319 client's public key and the client name), but performs otherwise-\r
320 applicable checks, and proceeds as normal according to [RFC4556].\r
321 For example, the AS MUST check if the client's Diffie-Hellman domain\r
322 parameters are acceptable. The Diffie-Hellman key agreement method\r
323 MUST be used and the reply key is derived according to Section\r
324 3.2.3.1 of [RFC4556]. If the clientPublicValue is not present in the\r
325 request, the KDC MUST return a KRB-ERROR with the code\r
326 KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556]. If all goes\r
327 well, an anonymous ticket is generated according to Section 3.1.3 of\r
328 [RFC4120] and a PA_PK_AS_REP [RFC4556] pre-authentication data is\r
329 included in the KDC reply according to [RFC4556]. If the KDC does\r
330 not have an asymmetric key pair, it MAY reply anonymously or reject\r
331 \r
332 \r
333 \r
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337 \r
338 \r
339 the authentication attempt. If the KDC replies anonymously, both the\r
340 signerInfos field and the certificates field of the SignedData\r
341 [RFC3852] of PA_PK_AS_REP in the reply are empty. The server name in\r
342 the anonymous KDC reply contains the name of the TGS.\r
343 \r
344 Upon receipt of the KDC reply that contains an anonymous ticket and a\r
345 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then\r
346 authenticate the KDC based on the KDC's signature in the\r
347 PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply\r
348 (the reply is anonymous), the client MUST reject the returned ticket\r
349 if it cannot authenticate the KDC otherwise.\r
350 \r
351 A KDC that supports anonymous PKINIT MUST indicate the support of\r
352 PKINIT according to Section 3.4 of [RFC4556].\r
353 \r
354 Note that in order to obtain an anonymous ticket with the anonymous\r
355 realm name, the client MUST set the client name as the anonymous\r
356 principal in the request when requesting an anonymous ticket in an AS\r
357 exchange. Anonymity PKINIT is the only way via which an anonymous\r
358 ticket with the anonymous realm as the client realm can be generated\r
359 in this specification.\r
360 \r
361 4.2. Anonymity Support in TGS Exchange\r
362 \r
363 The client requests an anonymous ticket by setting the anonymous KDC\r
364 option in a TGS exchange, and in that request the client can use a\r
365 normal Ticket Granting Ticket (TGT) with the client's identity, or an\r
366 anonymous TGT, or an anonymous cross realm TGT. If the client uses a\r
367 normal TGT, the client's identity is known to the TGS.\r
368 \r
369 Note that the client can completely hide the client's identity in an\r
370 AS exchange using anonymous PKINIT as described in the previous\r
371 section.\r
372 \r
373 If the ticket in the PA-TGS-REQ of the TGS request is an anonymous\r
374 one, the anonymous KDC option MUST be set in the request. Otherwise,\r
375 the KDC MUST return a KRB-ERROR message with the code\r
376 KDC_ERR_BADOPTION.\r
377 \r
378 When policy allows, the KDC issues an anonymous ticket. If the\r
379 ticket in the TGS request is an anonymous one, the client name and\r
380 the client realm are copied from that ticket; otherwise the ticket in\r
381 the TGS request is a normal ticket, the returned anonymous ticket\r
382 contains the client name as the anonymous principal and the client\r
383 realm as the true realm of the client. In all cases, according to\r
384 [RFC4120] the client name and the client realm in the EncTicketPart\r
385 of the reply MUST match with the corresponding client name and the\r
386 client realm of the anonymous ticket in the reply; the client MUST\r
387 \r
388 \r
389 \r
390 Zhu & Leach Expires April 11, 2009 [Page 7]\r
392 Internet-Draft Kerberos Anonymity Support October 2008\r
393 \r
394 \r
395 use the client name and the client realm returned in the KDC-REP in\r
396 subsequent message exchanges when using the obtained anonymous\r
397 ticket.\r
398 \r
399 Care MUST be taken by the TGS not to reveal the client's identity in\r
400 the authorization data of the returned ticket. When propagating\r
401 authorization data in the ticket or in the enc-authorization-data\r
402 field of the request, the TGS MUST ensure that the client\r
403 confidentiality is not violated in the returned anonymous ticket.\r
404 The TGS MUST process the authorization data recursively according to\r
405 Section 5.2.6 of [RFC4120] beyond the container levels such that all\r
406 embedded authorization elements are interpreted. The TGS SHOULD NOT\r
407 populate identity-based authorization data into an anonymous ticket\r
408 in that such authorization data typically reveals the client's\r
409 identity. The specification of a new authorization data type MUST\r
410 specify the processing rules of the authorization data when an\r
411 anonymous ticket is returned. If there is no processing rule defined\r
412 for an authorization data element or the authorization data element\r
413 is unknown, the TGS MUST process it when an anonymous ticket is\r
414 returned as follows:\r
415 \r
416 o If the authorization data element may reveal the client's\r
417 identity, it MUST be removed unless otherwise specified.\r
418 \r
419 o If the authorization data element, that could reveal's the\r
420 client's identity. is intended to restrict the use of the ticket\r
421 or limit the rights otherwise conveyed in the ticket, it cannot be\r
422 removed in order to hide the client's identity. In this case, the\r
423 authentication attempt MUST be rejected, and the TGS MUST return\r
424 an error message with the code KDC_ERR_POLICY. Note this is\r
425 applicable to both critical and optional authorization data.\r
426 \r
427 o If the authorization data element is unknown, the TGS MAY remove\r
428 it, or transfer it into the returned anonymous ticket, or reject\r
429 the authentication attempt, based on local policy for that\r
430 authorization data type unless otherwise specified. If there is\r
431 no policy defined for a given unknown authorization data type, the\r
432 authentication MUST be rejected. The error code is KDC_ERR_POLICY\r
433 when the authentication is rejected.\r
434 \r
435 The AD-INITIAL-VERIFIED-CAS authorization data as defined in\r
436 [RFC4556] contains the issuer name of the client certificate. If it\r
437 is undesirable to disclose such information about the client's\r
438 identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be\r
439 removed from an anonymous ticket.\r
440 \r
441 The TGS encodes the name of the previous realm into the transited\r
442 field according to Section 3.3.3.2 of [RFC4120]. Based on local\r
443 \r
444 \r
445 \r
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449 \r
450 \r
451 policy, the TGS MAY omit the previous realm if the cross realm TGT is\r
452 an anonymous one in order to hide the authentication path of the\r
453 client. The unordered set of realms in the transited field, if\r
454 present, can reveal which realm may potentially be the realm of the\r
455 client or the realm that issued the anonymous TGT. The anonymous\r
456 Kerberos realm name MUST NOT be present in the transited field of a\r
457 ticket. The true name of the realm that issued the anonymous ticket\r
458 MAY be present in the transited field of a ticket.\r
459 \r
460 4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for\r
461 Anonymity Support\r
462 \r
463 In both AS and TGS exchanges, the realm field in the KDC request is\r
464 always the realm of the target KDC, not the anonymous realm when the\r
465 client requests an anonymous ticket.\r
466 \r
467 Absent other information the KDC MUST NOT include any identifier in\r
468 the returned anonymous ticket that could reveal the client's identity\r
469 to the server.\r
470 \r
471 Unless anonymous PKINIT is used, if a client requires anonymous\r
472 communication then the client MUST check to make sure that the ticket\r
473 in the reply is actually anonymous by checking the presence of the\r
474 anonymous ticket flag in the flags field of the EncKDCRepPart. This\r
475 is because KDCs ignore unknown KDC options. A KDC that does not\r
476 understand the anonymous KDC option will not return an error, but\r
477 will instead return a normal ticket.\r
478 \r
479 The subsequent client and server communications then proceed as\r
480 described in [RFC4120].\r
481 \r
482 Note that the anonymous principal name and realm are only applicable\r
483 to the client in Kerberos messages, the server cannot be anonymous in\r
484 any Kerberos message per this specification.\r
485 \r
486 A server accepting an anonymous service ticket may assume that\r
487 subsequent requests using the same ticket originate from the same\r
488 client. Requests with different tickets are likely to originate from\r
489 different clients.\r
490 \r
491 Upon receipt of an anonymous ticket, the transited policy check is\r
492 preformed in the same way as that of a normal ticket if the client's\r
493 realm is not the anonymous realm; if the client realm is the\r
494 anonymous realm, absent other information any realm in the\r
495 authentication path is allowed by the cross-realm policy check.\r
496 \r
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506 \r
507 5. Interoperability Requirements\r
508 \r
509 Conforming implementations MUST support the anonymous principal with\r
510 a non-anonymous realm, and they MAY support the anonymous principal\r
511 with the anonymous realm using anonymous PKINIT.\r
512 \r
513 \r
514 6. GSS-API Implementation Notes\r
515 \r
516 GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to\r
517 represent the anonymous identity. In addition, Section 2.1.1 of\r
518 [RFC1964] defines the single string representation of a Kerberos\r
519 principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The\r
520 anonymous principal with the anonymous realm corresponds to the GSS-\r
521 API anonymous principal. A principal with the anonymous principal\r
522 name and a non-anonymous realm is an authenticated principal, hence\r
523 such a principal does not correspond to the anonymous principal in\r
524 GSS-API with the GSS_C_NT_ANONYMOUS name type. The [RFC1964] name\r
525 syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the\r
526 anonymous principal name with a non-anonymous realm name and for\r
527 displaying and exporting these names.\r
528 \r
529 At the GSS-API [RFC2743] level, an initiator/client requests the use\r
530 of an anonymous principal with the anonymous realm by asserting the\r
531 "anonymous" flag when calling GSS_Init_Sec_Context(). The GSS-API\r
532 implementation MAY provide implementation-specific means for\r
533 requesting the use of an anonymous principal with a non-anonymous\r
534 realm.\r
535 \r
536 GSS-API does not know or define "anonymous credentials", so the\r
537 (printable) name of the anonymous principal will rarely be used by or\r
538 relevant for the initiator/client. The printable name is relevant\r
539 for the acceptor/server when performing an authorization decision\r
540 based on the initiator name that is returned from the acceptor side\r
541 upon the successful security context establishment.\r
542 \r
543 A GSS-API initiator MUST carefully check the resulting context\r
544 attributes from the initial call to GSS_Init_Sec_Context() when\r
545 requesting anonymity, because (as in the GSS-API tradition and for\r
546 backwards compatibility) anonymity is just another optional context\r
547 attribute. It could be that the mechanism doesn't recognize the\r
548 attribute at all or that anonymity is not available for some other\r
549 reasons -- and in that case the initiator MUST NOT send the initial\r
550 security context token to the acceptor, because it will likely reveal\r
551 the initiators identity to the acceptor, something that can rarely be\r
552 "un-done".\r
553 \r
554 Portable initiators are RECOMMENDED to use default credentials\r
555 \r
556 \r
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561 \r
562 \r
563 whenever possible, and request anonymity only through the input\r
564 anon_req_flag [RFC2743] to GSS_Init_Sec_Context().\r
565 \r
566 \r
567 7. PKINIT Client Contribution to the Ticket Session Key\r
568 \r
569 The definition in this section was motivated by protocol analysis of\r
570 anonymous PKINIT (defined in this document) in building tunneling\r
571 channels [FAST] and subsequent channel bindings. In order to enable\r
572 applications of anonymous PKINIT to form channels, all\r
573 implementations of anonymous PKINIT need to meet the requirements of\r
574 this section. There is otherwise no connection to the rest of this\r
575 document.\r
576 \r
577 PKINIT is useful for constructing tunneling channels. To ensure that\r
578 an attacker cannot create a channel with a given name, it is\r
579 desirable that neither the KDC nor the client can unilaterally\r
580 determine the ticket session key. To achieve that end, a KDC\r
581 conforming to this definition MUST encrypt a randomly generated key,\r
582 called the KDC contribution key, in the PA_PKINIT_KX padata (defined\r
583 next in this section). The KDC contribution key is then combined\r
584 with the reply key to form the ticket session key of the returned\r
585 ticket. These two keys are then combined using the KRB-FX-CF2\r
586 operation defined in Section 7.1, where K1 is the KDC contribution\r
587 key, K2 is the reply key, the input pepper1 is American Standard Code\r
588 for Information Interchange (ASCII) [ASAX34] string "PKINIT", and the\r
589 input pepper2 is ASCII string "KeyExchange".\r
590 \r
591 PA_PKINIT_KX 135\r
592 -- padata for PKINIT that contains an encrypted\r
593 -- KDC contribution key.\r
594 \r
595 PA-PKINIT-KX ::= EncryptedData -- EncryptionKey\r
596 -- Contains an encrypted key randomly\r
597 -- generated by the KDC (known as the KDC contribution key).\r
598 -- Both EncryptedData and EncryptionKey are defined in [RFC4120]\r
599 \r
600 The PA_PKINIT_KX padata MUST be included in the KDC reply when\r
601 anonymous PKINIT is used; it SHOULD be included if PKINIT is used\r
602 with the Diffie-Helleman key exchange but the client is not\r
603 anonymous; it MUST NOT be included otherwise (e.g. when PKINIT is\r
604 used with the public key encryption as the key exchange).\r
605 \r
606 The padata-value field of the PA-PKINIT-KX type padata contains the\r
607 DER [X680] [X690] encoding of the Abstract Syntax Notation One\r
608 (ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is a\r
609 EncryptedData. The clear text data being encrypted is the DER\r
610 encoded Kerberos session key randomly generated by the KDC. The\r
611 \r
612 \r
613 \r
614 Zhu & Leach Expires April 11, 2009 [Page 11]\r
616 Internet-Draft Kerberos Anonymity Support October 2008\r
617 \r
618 \r
619 encryption key is the reply key and the key usage number is\r
620 KEY_USAGE_PA_PKINIT_KX (44).\r
621 \r
622 The client then decrypts the KDC contribution key and verifies the\r
623 ticket session key in the returned ticket is the combined key of the\r
624 KDC contribution key and the reply key as described above. A\r
625 conforming client MUST reject anonymous PKINIT authentication if the\r
626 PA_PKINIT_KX padata is not present in the KDC reply or if the ticket\r
627 session key of the returned ticket is not the combined key of the KDC\r
628 contribution key and the reply key when PA-PKINIT-KX is present in\r
629 the KDC reply.\r
630 \r
631 7.1. Combinging Two protocol Keys\r
632 \r
633 KRB-FX-CF2() combines two protocol keys based on the pseudo-random()\r
634 function defined in [RFC3961].\r
635 \r
636 Given two input keys, K1 and K2, where K1 and K2 can be of two\r
637 different enctypes, the output key of KRB-FX-CF2(), K3, is derived as\r
638 follows:\r
639 \r
640 KRB-FX-CF2(protocol key, protocol key, octet string,\r
641 octet string) -> (protocol key)\r
642 \r
643 PRF+(K1, pepper1) -> octet-string-1\r
644 PRF+(K2, pepper2) -> octet-string-2\r
645 KRB-FX-CF2(K1, K2, pepper1, pepper2) ->\r
646 random-to-key(octet-string-1 ^ octet-string-2)\r
647 \r
648 Where ^ denotes the exclusive-OR operation. PRF+() is defined as\r
649 follows:\r
650 \r
651 PRF+(protocol key, octet string) -> (octet string)\r
652 \r
653 PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) ||\r
654 pseudo-random( key, 2 || shared-info ) ||\r
655 pseudo-random( key, 3 || shared-info ) || ...\r
656 \r
657 Here the counter value 1, 2, 3 and so on are encoded as a one-octet\r
658 integer. The pseudo-random() operation is specified by the enctype\r
659 of the protocol key. PRF+() uses the counter to generate enough bits\r
660 as needed by the random-to-key() [RFC3961] function for the\r
661 encryption type specified for the resulting key; unneeded bits are\r
662 removed from the tail.\r
663 \r
664 \r
665 \r
666 \r
667 \r
668 \r
669 \r
670 Zhu & Leach Expires April 11, 2009 [Page 12]\r
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673 \r
674 \r
675 8. Security Considerations\r
676 \r
677 Since KDCs ignore unknown options, a client requiring anonymous\r
678 communication needs to make sure that the returned ticket is actually\r
679 anonymous. This is because a KDC that that does not understand the\r
680 anonymous option would not return an anonymous ticket.\r
681 \r
682 By using the mechanism defined in this specification, the client does\r
683 not reveal the client's identity to the server but the client\r
684 identity may be revealed to the KDC of the server principal (when the\r
685 server principal is in a different realm than that of the client),\r
686 and any KDC on the cross-realm authentication path. The Kerberos\r
687 client MUST verify the ticket being used is indeed anonymous before\r
688 communicating with the server, otherwise the client's identity may be\r
689 revealed unintentionally.\r
690 \r
691 In cases where specific server principals must not have access to the\r
692 client's identity (for example, an anonymous poll service), the KDC\r
693 can define server principal specific policy that insure any normal\r
694 service ticket can NEVER be issued to any of these server principals.\r
695 \r
696 If the KDC that issued an anonymous ticket were to maintain records\r
697 of the association of identities to an anonymous ticket, then someone\r
698 obtaining such records could breach the anonymity. Additionally, the\r
699 implementations of most (for now all) KDC's respond to requests at\r
700 the time that they are received. Traffic analysis on the connection\r
701 to the KDC will allow an attacker to match client identities to\r
702 anonymous tickets issued. Because there are plaintext parts of the\r
703 tickets that are exposed on the wire, such matching by a third party\r
704 observer is relatively straightforward. A service that is\r
705 authenticated by the anonymous principals may be able to infer the\r
706 identity of the client by examining and linking quasi-static protocol\r
707 information such as the IP address from which a request is received,\r
708 or by linking multiple uses of the same anonymous ticket.\r
709 \r
710 The client's real identity is not revealed when the client is\r
711 authenticated as the anonymous principal. Application servers MAY\r
712 reject the authentication in order to, for example, prevent\r
713 information disclosure or as part of Denial of Service (DOS)\r
714 prevention. Application servers MUST avoid accepting anonymous\r
715 credentials in situations where they must record the client's\r
716 identity; for example, when there must be an audit trail.\r
717 \r
718 \r
719 9. Acknowledgements\r
720 \r
721 JK Jaganathan helped editing early revisions of this document.\r
722 \r
723 \r
724 \r
725 \r
726 Zhu & Leach Expires April 11, 2009 [Page 13]\r
728 Internet-Draft Kerberos Anonymity Support October 2008\r
729 \r
730 \r
731 Clifford Neuman contributed the core notions of this document.\r
732 \r
733 Ken Raeburn reviewed the document and provided suggestions for\r
734 improvements.\r
735 \r
736 Martin Rex wrote the text for GSS-API considerations.\r
737 \r
738 Nicolas Williams reviewed the GSS-API considerations section and\r
739 suggested ideas for improvements.\r
740 \r
741 Sam Hartman and Nicolas Williams were great champions of this work.\r
742 \r
743 Miguel Garcia and Phillip Hallam-Baker reviewed the document and\r
744 provided helpful suggestions.\r
745 \r
746 In addition, the following individuals made significant\r
747 contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love\r
748 Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.\r
749 \r
750 \r
751 10. IANA Considerations\r
752 \r
753 This document defines a new 'anonymous' Kerberos well-known name and\r
754 a new 'anonymous' Kerberos well-known realm based on [KRBNAM]. IANA\r
755 is requested to add these two values to the Kerberos naming\r
756 registries that are created in [KRBNAM].\r
757 \r
758 \r
759 11. References\r
760 \r
761 11.1. Normative References\r
762 \r
763 [ASAX34] American Standard Code for Information Interchange, \r
764 ASA X3.4-1963, American Standards Association, June 17, \r
765 1963.\r
766 \r
767 [KRBNAM] Zhu, L., "Additional Kerberos Naming Constraints",\r
768 draft-ietf-krb-wg-naming (work in progress), 2008.\r
769 \r
770 [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",\r
771 RFC 1964, June 1996.\r
772 \r
773 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate\r
774 Requirement Levels", BCP 14, RFC 2119, March 1997.\r
775 \r
776 [RFC2743] Linn, J., "Generic Security Service Application Program\r
777 Interface Version 2, Update 1", RFC 2743, January 2000.\r
778 \r
779 [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",\r
780 RFC 3852, July 2004.\r
781 \r
782 [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for\r
783 \r
784 \r
785 \r
786 Zhu & Leach Expires April 11, 2009 [Page 14]\r
788 Internet-Draft Kerberos Anonymity Support October 2008\r
789 \r
790 \r
791 Kerberos 5", RFC 3961, February 2005.\r
792 \r
793 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The\r
794 Kerberos Network Authentication Service (V5)", RFC 4120,\r
795 July 2005.\r
796 \r
797 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial\r
798 Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.\r
799 \r
800 \r
801 [X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,\r
802 Information technology - Abstract Syntax Notation One\r
803 (ASN.1): Specification of basic notation.\r
804 \r
805 [X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,\r
806 Information technology - ASN.1 encoding Rules:\r
807 Specification of Basic Encoding Rules (BER), Canonical\r
808 Encoding Rules (CER) and Distinguished Encoding Rules\r
809 (DER). \r
810 \r
811 11.2. Informative References\r
812 \r
813 [FAST] Zhu, L. and S. Hartman, "A Generalized Framework for\r
814 Kerberos Pre-Authentication",\r
815 draft-ietf-krb-wg-preauth-framework (work in progress),\r
816 2008.\r
817 \r
818 \r
819 Authors' Addresses\r
820 \r
821 Larry Zhu\r
822 Microsoft Corporation\r
823 One Microsoft Way\r
824 Redmond, WA 98052\r
825 US\r
826 \r
827 Email: lzhu@microsoft.com\r
828 \r
829 \r
830 Paul Leach\r
831 Microsoft Corporation\r
832 One Microsoft Way\r
833 Redmond, WA 98052\r
834 US\r
835 \r
836 Email: paulle@microsoft.com\r
837 \r
838 \r
839 \r
840 \r
841 \r
842 \r
843 \r
844 \r
845 \r
846 \r
847 \r
848 \r
849 \r
850 \r
851 \r
852 \r
853 Zhu & Leach Expires April 11, 2009 [Page 15]\r
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856 \r
857 \r
858 Full Copyright Statement\r
859 \r
860 Copyright (C) The IETF Trust (2008).\r
861 \r
862 This document is subject to the rights, licenses and restrictions\r
863 contained in BCP 78, and except as set forth therein, the authors\r
864 retain all their rights.\r
865 \r
866 This document and the information contained herein are provided on an\r
867 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS\r
868 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND\r
869 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS\r
870 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF\r
871 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED\r
872 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.\r
873 \r
874 \r
875 Intellectual Property\r
876 \r
877 The IETF takes no position regarding the validity or scope of any\r
878 Intellectual Property Rights or other rights that might be claimed to\r
879 pertain to the implementation or use of the technology described in\r
880 this document or the extent to which any license under such rights\r
881 might or might not be available; nor does it represent that it has\r
882 made any independent effort to identify any such rights. Information\r
883 on the procedures with respect to rights in RFC documents can be\r
884 found in BCP 78 and BCP 79.\r
885 \r
886 Copies of IPR disclosures made to the IETF Secretariat and any\r
887 assurances of licenses to be made available, or the result of an\r
888 attempt made to obtain a general license or permission for the use of\r
889 such proprietary rights by implementers or users of this\r
890 specification can be obtained from the IETF on-line IPR repository at\r
891 http://www.ietf.org/ipr.\r
892 \r
893 The IETF invites any interested party to bring to its attention any\r
894 copyrights, patents or patent applications, or other proprietary\r
895 rights that may cover technology that may be required to implement\r
896 this standard. Please address the information to the IETF at\r
897 ietf-ipr@ietf.org.\r
898 \r
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