4 Network Working Group L. Hornquist Astrand
5 Internet-Draft Stockholm University
6 Expires: September 2, 2006 L. Zhu
11 PK-INIT algorithm agility
12 draft-ietf-krb-wg-pkinit-alg-agility-00
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41 Copyright (C) The Internet Society (2006).
45 The PK-INIT protocol have in several places hard coded crypto
46 algorithms. The protocol specification needs to be updated so it can
47 support negotiation to upgrading to newer versions of crypto
48 algorithms. This document addresses this issue.
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62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
63 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
64 3. paChecksum agility . . . . . . . . . . . . . . . . . . . . . . 5
65 4. CMS Digest Algorithm agility . . . . . . . . . . . . . . . . . 6
66 5. Certificate Signer Algorithm Identifier agility . . . . . . . 7
67 6. octetstring2key function agility . . . . . . . . . . . . . . . 8
68 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
69 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
70 9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
71 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
72 Intellectual Property and Copyright Statements . . . . . . . . . . 12
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118 The Kerberos PK-INIT document contains several hardcoded algorithms
119 that was know designed at design time that they had to be replaced by
120 something else at a later time, this document described how to use
121 other algorithms other then those that are hard-coded.
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172 2. Requirements notation
174 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
175 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
176 document are to be interpreted as described in [RFC2119].
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228 3. paChecksum agility
230 The paChecksum binds the PK-INIT part of the request to main body of
231 the Kerberos request (KDC-REQ-BODY). This is to makes sure an
232 attacker can not change the request from the client to the server.
233 The problem is that paChecksum is hardcoded to use SHA1-1, however,
234 there is a mechaism to provide algorithm agility for the paChecksum
235 within the PK-INIT prototcol. Newer clients can choose not send the
236 paChecksum field, but rather add some new fields after the existing
237 fields, older KDC will send back know failure-code so that newer
238 clients can fall back to the old protocol if local policy allows
241 If the attacker can preserve the checksum in paChecksum, an attacker
242 can, for example, change the KDC-REQ-BODY is to downgrade the
243 encryption types used, expend the expiration time, etc, and then try
244 to brute-force the request.
246 In the Public Key Encryption case of PK-INIT the reply contains a
247 checksum over the whole request in the asChecksum field, in this case
248 the client will detect any modifications to the request. Since the
249 asChecksum is using the associated checksum of the session key
250 encryption type, asChecksum field is algorithm agile.
252 One way to solve this problem is to add the asChecksum to the Diffie-
253 Hellman case reply too, and just ignore the paCheckSum field. The
254 KDC should still not issue tickets that are too weak, since that
255 exposes the problem. This is regardless of the using PK-INIT or not.
257 Questions for wg: Wait for Kerberos Extensions that will solve this
258 problem (ignore the problem for how), or use add asChecksum to DH
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284 4. CMS Digest Algorithm agility
286 The client can tell KDC what the supported CMS types are in the
287 requset packet, but there are no equivalent for KDC to the the client
288 what the digest algorithm are support in an reply.
290 Have KDC send the CMS list of supported encryption types in the
291 e-data field of KRB-ERROR when returning the
292 KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error.
294 DER encoded TS-SD-PARAMETERS specifies supported digest algorithms.
295 The list is in decreasing preference order.
299 TD-SD-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier
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340 5. Certificate Signer Algorithm Identifier agility
342 The KDC can reject a certificate based on the signers hash algorithm
343 with the error KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED, but doesn't tell
344 the client what algorithm are supported.
346 DER encoded TS-DC-PARAMETERS specifies supported certificate digest
347 algorithms. The AllowedAlgorithms is in decreasing preference order.
348 RejectedAlgorithm may be include my the KDC to tell what algorithm
349 was rejected in case the rejected certificate was part of a computed
354 TD-DC-PARAMETERS ::= SEQUENCE {
355 AllowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
356 RejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL
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396 6. octetstring2key function agility
398 The PK-INIT standard uses a home-grown string 2 key function in the
399 DH case. The function uses SHA-1 to mix and stretch the DH shared
402 Describe how the client announces that is supports the new String to
403 key function. Probably by stuffing it into the supportCMSTypes field
406 Use NIST SP 800 56B when its published.
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452 7. Security Considerations
454 This document describes negotiation of checksum types and other
455 cryptographic functions. Most of this negotiation is done
456 unauthenticated with no way to very
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508 8. IANA Considerations
510 No IANA considerations.
512 9. Normative References
514 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
515 Requirement Levels", BCP 14, RFC 2119, March 1997.
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566 Love Hornquist Astrand
575 Microsoft Corporation
580 Email: lzhu@microsoft.com
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