Network Working Group L. Hornquist Astrand Internet-Draft Stockholm University Expires: April 26, 2007 L. Zhu Microsoft Corporation October 23, 2006 PK-INIT algorithm agility draft-ietf-krb-wg-pkinit-alg-agility-01 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 26, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract The PK-INIT protocol have in several places hard coded crypto algorithms. The protocol specification needs to be updated so it can support negotiation to upgrading to newer versions of crypto algorithms. This document addresses this issue. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 1] Internet-Draft PK-INIT algorithm agility October 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 3. paChecksum agility . . . . . . . . . . . . . . . . . . . . . . 5 4. CMS Digest Algorithm agility . . . . . . . . . . . . . . . . . 6 5. Certificate Signer Algorithm Identifier agility . . . . . . . 7 6. octetstring2key function agility . . . . . . . . . . . . . . . 8 7. Security Considerations . . . . . . . . . . . . . . . . . . . 10 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 9. Normative References . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12 Intellectual Property and Copyright Statements . . . . . . . . . . 13 Hornquist Astrand & Zhu Expires April 26, 2007 [Page 2] Internet-Draft PK-INIT algorithm agility October 2006 1. Introduction The Kerberos PK-INIT document contains several hardcoded algorithms that was know designed at design time that they had to be replaced by something else at a later time, this document described how to use other algorithms other then those that are hard-coded. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 3] Internet-Draft PK-INIT algorithm agility October 2006 2. Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 4] Internet-Draft PK-INIT algorithm agility October 2006 3. paChecksum agility The paChecksum binds the PK-INIT part of the request to main body of the Kerberos request (KDC-REQ-BODY). This is to makes sure an attacker can not change the request from the client to the server. The problem is that paChecksum is hardcoded to use SHA1-1, however, there is a mechaism to provide algorithm agility for the paChecksum within the PK-INIT prototcol. Newer clients can choose not send the paChecksum field, but rather add some new fields after the existing fields, older KDC will send back know failure-code so that newer clients can fall back to the old protocol if local policy allows that. If the attacker can preserve the checksum in paChecksum, an attacker can, for example, change the KDC-REQ-BODY is to downgrade the encryption types used, expend the expiration time, etc, and then try to brute-force the request. In the Public Key Encryption case of PK-INIT the reply contains a checksum over the whole request in the asChecksum field, in this case the client will detect any modifications to the request. Since the asChecksum is using the associated checksum of the session key encryption type, asChecksum field is algorithm agile. One way to solve this problem is to add the asChecksum to the Diffie- Hellman case reply too, and just ignore the paCheckSum field. The KDC should still not issue tickets that are too weak, since that exposes the problem. This is regardless of the using PK-INIT or not. Questions for wg: Wait for Kerberos Extensions that will solve this problem (ignore the problem for how), or use add asChecksum to DH case. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 5] Internet-Draft PK-INIT algorithm agility October 2006 4. CMS Digest Algorithm agility The client can tell KDC what the supported CMS types are in the requset packet, but there are no equivalent for KDC to the the client what the digest algorithm are support in an reply. Have KDC send the CMS list of supported encryption types in the e-data field of KRB-ERROR when returning the KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error. DER encoded TS-SD-PARAMETERS specifies supported digest algorithms. The list is in decreasing preference order. TD-SD-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier Hornquist Astrand & Zhu Expires April 26, 2007 [Page 6] Internet-Draft PK-INIT algorithm agility October 2006 5. Certificate Signer Algorithm Identifier agility The KDC can reject a certificate based on the signers hash algorithm with the error KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED, but doesn't tell the client what algorithm are supported. DER encoded TS-DC-PARAMETERS specifies supported certificate digest algorithms. The AllowedAlgorithms is in decreasing preference order. RejectedAlgorithm may be include my the KDC to tell what algorithm was rejected in case the rejected certificate was part of a computed chain. TD-DC-PARAMETERS ::= SEQUENCE { AllowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier, RejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL } Hornquist Astrand & Zhu Expires April 26, 2007 [Page 7] Internet-Draft PK-INIT algorithm agility October 2006 6. octetstring2key function agility The PK-INIT standard uses a home-grown string to key function to convert the DH key to a kerberos protocol key. The function uses SHA-1 to mix and stretch the DH shared key. This is replaced by the specification defined in [SP80056A] to convert a DH key to a symmetric key. The client annouces it supports by inserting the id-Pkinit-SP800-56A oid into the supportedCMSTypes in the request packet. The KDC confirms this by setting the string2keyAlg in the updated DHRepInfo. The replacement string to key function is using [draft-dang-nistkdf] ASN.1 specification. SV is the concatenation of SVLen and SVData. Where SVLen is a 4 byte big endian encoded length of SVData and SVData is the DH shared secret Z. OtherInfo is DER encoded and filled out as follows. algorithmID is set to id-PkInit-S2kFunction. The two fields partyUInfo and partyVInfo is filled with DER encoded KRB5PrincipalName using the client and the KDC principal. suppPubInfo is filled by the DER encode pKinitSuppPubInfo. The enctype field is set to the enctype that string to key function is supposed to produce. The clientDHNonce and serverDHNonce elements of pKinitSuppPubInfo is filled in with clientDHNonce and serverDHNonce from th PK-INIT request if they where sent by the client or KDC. The field suppPrivInfo is not used. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 8] Internet-Draft PK-INIT algorithm agility October 2006 id-Pkinit-SP800-56A ::= OBJECT IDENTIFIER { ... TBD ... } id-PkInit-S2kFunction ::= OBJECT IDENTIFIER { ... TBD ... } pKinitSuppPubInfo ::= SEQUENCE { enctype [0] ENCTYPE, clientDHNonce [1] OCTET STRING OPTIONAL serverDHNonce [2] OCTET STRING OPTIONAL } OtherInfo ::= SEQUENCE { algorithmID AlgorithmIdentifier, partyUInfo [0] OCTET STRING CONTAINS (KRB5PrincipalName), partyVInfo [1] OCTET STRING CONTAINS (KRB5PrincipalName), suppPubInfo [2] OCTET STRING OPTIONAL, suppPrivInfo [3] OCTET STRING OPTIONAL } DHRepInfo ::= SEQUENCE { dhSignedData [0] IMPLICIT OCTET STRING, serverDHNonce [1] DHNonce OPTIONAL, ..., string2keyAlg [2] AlgorithmIdentifier } Hornquist Astrand & Zhu Expires April 26, 2007 [Page 9] Internet-Draft PK-INIT algorithm agility October 2006 7. Security Considerations This document describes negotiation of checksum types and other cryptographic functions. Most of this negotiation is done unauthenticated so care needs to be taken to accect resonable values. Implemetion must make it possible to change the what is resonable over time. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 10] Internet-Draft PK-INIT algorithm agility October 2006 8. IANA Considerations No IANA considerations. 9. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [SP80056A] Barker, E., Don, D., and M. Smid, "Recommendation for Pair-Wise Key Establishment Schemes Using Discrete Logarithm CryptographyMarch", March 2006. [draft-dang-nistkdf] Dang, Q. and P. Polk, "Hash-Based Key Derivation (HKD)", June 2006. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 11] Internet-Draft PK-INIT algorithm agility October 2006 Authors' Addresses Love Hornquist Astrand Stockholm University SE-106 91 STOCKHOLM SWEDEN Email: lha@it.su.se Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: lzhu@microsoft.com Hornquist Astrand & Zhu Expires April 26, 2007 [Page 12] Internet-Draft PK-INIT algorithm agility October 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Hornquist Astrand & Zhu Expires April 26, 2007 [Page 13]