NETWORK WORKING GROUP L. Zhu Internet-Draft P. Leach Expires: October 2, 2005 K. Jaganathan Microsoft Corporation March 31, 2005 Kerberos Cryptosystem Negotiation Extension draft-zhu-kerb-enctype-nego-01 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with RFC 3668. 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 October 2, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document specifies an extension by Kerberos to negotiate new encryption types between the client-server peers. Zhu, et al. Expires October 2, 2005 [Page 1] Internet-Draft Enctype Negotiation March 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 3. Negotiation Extension . . . . . . . . . . . . . . . . . . . . 3 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. Normative References . . . . . . . . . . . . . . . . . . . . . 4 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 5 A. Leveraging this Enctype Negotiation in Windows SPNEGO Implementations . . . . . . . . . . . . . . . . . . . . . . . 5 Intellectual Property and Copyright Statements . . . . . . . . 7 Zhu, et al. Expires October 2, 2005 [Page 2] Internet-Draft Enctype Negotiation March 2005 1. Introduction Under the current mechanism [CLAR], the KDC must limit the ticket session key enctype chosen for a given server to one it believes is supported by both the client and the server. If both the client and server understand a stronger enctype than the one selected by the KDC, they can not negotiate it. As the result, the protection of application traffic is often weaker than necessary when the server can support different sets of enctypes depending on the server application software being used. This document specifies an extension to Kerberos to allow clients and servers to negotiate a different and possible stronger cryptosystem to be used in subsequent communication. This extension utilizes an authorization data element in the authenticator of the AP-REQ message [CLAR]. The client sends the list of enctypes that it supports to the server, the server then informs the client its choice. The negotiated subkey is sent in the AP-REP message [CLAR]. 2. Conventions Used in This Document 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]. 3. Negotiation Extension If the client prefers an enctype over that of the service ticket session key, then it MUST send the list of enctypes it supports (including the one selected by the KDC) in decreasing preference order. The client sends the enctype list via the authorization-data of the authenticator in the AP-REQ [CLAR]. A new authorization data element type AD-ETYPE-NEGOTIATION (129) is defined. This authorization data element itself is enclosed in the AD-IF-RELEVANT container, thus a correctly implemented server that does not understand this element should ignore it [CLAR]. The value of this authorization element contains the DER [X60] encoding of the following ASN.1 type: EtypeList ::= SEQUENCE OF Int32 -- Specifies the enctypes supported by the client. -- This enctype list is in decreasing preference order -- (favorite choice first). -- Int32 is defined in [CLAR]. Zhu, et al. Expires October 2, 2005 [Page 3] Internet-Draft Enctype Negotiation March 2005 If the EtypeList is present and the server prefers an enctype from the client's enctype list over that of the AP-REQ authenticator subkey (if that is present) or the service ticket session key, the server MUST create a subkey using that enctype. This negotiated subkey is sent in the subkey field of AP-REP message and it MUST be used for subsequent communication. This negotiation extension MUST NOT be used when the client does not expect the subkey in the AP-REP message from the server. Note that to preserve the quality of randomness provided by the KDC, implementations of this extension SHOULD consider using the service ticket session key value as a source of additional entropy when generating the negotiated subkey. If the AP-REQ authenticator subkey is present, it MAY also be used as a source of entropy. The policy by which the client or the server chooses an enctype (i.e., how the preference order for the supported enctypes is selected) is an implementation-specific local matter. 4. Security Considerations The client's enctype list and the server's reply enctype are part of encrypted data, thus the security considerations are the same as those of the Kerberos encrypted data. In all cases, the communicating peers are exposed to the denial of service threat. 5. IANA Considerations No IANA actions are required for this document. 6. Normative References [CLAR] RFC-Editor: To be replaced by RFC number for draft-ietf- krb-wg-kerberos-clarifications. Work in Progress. [GSS-CFX] RFC-Editor: To be replaced by RFC number for draft-ietf- krb-wg-gssapi-cfx. Work in Progress. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2743] Linn, J., "Generic Security Service Application Program Interface Version 2, Update 1", RFC 2743, January 2000. Zhu, et al. Expires October 2, 2005 [Page 4] Internet-Draft Enctype Negotiation March 2005 [SPNEGOBIS] RFC-Editor: To be replaced by RFC number for draft-ietf- kitten-2478bis. Work in progress. [X690] ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER), ITU-T Recommendation X.690 (1997) | ISO/IEC International Standard 8825-1:1998. Authors' Addresses Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: lzhu@microsoft.com Paul Leach Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: paulle@microsoft.com Karthik Jaganathan Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: karthikj@microsoft.com Appendix A. Leveraging this Enctype Negotiation in Windows SPNEGO Implementations The SPNEGO implementations in Windows 2000, Windows XP and Windows 2003 do not generate or verify the mechlistMIC field when it is required [SPNEGOBIS]. When the SPNEGO implementations that are updated according to [SPNEGOBIS], an SSPI initiator or acceptor needs to determine if the peer is updated, so that it can generate the mechlistMIC token when the peer can process it. With the bidirectional negotiation, the updated SPNEGO implementation can achieve the following two goals: Zhu, et al. Expires October 2, 2005 [Page 5] Internet-Draft Enctype Negotiation March 2005 o It can remain backward compatible with legacy implementations, if local policy allows unsafe and unprotected negotiation with downlevel implementations when the mechlistMIC token exchange would otherwise be required by [SPNEGOBIS]. o The mechanism negotiation is protected according to [SPNEGOBIS] when both peers are updated. However, the updated SPNEGO implementation itself can not securely inform the peer whether the local implementation is updated, thus it has to obtain such information from the negotiated mechanism. For Windows SPNEGO implementations, both the initiator and the acceptor are assumed to have been updated if a "newer" [CLAR] or different enctype is negotiated for use by the Kerberos GSS-API mechanism. Zhu, et al. Expires October 2, 2005 [Page 6] Internet-Draft Enctype Negotiation March 2005 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 (2005). 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. Zhu, et al. Expires October 2, 2005 [Page 7]