Network Working Group L. Zhu Request for Comments: 4537 P. Leach Updates: 4120 K. Jaganathan Category: Standards Track Microsoft Corporation June 2006 Kerberos Cryptosystem Negotiation Extension Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2006).
AbstractThis document specifies an extension to the Kerberos protocol as defined in RFC 4120, in which the client can send a list of supported encryption types in decreasing preference order, and the server then selects an encryption type that is supported by both the client and the server. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . . 2 3. Negotiation Extension . . . . . . . . . . . . . . . . . . . . . 2 4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 4 6. Normative References . . . . . . . . . . . . . . . . . . . . . 4
RFC4120], the Kerberos Distribution Center (KDC) must limit the ticket session key encryption type (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 cannot 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 the Kerberos protocol to allow clients and servers to negotiate use of a different and possibly stronger cryptosystem in subsequent communication. This extension utilizes an authorization data element in the authenticator of the AP-REQ message [RFC4120]. The client sends the list of enctypes that it supports to the server; the server then informs the client of its choice. The negotiated subkey is sent in the AP-REP message [RFC4120]. RFC2119]. RFC4120]. A new authorization data element type AD-ETYPE-NEGOTIATION is defined. AD-ETYPE-NEGOTIATION 129
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 [RFC4120]. The value of this authorization element contains the DER [X680] [X690] 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 [RFC4120]. 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 is then used as the protocol key or base key [RFC3961] for subsequent communication. If the enctype of the ticket session key is included in the enctype list sent by the client, it SHOULD be the last on the list; otherwise, this enctype MUST NOT be negotiated if it was not included in the list. This negotiation extension SHOULD NOT be used when the client does not expect the subkey in the AP-REP message from the server. A note on key generation: The KDC has a strong Pseudo-Random Number Generator (PRNG); as such, the client can take advantage of the randomness provided by the KDC by reusing the KDC key data when generating keys. Implementations SHOULD use 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 server MAY ignore the preference order indicated by the client. 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 a local matter.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3961] Raeburn, K., "Encryption and Checksum Specifications for Kerberos 5", RFC 3961, February 2005. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002, Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation. [X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002, Information technology - ASN.1 encoding Rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER).
Full 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. 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. Intellectual Property 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. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at firstname.lastname@example.org. Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).