NETWORK WORKING GROUP S. Emery Internet-Draft Sun Microsystems Updates: 4121 (if approved) March 2007 Intended status: Standards Track Expires: September 2, 2007 Kerberos Version 5 GSS-API Channel Binding Hash Agility draft-ietf-krb-wg-gss-cb-hash-agility-02.txt 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 September 2, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Emery Expires September 2, 2007 [Page 1] Internet-Draft Channel Binding Hash Agility March 2007 Abstract Currently, the Kerberos Version 5 Generic Security Services Application Programming Interface (GSS-API) mechanism [RFC4121] does not have the ability to utilize better hash algorithms used to generate channel binding identities. The current mechanism for doing this is hard coded to use MD5 only. The purpose of this document is to outline changes required to update the protocol so that more secure algorithms can be used to create channel binding identities. The extensibility of this solution also provides an eventual replacement of identities based solely on hash algorithms. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 4 3. Channel binding hash agility . . . . . . . . . . . . . . . . . 5 4. Security considerations . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 7. Normative References . . . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Emery Expires September 2, 2007 [Page 2] Internet-Draft Channel Binding Hash Agility March 2007 1. Introduction With the recently discovered weaknesses in the MD5 hash algorithm there is a need to move stronger hash alogrithms. Kerberos Version 5 Generic Security Services Application Programming Interface (GSS-API) mechanism [RFC4121] uses MD5 to calculate channel binding identities that are required to be unique. This document specifies an update to the mechanism that allows it to create channel binding identities based on negotiating algorithms securely. This will prevent lengthy standardizations in the future when new attacks arise and will allow an incremental update to the protocol so that this will interoperate with older implementations. Emery Expires September 2, 2007 [Page 3] Internet-Draft Channel Binding Hash Agility March 2007 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]. The term "little endian order" is used for brevity to refer to the least-significant-octet-first encoding, while the term "big endian order" is for the most-significant-octet-first encoding. Emery Expires September 2, 2007 [Page 4] Internet-Draft Channel Binding Hash Agility March 2007 3. Channel binding hash agility When generating a channel binding identifier, Bnd, a hash is computed from the channel binding information. Initiators MUST populate the Bnd field in order to maintain interoperability with existing acceptors. In addition, initiators MUST populate the extension field, Exts, with TYPED-DATA as defined in [RFC4120]. The 0x8003 GSS checksum MUST have the following structure: Octet Name Description ----------------------------------------------------------------- 0..3 Lgth Number of octets in Bnd field; Represented in little-endian order; Currently contains hex value 10 00 00 00 (16). 4..19 Bnd Channel binding information, as described in section 4.1.1.2 [RFC4121]. 20..23 Flags Four-octet context-establishment flags in little-endian order as described in section 4.1.1.1 [RFC4121]. 24..25 DlgOpt The delegation option identifier (=1) in little-endian order [optional]. This field and the next two fields are present if and only if GSS_C_DELEG_FLAG is set as described in section 4.1.1.1 [RFC4121]. 26..27 Dlgth The length of the Deleg field in little-endian order [optional]. 28..(n-1) Deleg KRB_CRED message (n = Dlgth + 28) [optional]. n..last Exts Extensions where Exts is the concatenation of zero, one or more individual extensions, each of which consists of: type -- big endian order unsigned integer, 32-bits length -- big endian order unsigned integer, 32-bits data -- octet string of length octets in that order When channel binding is used the Exts MUST include the following extension: data-type 0x00000000 data-value The output obtained by applying the Kerberos V get_mic() operation [RFC3961], using the sub-session key from the authenticator and key usage number TBD, to the channel binding data as described in [RFC4121], section 4.1.1.2 (using get_mic instead of MD5). Emery Expires September 2, 2007 [Page 5] Internet-Draft Channel Binding Hash Agility March 2007 Initiators that are unwilling to use a MD5 hash of the channel bindings should set the Bnd field to all ones (1). Emery Expires September 2, 2007 [Page 6] Internet-Draft Channel Binding Hash Agility March 2007 4. Security considerations Initiators do not know if the acceptor had ignored channel bindings or whether it validated the MD5 hash of the channel bindings [RFC4121]. Ultimately, it is up to the application whether to use channel binding or not. This is dependent upon the security policy of these applications. Emery Expires September 2, 2007 [Page 7] Internet-Draft Channel Binding Hash Agility March 2007 5. IANA Considerations The IANA is hereby requested to create a new registry of "Kerberos V GSS-API mechanism extension types" with four-field entries (type number, type name, description, and normative reference) and, initially, a single registration: 0x00000000, "Channel Binding MIC," "Extension for hash function-agile channel binding," . Registration of additional extensions SHALL be by IESG Protocol Action. Emery Expires September 2, 2007 [Page 8] Internet-Draft Channel Binding Hash Agility March 2007 6. Acknowledgements Larry Zhu helped in the review of this document overall and provided the suggestions of typed data and server acknowledgement. Nicolas Williams and Sam Hartman suggested that the Bnd and Exts fields be populated simultaneously. Nicolas Williams and Jeffrey Hutzelman had also suggested a number changes to this document. Emery Expires September 2, 2007 [Page 9] Internet-Draft Channel Binding Hash Agility March 2007 7. Normative References [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. [RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Mechanism: Version 2", RFC 4121, July 2005. Emery Expires September 2, 2007 [Page 10] Internet-Draft Channel Binding Hash Agility March 2007 Author's Address Shawn Emery Sun Microsystems 500 Eldorado Blvd M/S UBRM05-171 Broomfield, CO 80021 US Email: shawn.emery@sun.com Emery Expires September 2, 2007 [Page 11] Internet-Draft Channel Binding Hash Agility March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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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 ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Emery Expires September 2, 2007 [Page 12]