NETWORK WORKING GROUP S. Emery Internet-Draft Sun Intended status: Standards Track October 2006 Expires: April 4, 2007 Kerberos Version 5 GSS-API Channel Binding Hash Agility draft-ietf-krb-wg-gss-cb-hash-agility-00.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 April 4, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Emery Expires April 4, 2007 [Page 1] Internet-Draft Channel Binding Hash Agility October 2006 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. Channel binding hash agility . . . . . . . . . . . . . . . . . 4 3. Channel binding extension hash parameters . . . . . . . . . . 6 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 April 4, 2007 [Page 2] Internet-Draft Channel Binding Hash Agility October 2006 1. Introduction With the recently discovered weaknesses in the MD5 (SHA1 based) hash algorithm there is a need to move to crypto-agility. 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. Emery Expires April 4, 2007 [Page 3] Internet-Draft Channel Binding Hash Agility October 2006 2. Channel binding hash agility When generating a channel binding identifier, Bnd, a hash is computed from the channel binding information. Newer clients (initiators) shall continue to populate the Bnd field in order to remain compatible with older servers (acceptors). In addition, newer clients shall populate the extension field, Exts, with TYPED-DATA as defined in [RFC4120]. The 0x8003 GSS checksum would 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 Type Extensions in ASN.1 DER encoding where Extensions ::= TYPED-DATA -- as defined in [RFC4120] The TYPED-DATA would have the following information: data-type This field specifies the type of channel binding extensions. TBD is specified when the data-value contains channel binding hash information. data-value This field contains specific channel binding information relative to data-type. When data-type is TBD then data-value contains the output obtained from the get_mic() operation as specified in [RFC3961]. The parameters used are described in section 2.2. Emery Expires April 4, 2007 [Page 4] Internet-Draft Channel Binding Hash Agility October 2006 When a newer server receives a token sent by a newer client the Bnd field will be ignored. The newer server will then show that it understands the extension by sending an AP-REP with the following structure: EncAPRepPart ::= [APPLICATION 27] SEQUENCE { ctime [0] KerberosTime, cusec [1] Microseconds, subkey [2] EncryptionKey OPTIONAL, seq-number [3] UInt32 OPTIONAL, extensions [4] TYPED-DATA, ... } where extensions is the same data-type that the client had sent in the AP-REQ. Emery Expires April 4, 2007 [Page 5] Internet-Draft Channel Binding Hash Agility October 2006 3. Channel binding extension hash parameters The MAC value used as the channel binding identifier is calculated with the following parameters: (1) The session key is used to derive the specific key that is used exclusively for the channel binding identifiers. The key usage is a 32 bit integer TBD. The specific key Kc, is therefore derived as: Kc = key_generation(session-key, TBD | 0x99) where key_generation is the key-derivation function where 0x99 is the octet that is concatenated to the key usage (2) The message parameter is created the same way as described in section 4.1.1.2 of [RFC4121]. Emery Expires April 4, 2007 [Page 6] Internet-Draft Channel Binding Hash Agility October 2006 4. Security considerations Servers can use a down-grade attack by ignoring the channel binding extensions, but client policy can prevent these attacks if the client has specific requirements. Emery Expires April 4, 2007 [Page 7] Internet-Draft Channel Binding Hash Agility October 2006 5. IANA Considerations Question for wg: Do we need to be concerned with typed hole type values? Emery Expires April 4, 2007 [Page 8] Internet-Draft Channel Binding Hash Agility October 2006 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. Emery Expires April 4, 2007 [Page 9] Internet-Draft Channel Binding Hash Agility October 2006 7. Normative References [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 April 4, 2007 [Page 10] Internet-Draft Channel Binding Hash Agility October 2006 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 April 4, 2007 [Page 11] Internet-Draft Channel Binding Hash Agility October 2006 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. 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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 ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Emery Expires April 4, 2007 [Page 12]