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NETWORK WORKING GROUP S. Emery
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Internet-Draft Sun Microsystems
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Updates: 4121 (if approved) July 14, 2008
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Intended status: Standards Track
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Expires: January 15, 2009
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Kerberos Version 5 GSS-API Channel Binding Hash Agility
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draft-ietf-krb-wg-gss-cb-hash-agility-04.txt
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By submitting this Internet-Draft, each author represents that any
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applicable patent or other IPR claims of which he or she is aware
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have been or will be disclosed, and any of which he or she becomes
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aware will be disclosed, in accordance with Section 6 of BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that
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other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
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and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
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material or to cite them other than as "work in progress."
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt.
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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html.
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This Internet-Draft will expire on January 15, 2009.
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Currently, channel bindings are implemented using a MD5 hash in the
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Kerberos Version 5 Generic Security Services Application Programming
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Interface (GSS-API) mechanism [RFC4121]. This document updates
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RFC4121 to allow the channel binding restriction to be lifted using
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algorithms negotiated based on Kerberos crypto framework as defined
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in RFC3961. In addition, because this update makes use of the last
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extensible field in the Kerberos client-server exchange message,
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extensions are defined to allow future protocol extensions.
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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2. Conventions Used in This Document . . . . . . . . . . . . . . 4
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3. Channel binding hash agility . . . . . . . . . . . . . . . . . 5
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4. Security considerations . . . . . . . . . . . . . . . . . . . 7
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5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
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6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
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7. Normative References . . . . . . . . . . . . . . . . . . . . . 10
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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Intellectual Property and Copyright Statements . . . . . . . . . . 12
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With the recently discovered weaknesses in the MD5 hash algorithm
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there is a need to use stronger hash algorithms. Kerberos Version 5
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Generic Security Services Application Programming Interface (GSS-API)
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mechanism [RFC4121] uses MD5 to calculate channel binding verifiers.
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This document specifies an update to the mechanism that allows it to
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create channel binding information based on negotiating algorithms
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securely. This will allow deploying new algorithms incrementally
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without break interoperability with older implementations, when new
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attacks arise in the future.
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2. Conventions Used in This Document
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
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The term "little endian order" is used for brevity to refer to the
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least-significant-octet-first encoding, while the term "big endian
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order" is for the most-significant-octet-first encoding.
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3. Channel binding hash agility
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When generating a channel binding verifier, Bnd, a hash is computed
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from the channel binding fields. Initiators MUST populate the Bnd
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field in order to maintain interoperability with existing acceptors.
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In addition, initiators MUST populate the extension field, Exts, with
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TYPED-DATA as defined in [RFC4120]. All fields before Exts, do not
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change from what is described in [RFC4121], they are listed for
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convenience. The 0x8003 GSS checksum MUST have the following
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Octet Name Description
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-----------------------------------------------------------------
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0..3 Lgth Number of octets in Bnd field; Represented
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in little-endian order; Currently contains
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hex value 10 00 00 00 (16).
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4..19 Bnd Channel binding information, as described in
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section 4.1.1.2 [RFC4121].
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20..23 Flags Four-octet context-establishment flags in
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little-endian order as described in section
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24..25 DlgOpt The delegation option identifier (=1) in
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little-endian order [optional]. This field
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and the next two fields are present if and
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only if GSS_C_DELEG_FLAG is set as described
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in section 4.1.1.1 [RFC4121].
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26..27 Dlgth The length of the Deleg field in
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little-endian order [optional].
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28..(n-1) Deleg KRB_CRED message (n = Dlgth + 28) [optional].
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n..last Exts Extensions
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where Exts is the concatenation of zero, one or more individual
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extensions, each of which consists of, in order:
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type -- big endian order unsigned integer, 32-bits, which contains
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the type of extension
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data -- octet string of extension information
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If multiple extensions are present then there MUST be at most one
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instance of a given extension type.
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When channel binding is used the Exts MUST include the following
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The output obtained by applying the Kerberos V get_mic()
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operation [RFC3961], using the sub-session key from the
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authenticator and key usage number 43, to the channel binding
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data as described in [RFC4121], section 4.1.1.2 (using get_mic
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Initiators that are unwilling to use a MD5 hash of the channel
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bindings MUST set the Bnd field to sixteen octets of hex value FF.
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4. Security considerations
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Initiators do not know if the acceptor had ignored channel bindings
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or whether it validated the MD5 hash of the channel bindings
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Ultimately, it is up to the application whether to use channel
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binding or not. This is dependent upon the security policy of these
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5. IANA Considerations
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The IANA is hereby requested to create a new registry of "Kerberos V
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GSS-API mechanism extension types" with four-field entries (type
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number, type name, description, and normative reference) and,
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initially, a single registration: 0x00000000, "Channel Binding MIC,"
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"Extension for the verifier of the channel bindings," <this RFC>.
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Using the guidelines for allocation as described in [RFC5226], type
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number assignments are as follows:
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0x00000000 - 0x000003FF IETF Consensus
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0x00000400 - 0xFFFFF3FF Specification Required
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0xFFFFF400 - 0xFFFFFFFF Private Use
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Larry Zhu helped in the review of this document overall and provided
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the suggestions of typed-data.
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Nicolas Williams and Sam Hartman suggested that the Bnd and Exts
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fields be populated simultaneously.
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Nicolas Williams and Jeffrey Hutzelman had also suggested a number
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changes to this document.
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7. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
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Kerberos 5", RFC 3961, February 2005.
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[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
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Kerberos Network Authentication Service (V5)", RFC 4120,
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[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
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Version 5 Generic Security Service Application Program
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Interface (GSS-API) Mechanism: Version 2", RFC 4121,
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
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IANA Considerations Section in RFCs", BCP 26, RFC 5226,
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Email: shawn.emery@sun.com
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Full Copyright Statement
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Copyright (C) The IETF Trust (2008).
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This document is subject to the rights, licenses and restrictions
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contained in BCP 78, and except as set forth therein, the authors
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retain all their rights.
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This document and the information contained herein are provided on an
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"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
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OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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Intellectual Property
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pertain to the implementation or use of the technology described in
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