draft-ietf-krb-wg-gss-cb-hash-agility-04.txt [plain text]
NETWORK WORKING GROUP S. Emery
Internet-Draft Sun Microsystems
Updates: 4121 (if approved) July 14, 2008
Intended status: Standards Track
Expires: January 15, 2009
Kerberos Version 5 GSS-API Channel Binding Hash Agility
draft-ietf-krb-wg-gss-cb-hash-agility-04.txt
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Abstract
Currently, channel bindings are implemented using a MD5 hash in the
Kerberos Version 5 Generic Security Services Application Programming
Interface (GSS-API) mechanism [RFC4121]. This document updates
RFC4121 to allow the channel binding restriction to be lifted using
algorithms negotiated based on Kerberos crypto framework as defined
in RFC3961. In addition, because this update makes use of the last
extensible field in the Kerberos client-server exchange message,
extensions are defined to allow future protocol extensions.
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
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1. Introduction
With the recently discovered weaknesses in the MD5 hash algorithm
there is a need to use stronger hash algorithms. Kerberos Version 5
Generic Security Services Application Programming Interface (GSS-API)
mechanism [RFC4121] uses MD5 to calculate channel binding verifiers.
This document specifies an update to the mechanism that allows it to
create channel binding information based on negotiating algorithms
securely. This will allow deploying new algorithms incrementally
without break interoperability with older implementations, when new
attacks arise in the future.
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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.
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3. Channel binding hash agility
When generating a channel binding verifier, Bnd, a hash is computed
from the channel binding fields. 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]. All fields before Exts, do not
change from what is described in [RFC4121], they are listed for
convenience. 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, in order:
type -- big endian order unsigned integer, 32-bits, which contains
the type of extension
data -- octet string of extension information
If multiple extensions are present then there MUST be at most one
instance of a given extension type.
When channel binding is used the Exts MUST include the following
extension:
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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 43, to the channel binding
data as described in [RFC4121], section 4.1.1.2 (using get_mic
instead of MD5).
Initiators that are unwilling to use a MD5 hash of the channel
bindings MUST set the Bnd field to sixteen octets of hex value FF.
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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.
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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 the verifier of the channel bindings," <this RFC>.
Using the guidelines for allocation as described in [RFC5226], type
number assignments are as follows:
0x00000000 - 0x000003FF IETF Consensus
0x00000400 - 0xFFFFF3FF Specification Required
0xFFFFF400 - 0xFFFFFFFF Private Use
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6. Acknowledgements
Larry Zhu helped in the review of this document overall and provided
the suggestions of typed-data.
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.
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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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
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Author's Address
Shawn Emery
Sun Microsystems
500 Eldorado Blvd
M/S UBRM05-171
Broomfield, CO 80021
US
Email: shawn.emery@sun.com
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