draft-ietf-krb-wg-gss-cb-hash-agility-03.txt [plain text]
NETWORK WORKING GROUP S. Emery
Internet-Draft Sun Microsystems
Updates: 4121 (if approved) November 9, 2007
Intended status: Standards Track
Expires: May 12, 2008
Kerberos Version 5 GSS-API Channel Binding Hash Agility
draft-ietf-krb-wg-gss-cb-hash-agility-03.txt
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Copyright (C) The IETF Trust (2007).
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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
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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.
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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 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).
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Initiators that are unwilling to use a MD5 hash of the channel
bindings should set the Bnd field to all ones (1).
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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 hash function-agile channel binding," <this RFC>.
Registration of additional extensions SHALL be by IESG Protocol
Action.
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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.
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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.
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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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