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CVS shishi/doc/specifications |
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Wed, 19 Oct 2005 01:20:24 +0200 |
Update of /home/cvs/shishi/doc/specifications
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draft-zhu-kerb-anon-00.txt
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2005/10/18 23:20:24 1.1
NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan
Expires: April 18, 2006 Microsoft Corporation
October 15, 2005
Anonymity Support for Kerberos
draft-zhu-kerb-anon-00
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 18, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines the use of anonymous Kerberos tickets for the
purpose of authenticating the servers and enabling secure
communication between a client and a server, without identifying the
client to the server.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
In certain situations or environments, the Kerberos [RFC4120] client
may wish to authenticate a server and/or protect communications
without revealing its own identity. For example, consider an
application which provides read access to a research database, and
which permits queries by arbitrary requestors. A client of such a
service might wish to authenticate the service, to establish trust in
the information received from it, but might not wish to disclose its
identity to the service for privacy reasons.
To accomplish this, a Kerberos mechanism is specified in this
document by which a client requests an anonymous ticket and use that
to authenticate the server and secure subsequent client-server
communications. This provides Kerberos with functional equivalence
to TLS [RFC2246] in environments where Kerberos is a more attractive
authentication mechanism.
Using this mechanism, the client has to reveal its identity in its
initial request to its own Key Distribution Center (KDC) [RFC4120],
and then it can remain anonymous thereafter to KDCs on the cross-
realm authentication path, if any, and to the server with which it
communicates.
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].
3. Definitions
An anonymous ticket is a ticket that has all of the following
properties:
o The name-type of the client principal name is NT-UNKNOWN
[RFC4120], and the name-string is an empty SEQUENCE (this client
principal name is referred hereafter as the anonymous client
principal name in this document).
o The client realm name is an empty KerberosString [RFC4120].
o The tr-type field of the transited field [RFC4120] is NO-
TRANSITED-INFO (as defined later in this section) and the contents
field is an empty OCTET STRING. No transited policy is defined
for anonymous tickets.
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o It contains no information that can reveal the client's identity.
o The anonymous ticket flag (as defined later in this section) is
set.
The anonymous ticket flag is defined as bit 14 (with the first bit
being bit 0) in the TicketFlags:
TicketFlags ::= KerberosFlags
-- anonymous(14)
-- TicketFlags and KerberosFlags are defined in [RFC4120]
The anonymous ticket flag MUST NOT be set by implementations of this
specification if the ticket is not an anonymous ticket as defined.
The request-anonymous KDC option is defined as bit 14 (with the first
bit being bit 0) in the KDCOptions:
KDCOptions ::= KerberosFlags
-- request-anonymous(14)
-- KDCOptions and KerberosFlags are defined in [RFC4120]
The anonymous transited encoding type is defined as follows:
NO-TRANSITED-INFO 0
This transited encoding type indicates there is no information
available about the authentication path.
4. Protocol Description
In order to request an anonymous ticket, the client sets the request-
anonymous KDC option in an AS or TGS request [RFC4120]. Note that if
the service ticket in the PA-TGS-REQ [RFC4120] is anonymous, the
request-anonymous KDC option MUST be set in the request.
When policy allows, the KDC issues an anonymous ticket. The KDC that
implements this specification MUST NOT carry information that can
reveal the client's identity, from the TGS request into the returned
anonymous ticket.
It should be noted that unless otherwise specified by this document
the client principal name and the client realm in the Kerberos
messages [RFC4120] should be the client name and client realm that
can uniquely identify the client principal to the KDC, not the
anonymous client principal name and the empty realm name. For
example, the client name and realm in the request body and the
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Internet-Draft Kerberos Anonymity Support October 2005
EncKDCRepPart of the reply [RFC4120] are identifiers of the client
principal. In other words, the client name and client realm in the
EncKDCRepPart does not match with that of the returned anonymous
ticket.
If either local policy prohibits issuing of anonymous tickets or it
is inappropriate to remove information (such as restrictions) from
the TGS request in order to produce an anonymous ticket, the KDC MUST
return an error message with the code KDC_ERR_POLICY [RFC4120].
If a client requires anonymous communication then the client should
check to make sure that the resulting ticket is actually anonymous by
checking the presence of the anonymous ticket flag. Because KDCs
ignore unknown KDC options, a KDC that does not understand the
request-anonymous KDC option will not return an error, but will
instead return a normal ticket.
The subsequent client and server communications then proceed as
described in [RFC4120]. The client principal name in the
Authenticator of the KRB_AP_REQ MUST be the anonymous client
principal name and the client realm of the Authenticator MUST be an
empty KerberosString [RFC4120].
A server accepting such an anonymous service ticket may assume that
subsequent requests using the same ticket originate from the same
client. Requests with different tickets are likely to originate from
different clients.
Interoperability and backward-compatibility notes: the KDC is given
the task of rejecting a request for an anonymous ticket when the
anonymous ticket is not acceptable by the server.
5. Security Considerations
Since KDCs ignore unknown options, a client requiring anonymous
communication needs to make sure that the ticket is actually
anonymous. A KDC that that does not understand the anonymous option
would not return an anonymous ticket.
By using the mechanism defined in this specification, the client does
not reveal its identity to the server but its identity may be
revealed to the KDC of the server principal (when the server
principal is in a different realm than that of the client), and any
KDC on the cross-realm authentication path. The Kerberos client MUST
verify the ticket being used are indeed anonymous before
communicating with the cross-realm KDC or the server, otherwise the
client's identity may be revealed to the server unintentionally.
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6. Acknowledgements
Most of this document is based on earlier versions of [RFC4120].
The authors would like to thank Sam Hartman for his comments and
suggestions.
7. IANA Considerations
No IANA actions are required for this document.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
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Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: address@hidden
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: address@hidden
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: address@hidden
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Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
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