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NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan
Expires: December 5, 2006 Microsoft Corporation
June 3, 2006
Anonymity Support for Kerberos
draft-ietf-krb-wg-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
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This Internet-Draft will expire on December 5, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
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 . . . . . . . . . . . . . . . . . . . . . 5
5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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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 client's principal name is the anonymous Kerberos principal
name. The anonymous Kerberos principal name is defined as
follows: it is a reserved Kerberos principal name as defined in
[KRBNAM], the name-type is KRB_NT_RESRVED [KRBNAM], and the name-
string is a sequence of two KerberosString components: "RESERVED",
"ANONYMOUS".
o The client's realm name is the anonymous kerberos realm name. The
anonymous Kerberos realm name is defined as follows: it is a
reserved realm name as defined in [KRBNAM] and the realm name is
the literal "RESERVED:ANONYMOUS".
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o The authtime field in the ticket is set to the time of the ticket
request, not the time of the initial authentication for the
principal who has made the request.
o The transited field [RFC4120] can either contain the client's
authentication path or contain the anonymous authentication path
defined as follows: the tr-type field of the transited field is
NO-TRANSITED-INFO (as defined later in this section) and the
contents field is an empty OCTET STRING. If a TGS request
contains an anonymous ticket with a "normal" authentication path
(i.e. the transited field does not contain the anonymous
authentication path as defined above), then the reply ticket, if
any, MUST NOT contain the anonymous authentication path. For
application servers, no transited policy is defined for the
anonymous authentication path, but all of the transited checks
would still apply if an anonymous ticket contains a "normal"
authentication path. Note that the "normal" authentication path
in an anonymous ticket can be a partial path, thus it may not be
sufficient to identify the originating client realm.
o It contains no information that can reveal the client's identity
other than, at most, the client's realm or the realm(s) on the
authentication path.
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 in
this section.
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
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This transited encoding type indicates that there is no information
available about the authentication path.
Note that the server principal name and the server realm in a cross-
realm referral TGT are not dependent on whether the client is the
anonymous principal or not.
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
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].
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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. GSS-API Implementation Notes
At the GSS-API [RFC2743] level, the use of an anonymous principal by
the initiator/client requires a software change of the initiator/
client software (to assert the "anonymous" flag when calling
GSS_Init_Sec_Context().
GSS-API does not know or define "anonymous credentials", so the
(printable) name of the anonymous principal will rarely be used by or
relevant for the initator/client. The printable name is relevant for
the acceptor/server when performing an authorization decision based
on the name that pops up from GSS_Accept_Sec_Context() upon
successful security context establishment.
A GSS-API initiator MUST carefully check the resulting context
attributes from the initial call to GSS_Init_Sec_Context() when
requesting anonymity, because (as in the GSS-API tradition and for
backwards compatibility) anonymity is just another optional context
attribute. It could be that the mechanism doesn't recognize the
attribute at all or that anonymity is not available for some other
reasons -- and in that case the initiator must NOT send the initial
security context token to the acceptor, because it will likely reveal
the initiators identity to the acceptor, something that can rarely be
"un-done".
GSS-API defines name_type GSS_C_NT_ANONYMOUS [RFC2743] to represent
an anonymous identity. In addition, according to Section 2.1.1 of
[RFC1964] the string representation of the anonymous client principal
name can be "RESERVED/ANONYMOUS" or "RESERVED/
ANONYMOUS@RESERVED:ANONYMOUS" with the name_type
GSS_KRB5_NT_PRINCIPAL_NAME. Implementations conforming to this
specification MUST be able to accept the GSS_C_NT_ANONYMOUS name form
and the GSS_KRB5_NT_PRINCIPAL_NAME name forms, and consider them
equivalent.
Portable initiators are RECOMMENDED to use default credentials
whenever possible, and request anonymity only through the input
anon_req_flag to GSS_Init_Sec_Context().
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6. Security Considerations
Since KDCs ignore unknown options [RFC4120], 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.
In cases where specific server principals must not have access to the
client's identity (for example, an anonymous poll service), the KDC
can define server principal specific policy that insure any normal
service ticket can NEVER be issued to any of these server principals.
7. Acknowledgements
The authors would like to thank the following individuals for their
insightful comments and fruitful discussions: Sam Hartman, Martin
Rex, Nicolas Williams, Jeffery Altman, Tom Yu, Chaskiel M Grundman,
Love Hoernquist Aestrand, Jeffery Hutzelman, and Clifford Neuman.
8. IANA Considerations
No IANA actions are required for this document.
9. Normative References
[KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints",
draft-ietf-krb-wg-naming, work in progress.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[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.
[RFC2743] Linn, J., "Generic Security Service Application Program
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Interface Version 2, Update 1", RFC 2743, January 2000.
[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: lzhu@microsoft.com
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: paulle@microsoft.com
Karthik Jaganathan
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: karthikj@microsoft.com
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