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NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan
Expires: January 17, 2007 Microsoft Corporation
July 16, 2006
Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-01
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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Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 17, 2007.
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 . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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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
The anonymous Kerberos realm name is a reserved realm name as defined
in [KRBNAM] and its value is the literal "RESERVED:ANONYMOUS".
The anonymous Kerberos principal name is a reserved Kerberos
principal name as defined in [KRBNAM], its name-type [RFC4120] is
KRB_NT_RESRVED [KRBNAM], and its name-string [RFC4120] is a sequence
of two KerberosString components: "RESERVED", "ANONYMOUS".
In this specification, only the client name or the client realm can
be anonymous; the server name or the server realm can not be
anonymous.
The transited field [RFC4120] of a ticket is an anonymous
authentication path if the tr-type field of the TransitedEncoding
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type [RFC4120] is NO-TRANSITED-INFO and the contents field is an
empty OCTET STRING.
NO-TRANSITED-INFO TBA
This transited encoding type indicates that there is no information
available about the authentication path.
The anonymous ticket flag is defined as bit TBA (with the first bit
being bit 0) in the TicketFlags:
TicketFlags ::= KerberosFlags
-- anonymous(TBA)
-- TicketFlags and KerberosFlags are defined in [RFC4120]
An anonymous ticket is a ticket that has all of the following
properties:
o The cname field [RFC4120] contains the anonymous Kerberos
principal name.
o The crealm field [RFC4120] contains either the realm name of the
client who made the request or the anonymous kerberos realm name,
based on the local policy of the KDC.
o The transited field [RFC4120] can contain either the client's
"normal" authentication path according to Section 3.3.3.2 of
[RFC4120] or the anonymous authentication path.
o It contains no information that can reveal the client's identity.
However the ticket can contain the client realm and the realms on
the authentication path, and the authorization data may provide
additional information of the client. For example, an anonymous
principal that is only identifiable within a particular group of
users can be implemented by using authorization data.
o The anonymous ticket flag is set.
Notes: The anonymous ticket flag MUST NOT be set by implementations
of this specification if the ticket is not an anonymous ticket. 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.
The request-anonymous KDC option is defined as bit TBA (with the
first bit being bit 0) in the KDCOptions:
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KDCOptions ::= KerberosFlags
-- request-anonymous(TBA)
-- KDCOptions and KerberosFlags are defined in [RFC4120]
4. Protocol Description
In order to request an anonymous ticket, the client sets the request-
anonymous KDC option in an Authentication Exchange (AS) or Ticket
Granting Service (TGS) request [RFC4120]. The client can request an
anonymous TGT based on a normal TGT. Note that if the ticket in the
PA-TGS-REQ [RFC4120] is anonymous, the request-anonymous KDC option
MUST be set in the request.
When propagating authorization data, care MUST be taken by the TGS to
ensure that the client confidentiality is not violated: the TGS MUST
either fail the request or remove authorization data that may reveal
the client's identity. An optional authorization element unknown by
the TGS MUST be removed if it can be ignored (such as ones enclosed
in the AD-IF-RELEVANT or the AD-KDCIssued containers [RFC4120]). The
TGS can strip critical unknown authorization data if such data do not
convey any rights based on the requesting client's identity. Here is
a table of the known authorization-data elements, flagged with
whether they interfere with client anonymity and recommendations for
how to process them.
ad-type References Can Breach Confidentiality?
------------------------------------------------------------------
AD-IF-RELEVANT RFC4120 Yes, remove if unknown
AD-KDCIssued RFC4120 Yes, remove if unknown
AD-AND-OR RFC4120 Yes, remove if unknown
AD-MANDATORY-FOR-KDC RFC4120 Yes, fail the request if unknown
If it is inappropriate to remove an authorization element 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].
When policy allows, the KDC issues an anonymous ticket. The client
realm in the anonymous ticket can be the anonymous realm name based
on local policy. The client name and the client realm the
EncKDCRepPart of the reply [RFC4120] MUST match with the
corresponding client name and the client realm of the anonymous reply
ticket. The client then MUST use the client name and the client
realm returned in the EncKDCRepPart in subsequent message exchanges
when using that anonymous ticket.
If there is a key known by both the client and the KDC for encrypting
the KDC reply, the cname field in the request [RFC4120] can be
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anonymous. If the client is anonymous and the KDC does not have a
key to encrypt the reply, the KDC MUST return an error message with
the code KDC_ERR_NULL_KEY [RFC4120]. For AS exchange, if the reply
key is selected from the client keys (for example, as described in
Section 3.1.3 of [RFC4120]), then the client principal MUST NOT be
anonymous. The client can use the client keys to request an
anonymous TGT in the AS request. The anonymous client name, for
example, can be used in conjunction with PKINIT [RFC4556]. An
anonymous PKINIT client can authenticate the KDC based on the KDC
certificate. For TGS exchange, the reply key is selected according
to Section 3.3.3 of [RFC4120] as normal.
The KDC fills out the transited field of the anonymous ticket in the
reply as follows: If the service ticket in a TGS request is an
anonymous ticket with a "normal" authentication path, then the
authentication path in the reply ticket MUST also contain a "normal"
authentication path: the TGS MUST add the name of the previous realm.
However, if the service ticket in a TGS request is an anonymous
ticket with an anonymous authentication path, then the reply ticket
can contain either an anonymous authentication path or a "normal"
authentication path, based on the local policy of the KDC. Thus a
"normal" authentication path in an anonymous ticket can be a partial
path: it may not include all the intermediate realms on the
authentication path.
The KDC fills out the authtime field of the anonymous ticket in the
reply as follows: If the anonymous ticket is returned in an AS
exchange, the authtime field of the ticket contains the request time.
If the anonymous ticket is returned in a TGS exchange, the authtime
field contains the time of the initial authentication for the
principal who has made the request. An anonymous ticket can be
renewed, and the authtime field of a renewed ticket is the authtime
in the anonymous ticket that the renewed ticket was based on.
If a client requires anonymous communication then the client MUST
check to make sure that the ticket in the reply 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]. No transited policy checking is needed for
the anonymous authentication path. However, transited policy checks
defined in Section 2.7 of [RFC4120] would apply to an anonymous
ticket that contains a "normal" authentication path.
A server accepting an anonymous service ticket may assume that
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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 the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
represent the anonymous identity. In addition, Section 2.1.1 of
[RFC1964] defines the single string representation of a Kerberos
principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For
the anonymous principals, the name component within the exportable
name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm
name according to Section 2.1.1 of [RFC1964]. In this specification
only the client/initiator can be the anonymous identity.
Portable initiators are RECOMMENDED to use default credentials
whenever possible, and request anonymity only through the input
anon_req_flag [RFC2743] 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 is 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.
If the KDC that issued an anonymous ticket were to maintain records
of the association of identities to an anonymous ticket, then someone
obtaining such records could breach the anonymity. Additionally, the
implementation of most (for now all) KDC's respond to requests at the
time that they are received. Traffic analasys on the connection to
the KDC will allow an attacket to match client identities to
anonymous tickets issued. Because there are plaintext parts of the
tickets that are exposed on the wire, such matching by a third party
observer is relatively straigtforward.
7. Acknowledgements
The authors would like to thank the following individuals for their
insightful comments and fruitful discussions: Sam Hartman, Clifford
Neuman, Martin Rex, Nicolas Williams, Jeffery Altman, Tom Yu,
Chaskiel M Grundman, Love Hoernquist Aestrand, and Jeffery Hutzelman.
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.
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[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
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.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
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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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