draft-ietf-krb-wg-anon-02.txt [plain text]
NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan
Intended status: Standards Track Microsoft Corporation
Expires: April 14, 2007 October 11, 2006
Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-02
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines extensions to the Kerberos protocol for the
Kerberos client to authenticate the Kerberos Key Distribution Center
and the Kerberos server, without revealing the client's identity.
These extensions can be used to secure communication between the
anonymous client and 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 . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
In certain situations, 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.
Extensions to [RFC4120] are specified in this document by which a
client can authenticate the KDC and request an anonymous ticket. The
client can use the anonymous ticket to authenticate the server and
protect subsequent client-server communications. These extensions
provide Kerberos with functional equivalence to Transport Layer
Security (TLS) [RFC4346].
By using the extensions defined in this specification, the client MAY
reveal its identity in its initial request to its own KDC, but it can
remain anonymous thereafter to KDCs on the cross-realm authentication
path, 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 based on
[KRBNAM]. The value is the literal "RESERVED:ANONYMOUS".
The anonymous Kerberos principal name is a reserved Kerberos
principal name based on [KRBNAM]. The value of the name-type field
is KRB_NT_RESRVED [KRBNAM], and the value of the name-string field is
a sequence of two KerberosString components: "RESERVED", "ANONYMOUS".
Note that in this specification, the anonymous principal name and
realm are only applicable to the client in Kerberos messages, the
server MUST NOT be anonymous in any Kerberos message.
The transited field [RFC4120] of a ticket is an anonymous
authentication path if the tr-type field of the TransitedEncoding
type [RFC4120] is NO-TRANSITED-INFO and the contents field is an
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empty OCTET STRING.
NO-TRANSITED-INFO TBA
This means that no information of the authentication path is
disclosed.
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 client's realm name
or the anonymous realm name.
o The transited field [RFC4120] can contain either the client's
authentication path as described in Section 3.3.3.2 of [RFC4120]
or the anonymous authentication path.
o The anonymous ticket contains no information that can reveal the
client's identity. However the ticket MAY contain the client
realm and the realms on the authentication path, and authorization
data that MAY provide information related to the client's
identity. For example, an anonymous principal that is only
identifiable within a particular group of users can be implemented
using authorization data and such authorization data, if included
in the anonymous ticket, shall disclose the client's membership of
that group.
o The anonymous ticket flag is set.
The request-anonymous KDC option is defined as bit TBA (with the
first bit being bit 0) in the KDCOptions:
KDCOptions ::= KerberosFlags
-- request-anonymous(TBA)
-- KDCOptions and KerberosFlags are defined in [RFC4120]
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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. If the client wishes to
authenticate the KDC anonymously, it sets the client name as
anonymous in the AS exchange and provides a PA_PK_AS_REQ pre-
authentication data [RFC4556] where both the signerInfos field and
the certificates field of the SignedData [RFC3852] of PA_PK_AS_REQ
are empty. Because the anonymous client does not have an associated
asymmetric key pair, the client MUST use the Diffie-Hellman key
agreement method by filling in the Diffie-Hellman domain parameters
in the clientPublicValue [RFC4556].
If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is
anonymous, or if the client in the AS request is anonymous, the
request-anonymous KDC option MUST be set in the request.
Upon receiving the AS request with a PA_PK_AS_REQ from the anonymous
client, the KDC skips the checks for the client's signature and the
client's public key (such as the verification of the binding between
the client's public key and the client name), but performs otherwise-
applicable checks, and proceeds as normal according to [RFC4556].
For example, the AS MUST check if the client's Diffie-Hellman domain
parameters are acceptable. The Diffie-Hellman key agreement method
MUST be used and the reply key is derived according to Section
3.2.3.1 of [RFC4556]. If the clientPublicValue is not present in the
request, the KDC MUST return a KRB-ERROR [RFC4120] with the code
KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556] and there is no
accompanying e-data. The client that made the anonymous request can
authenticate the KDC based on the KDC's signature in the reply. If
the KDC does not have an asymmetric key pair, it MAY reply
anonymously. In which case, both the signerInfos field and the
certificates field of the SignedData [RFC3852] of PA_PK_AS_REP in the
reply are empty. The server name in an anonymous reply contains the
name of the TGS. Upon receipt of an anonymous KDC reply, the client
MUST reject the returned ticket if it can not authenticate the KDC
otherwise.
The client can use its keys to mutually authenticate with the KDC,
and request an anonymous TGT in the AS request. And in that case,
the reply key is selected as normal according to Section 3.1.3 of
[RFC4120].
For the TGS exchange, the reply key is selected as normal according
to Section 3.3.3 of [RFC4120].
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When policy allows, the KDC issues an anonymous ticket. Based on
local policy, the client realm in the anonymous ticket can be the
anonymous realm name or the realm of the KDC. However, in all cases,
the client name and the client realm in the EncKDCRepPart of the
reply [RFC4120] MUST match with the corresponding client name and the
client realm of the anonymous ticket in the reply. The client MUST
use the client name and the client realm returned in the
EncKDCRepPart in subsequent message exchanges when using the obtained
anonymous ticket.
During the TGS 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
structure). The TGS can only strip critical unknown authorization
data if the ticket does not convey any rights such as those conveyed
by a KDCIssued authorization data element. If a ticket contains a
KDCIssued authorization data element, then no other authorization
data elements may be removed if they could serve to limit the rights
conveyed by the KDCIssued element. Here is a table of the known
authorization-data elements, tagged 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, fail the request if unknown
AD-AND-OR RFC4120 Yes, remove if unknown
AD-MANDATORY-FOR-KDC RFC4120 Yes, fail the request if unknown
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 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.
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If the anonymous ticket is returned in a TGS exchange, the authtime
field contains the authtime of the ticket in the PA-TGS-REQ
[RFC4120]. An anonymous ticket can be renewed, and the authtime
field of a renewed ticket is the authtime in the anonymous ticket on
which the renewed ticket was based.
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].
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] and there is no accompanying e-data.
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. This is
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
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 the initiator/client 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 initiator/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
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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]. Note that in this
specification only the client/initiator can be anonymous.
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().
6. Security Considerations
Since KDCs ignore unknown options [RFC4120], a client requiring
anonymous communication needs to make sure that the ticket is
actually anonymous. This is because 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 server, otherwise the client's identity may be revealed
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.
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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
implementations of most (for now all) KDC's respond to requests at
the time that they are received. Traffic analysis on the connection
to the KDC will allow an attacker 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 straightforward.
7. Acknowledgements
Clifford Neuman contributed the core notions of this document.
Martin Rex wrote the text for GSS-API considerations.
Nicolas Williams reviewed the GSS-API considerations section and
suggested ideas for improvements.
Sam Hartman and Nicolas Williams were great champions of this work.
In addition, the following individuals made significant
contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love
Hoernquist Aestrand, and Jeffery Hutzelman.
8. IANA Considerations
Section 3 defines the anonymous Kerberos name and the anonymous
Kerberos realm based on [KRBNAM]. The IANA registry for [KRBNAM]
need to be updated to add references to 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.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
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RFC 3852, July 2004.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
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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