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NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) Microsoft Corporation
Intended status: Standards Track July 7, 2007
Expires: January 8, 2008
Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-04
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 January 8, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
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 . . . . . . . . . . . . . . . . . . . . . 4
5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
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 Key Distribution Center (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 defined as a well-known realm
name based on [KRBNAM]. The value is the literal "WELLKNOWN:
ANONYMOUS". An anonymous Kerberos realm name MUST NOT be present in
the transited field [RFC4120] of a ticket.
The anonymous Kerberos principal name is defined as a well-known
Kerberos principal name based on [KRBNAM]. The value of the name-
type field [RFC4120] is KRB_NT_WELLKNOWN [KRBNAM], and the value of
the name-string field [RFC4120] is a sequence of two KerberosString
components: "WELLKNOWN", "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.
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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]
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 the client's realm name, or
the name of the realm that issued the initial ticket for the
client principal, or the anonymous realm name.
o The anonymous ticket contains no information that can reveal the
client's identity. However the ticket may contain the client
realm, intermediate realms on the client's authentication path,
and authorization data that may provide information related to the
client's identity. For example, an anonymous principal that is
identifiable only 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 anonymous KDC option is defined as bit 14 (with the first bit
being bit 0) in the KDCOptions:
KDCOptions ::= KerberosFlags
-- anonymous(14)
-- KDCOptions and KerberosFlags are defined in [RFC4120]
As described in Section 4, the anonymous KDC option is set to request
an anonymous ticket.
4. Protocol Description
In order to request an anonymous ticket, the client sets the
anonymous KDC option in an Authentication Exchange (AS) or Ticket
Granting Service (TGS) request [RFC4120]. The client can request an
anonymous Ticket Granting Ticket (TGT) based on a normal TGT. Unless
otherwise specified, the client can obtain an anonymous ticket with
the anonymous realm name only by requesting an anonymous ticket in an
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AS exchange with the client realm set as anonymous in the request.
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 the PA_PK_AS_REQ are empty. Because the anonymous
client does not have an associated asymmetric key pair, the client
MUST choose 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
anonymous KDC option MUST be set in the request. Otherwise, the KDC
MUST return a KRB-ERROR message with the code KDC_ERR_BADOPTION
[RFC4120], and there is no accompanying e-data defined in this
document.
Upon receiving the AS request with a PA_PK_AS_REQ [RFC4556] from the
anonymous client, the KDC processes the request according to Section
3.1.2 of [RFC4120]. 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. If all goes well, an anonymous ticket is
generated according to Section 3.1.3 of [RFC4120] and a PA_PK_AS_REP
[RFC4556] pre-authentication data is included in the KDC reply
according to [RFC4556]. If the KDC does not have an asymmetric key
pair, it MAY reply anonymously or reject the authentication attempt.
If the KDC replies anonymously, 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 the anonymous KDC reply contains
the name of the TGS.
Upon receipt of the KDC reply that contains an anonymous ticket and a
PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
authenticate the KDC based on the KDC's signature in the
PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply
(the reply is anonymous), the client MUST reject the returned ticket
if it cannot authenticate the KDC otherwise.
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The client can use the client keys to mutually authenticate with the
KDC, 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].
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.
When propagating authorization data in the ticket or in the enc-
authorization-data field [RFC4120] of the request, the TGS MUST
ensure that the client confidentiality is not violated in the
returned anonymous ticket. The TGS MUST process the authorization
data recursively according to Section 5.2.6 of [RFC4120] beyond the
container levels such that all embedded authorization elements are
interpreted. Identity-based authorization data SHOULD NOT be present
in an anonymous ticket in that it typically reveals the client's
identity. The specification of a new authorization data type MUST
specify the processing rules of the authorization data when an
anonymous ticket is returned. If there is no processing rule defined
for an authorization data element or the authorization data element
is unknown, the TGS MUST process it when an anonymous ticket is
returned as follows:
o If the authorization data element may reveal the client's
identity, it MUST be removed unless otherwise specified.
o If the authorization data element is intended to restrict the use
of the ticket or limit the rights otherwise conveyed in the
ticket, it cannot be removed in order to hide the client's
identity. In this case, the authentication attempt MUST be
rejected, and the KDC MUST return an error message with the code
KDC_ERR_POLICY [RFC4120]. There is no accompanying e-data defined
in this document. Note this is applicable to both critical and
optional authorization data.
o If the authorization data element is unknown, the TGS MAY remove
it, or transfer it into the returned anonymous ticket, or reject
the authentication attempt, based on local policy for that
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authorization data type unless otherwise specified. If there is
no policy defined for a given unknown authorization data type, the
authentication MUST be rejected. The error code is KDC_ERR_POLICY
when the authentication is rejected.
The AD-INITIAL-VERIFIED-CAS authorization data [RFC4556] MAY be
removed from an anonymous ticket based on local policy of the TGS.
The TGS MUST add the name of the previous realm according to Section
3.3.3.2 of [RFC4120]. If the client's realm is the anonymous realm,
the abbreviation forms [RFC4120] that build on the preceding name
cannot be used at the start of the transited encoding. The null-
subfield form (e.g., encoding ending with ",") [RFC4120] could not be
used next to the anonymous realm that can potentially be at the
beginning where the client realm is filled in.
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 authtime of the ticket in the PA-TGS-REQ pre-
authentication data [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 the client is anonymous and the KDC does not have a key to encrypt
the reply (this can happen when, for example, the KDC does not
support PKINIT [RFC4556]), the KDC MUST return an error message with
the code KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying
e-data defined in this document.
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 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].
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.
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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 initiator name that is returned from the acceptor side
upon the 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
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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.
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
JK Jaganathan helped editing early revisions of this document.
Clifford Neuman contributed the core notions of this document.
Ken Raeburn reviewed the document and provided suggestions for
improvements.
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.
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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)",
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
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Full Copyright Statement
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This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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