One document matched: draft-ietf-krb-wg-anon-07.txt
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NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach
Updates: 4120 (if approved) Microsoft Corporation
Intended status: Standards Track July 27, 2008
Expires: January 28, 2009
Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-07
Status of this Memo
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Abstract
This document defines extensions to the Kerberos protocol for the
Kerberos client to authenticate the Kerberos Key Distribution Center
(KDC) and the Kerberos server, without revealing the client's
identity to the server or to the KDC. It updates RFC 4120. 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 . . . . . . . . . . . . . . . . . . . . . 10
9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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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 Kerberos 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.
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".
The anonymous Kerberos principal name is defined as a well-known
Kerberos principal name based on [KRBNAM]. The value of the name-
type field is KRB_NT_WELLKNOWN [KRBNAM], and the value of the name-
string field is a sequence of two KerberosString components:
"WELLKNOWN", "ANONYMOUS".
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]
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This is a new ticket flag that is used to indicate a ticket is an
anonymous one.
An anonymous ticket is a ticket that has all of the following
properties:
o The cname field contains the anonymous Kerberos principal name.
o The crealm field contains the client's realm name, or the name of
the realm that issued the initial ticket for the client principal
(when the anonymous ticket is obtained using anonymous Public Key
Cryptography for Initial Authentication in Kerberos (PKINIT) as
defined in Section 4, 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 Service exchange (AS) or
Ticket Granting Service (TGS) exchange. 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
AS exchange with the client realm set as anonymous in the request.
If the client wishes to authenticate to the KDC anonymously, it sets
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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].
This use of the anonymous client name in conjunction with PKINIT is
referred to as anonymous PKINIT. If anonymous PKINIT is used, the
realm name in the returned anonymous ticket MUST be the anonymous
realm.
If the ticket in the PA-TGS-REQ 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.
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 with the code KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED
[RFC4556]. 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.
A KDC that supports anonymous PKINIT MUST indicate the support of
PKINIT according to Section 3.4 of [RFC4556].
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 EncTicketPart of the
reply 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 KDC-REP in
subsequent message exchanges when using the obtained anonymous
ticket.
When propagating authorization data in the ticket or in the enc-
authorization-data field 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. AS or
TGS SHOULD NOT populate identity-based authorization data into an
anonymous ticket in that such authorization data 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. 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
authorization data type unless otherwise specified. If there is
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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 as defined in
[RFC4556] contains the issuer name of the client certificate. If it
is undesirable to disclose such information about the client's
identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be
removed from an anonymous ticket based on local policy of the TGS.
The TGS encodes the name of the previous realm into the transited
field according to Section 3.3.3.2 of [RFC4120]. Based on local
policy, the TGS MAY omit the previous realm if the cross realm TGT is
an anonymous one to hide the authentication path of the client. The
unordered set of realms in the transited field, if present, can
reveal which realm may potentially be the realm of the client or the
realm that issued the anonymous TGT. The anonymous Kerberos realm
name MUST NOT be present in the transited field of a ticket. The
true name of the realm that issued the anonymous ticket MAY be
present in the transited field of a ticket.
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].
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 in the flags
field of the EncKDCRepPart. 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].
Note that the anonymous principal name and realm are only applicable
to the client in Kerberos messages, the server cannot be anonymous in
any Kerberos message per this specification.
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, a client requiring anonymous
communication needs to make sure that the returned 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.
The client's real identity is not revealed when the client is
authenticated as the anonymous principal. Application servers MAY
reject the authentication in order to, for example, prevent
information disclosure or as part of Denial of Service (DOS)
prevention. Application servers MUST avoid accepting anonymous
credentials in situations where they must record the client's
identity; for example, when there must be an audit trail.
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.
Miguel Garcia and Phillip Hallam-Baker reviewed the document and
provided helpful suggestions.
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In addition, the following individuals made significant
contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love
Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.
8. IANA Considerations
This document defines a new 'anonymous' Kerberos well-known name and
a new 'anonymous' Kerberos well-known realm based on [KRBNAM]. IANA
is requested to add these two values to the Kerberos naming
registries that are created in [KRBNAM].
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.
[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
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Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: paulle@microsoft.com
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