draft-ietf-krb-wg-anon-00.txt   draft-ietf-krb-wg-anon-01.txt 
NETWORK WORKING GROUP L. Zhu NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach Internet-Draft P. Leach
Updates: 4120 (if approved) K. Jaganathan Updates: 4120 (if approved) K. Jaganathan
Expires: December 5, 2006 Microsoft Corporation Expires: January 17, 2007 Microsoft Corporation
June 3, 2006 July 16, 2006
Anonymity Support for Kerberos Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-00 draft-ietf-krb-wg-anon-01
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware 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 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. aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on December 5, 2006. This Internet-Draft will expire on January 17, 2007.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
This document defines the use of anonymous Kerberos tickets for the This document defines the use of anonymous Kerberos tickets for the
purpose of authenticating the servers and enabling secure purpose of authenticating the servers and enabling secure
communication between a client and a server, without identifying the communication between a client and a server, without identifying the
client to the server. client to the server.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5
5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 6 5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7 9. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 11
1. Introduction 1. Introduction
In certain situations or environments, the Kerberos [RFC4120] client In certain situations or environments, the Kerberos [RFC4120] client
may wish to authenticate a server and/or protect communications may wish to authenticate a server and/or protect communications
without revealing its own identity. For example, consider an without revealing its own identity. For example, consider an
application which provides read access to a research database, and application which provides read access to a research database, and
which permits queries by arbitrary requestors. A client of such a which permits queries by arbitrary requestors. A client of such a
service might wish to authenticate the service, to establish trust in service might wish to authenticate the service, to establish trust in
the information received from it, but might not wish to disclose its the information received from it, but might not wish to disclose its
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communicates. communicates.
2. Conventions Used in This Document 2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Definitions 3. Definitions
An anonymous ticket is a ticket that has all of the following The anonymous Kerberos realm name is a reserved realm name as defined
properties: in [KRBNAM] and its value is the literal "RESERVED:ANONYMOUS".
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 The anonymous Kerberos principal name is a reserved Kerberos
anonymous Kerberos realm name is defined as follows: it is a principal name as defined in [KRBNAM], its name-type [RFC4120] is
reserved realm name as defined in [KRBNAM] and the realm name is KRB_NT_RESRVED [KRBNAM], and its name-string [RFC4120] is a sequence
the literal "RESERVED:ANONYMOUS". of two KerberosString components: "RESERVED", "ANONYMOUS".
o The authtime field in the ticket is set to the time of the ticket In this specification, only the client name or the client realm can
request, not the time of the initial authentication for the be anonymous; the server name or the server realm can not be
principal who has made the request. anonymous.
o The transited field [RFC4120] can either contain the client's The transited field [RFC4120] of a ticket is an anonymous
authentication path or contain the anonymous authentication path authentication path if the tr-type field of the TransitedEncoding
defined as follows: the tr-type field of the transited field is type [RFC4120] is NO-TRANSITED-INFO and the contents field is an
NO-TRANSITED-INFO (as defined later in this section) and the empty OCTET STRING.
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 NO-TRANSITED-INFO TBA
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 This transited encoding type indicates that there is no information
set. available about the authentication path.
The anonymous ticket flag is defined as bit 14 (with the first bit The anonymous ticket flag is defined as bit TBA (with the first bit
being bit 0) in the TicketFlags: being bit 0) in the TicketFlags:
TicketFlags ::= KerberosFlags TicketFlags ::= KerberosFlags
-- anonymous(14) -- anonymous(TBA)
-- TicketFlags and KerberosFlags are defined in [RFC4120] -- TicketFlags and KerberosFlags are defined in [RFC4120]
The anonymous ticket flag MUST NOT be set by implementations of this An anonymous ticket is a ticket that has all of the following
specification if the ticket is not an anonymous ticket as defined in properties:
this section.
The request-anonymous KDC option is defined as bit 14 (with the first o The cname field [RFC4120] contains the anonymous Kerberos
bit being bit 0) in the KDCOptions: principal name.
KDCOptions ::= KerberosFlags o The crealm field [RFC4120] contains either the realm name of the
-- request-anonymous(14) client who made the request or the anonymous kerberos realm name,
-- KDCOptions and KerberosFlags are defined in [RFC4120] based on the local policy of the KDC.
The anonymous transited encoding type is defined as follows: 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.
NO-TRANSITED-INFO 0 o It contains no information that can reveal the client's identity.
This transited encoding type indicates that there is no information However the ticket can contain the client realm and the realms on
available about the authentication path. 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.
Note that the server principal name and the server realm in a cross- o The anonymous ticket flag is set.
realm referral TGT are not dependent on whether the client is the
anonymous principal or not. 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:
KDCOptions ::= KerberosFlags
-- request-anonymous(TBA)
-- KDCOptions and KerberosFlags are defined in [RFC4120]
4. Protocol Description 4. Protocol Description
In order to request an anonymous ticket, the client sets the request- 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 anonymous KDC option in an Authentication Exchange (AS) or Ticket
the service ticket in the PA-TGS-REQ [RFC4120] is anonymous, the Granting Service (TGS) request [RFC4120]. The client can request an
request-anonymous KDC option MUST be set in the request. 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 policy allows, the KDC issues an anonymous ticket. The KDC that When propagating authorization data, care MUST be taken by the TGS to
implements this specification MUST NOT carry information that can ensure that the client confidentiality is not violated: the TGS MUST
reveal the client's identity, from the TGS request into the returned either fail the request or remove authorization data that may reveal
anonymous ticket. 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.
It should be noted that unless otherwise specified by this document ad-type References Can Breach Confidentiality?
the client principal name and the client realm in the Kerberos ------------------------------------------------------------------
messages [RFC4120] should be the client name and client realm that AD-IF-RELEVANT RFC4120 Yes, remove if unknown
can uniquely identify the client principal to the KDC, not the AD-KDCIssued RFC4120 Yes, remove if unknown
anonymous client principal name and the empty realm name. For AD-AND-OR RFC4120 Yes, remove if unknown
example, the client name and realm in the request body and the AD-MANDATORY-FOR-KDC RFC4120 Yes, fail the request if unknown
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 If it is inappropriate to remove an authorization element from the
is inappropriate to remove information (such as restrictions) from TGS request in order to produce an anonymous ticket, the KDC MUST
the TGS request in order to produce an anonymous ticket, the KDC MUST
return an error message with the code KDC_ERR_POLICY [RFC4120]. return an error message with the code KDC_ERR_POLICY [RFC4120].
If a client requires anonymous communication then the client should When policy allows, the KDC issues an anonymous ticket. The client
check to make sure that the resulting ticket is actually anonymous by realm in the anonymous ticket can be the anonymous realm name based
checking the presence of the anonymous ticket flag. Because KDCs 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
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 ignore unknown KDC options, a KDC that does not understand the
request-anonymous KDC option will not return an error, but will request-anonymous KDC option will not return an error, but will
instead return a normal ticket. instead return a normal ticket.
The subsequent client and server communications then proceed as The subsequent client and server communications then proceed as
described in [RFC4120]. The client principal name in the described in [RFC4120]. No transited policy checking is needed for
Authenticator of the KRB_AP_REQ MUST be the anonymous client the anonymous authentication path. However, transited policy checks
principal name and the client realm of the Authenticator MUST be an defined in Section 2.7 of [RFC4120] would apply to an anonymous
empty KerberosString [RFC4120]. ticket that contains a "normal" authentication path.
A server accepting such an anonymous service ticket may assume that A server accepting an anonymous service ticket may assume that
subsequent requests using the same ticket originate from the same subsequent requests using the same ticket originate from the same
client. Requests with different tickets are likely to originate from client. Requests with different tickets are likely to originate from
different clients. different clients.
Interoperability and backward-compatibility notes: the KDC is given Interoperability and backward-compatibility notes: the KDC is given
the task of rejecting a request for an anonymous ticket when the the task of rejecting a request for an anonymous ticket when the
anonymous ticket is not acceptable by the server. anonymous ticket is not acceptable by the server.
5. GSS-API Implementation Notes 5. GSS-API Implementation Notes
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attributes from the initial call to GSS_Init_Sec_Context() when attributes from the initial call to GSS_Init_Sec_Context() when
requesting anonymity, because (as in the GSS-API tradition and for requesting anonymity, because (as in the GSS-API tradition and for
backwards compatibility) anonymity is just another optional context backwards compatibility) anonymity is just another optional context
attribute. It could be that the mechanism doesn't recognize the attribute. It could be that the mechanism doesn't recognize the
attribute at all or that anonymity is not available for some other attribute at all or that anonymity is not available for some other
reasons -- and in that case the initiator must NOT send the initial reasons -- and in that case the initiator must NOT send the initial
security context token to the acceptor, because it will likely reveal security context token to the acceptor, because it will likely reveal
the initiators identity to the acceptor, something that can rarely be the initiators identity to the acceptor, something that can rarely be
"un-done". "un-done".
GSS-API defines name_type GSS_C_NT_ANONYMOUS [RFC2743] to represent GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
an anonymous identity. In addition, according to Section 2.1.1 of represent the anonymous identity. In addition, Section 2.1.1 of
[RFC1964] the string representation of the anonymous client principal [RFC1964] defines the single string representation of a Kerberos
name can be "RESERVED/ANONYMOUS" or "RESERVED/ principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For
ANONYMOUS@RESERVED:ANONYMOUS" with the name_type the anonymous principals, the name component within the exportable
GSS_KRB5_NT_PRINCIPAL_NAME. Implementations conforming to this name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm
specification MUST be able to accept the GSS_C_NT_ANONYMOUS name form name according to Section 2.1.1 of [RFC1964]. In this specification
and the GSS_KRB5_NT_PRINCIPAL_NAME name forms, and consider them only the client/initiator can be the anonymous identity.
equivalent.
Portable initiators are RECOMMENDED to use default credentials Portable initiators are RECOMMENDED to use default credentials
whenever possible, and request anonymity only through the input whenever possible, and request anonymity only through the input
anon_req_flag to GSS_Init_Sec_Context(). anon_req_flag [RFC2743] to GSS_Init_Sec_Context().
6. Security Considerations 6. Security Considerations
Since KDCs ignore unknown options [RFC4120], a client requiring Since KDCs ignore unknown options [RFC4120], a client requiring
anonymous communication needs to make sure that the ticket is anonymous communication needs to make sure that the ticket is
actually anonymous. A KDC that that does not understand the actually anonymous. A KDC that that does not understand the
anonymous option would not return an anonymous ticket. anonymous option would not return an anonymous ticket.
By using the mechanism defined in this specification, the client does By using the mechanism defined in this specification, the client does
not reveal its identity to the server but its identity may be not reveal its identity to the server but its identity may be
revealed to the KDC of the server principal (when the server revealed to the KDC of the server principal (when the server
principal is in a different realm than that of the client), and any principal is in a different realm than that of the client), and any
KDC on the cross-realm authentication path. The Kerberos client MUST KDC on the cross-realm authentication path. The Kerberos client MUST
verify the ticket being used are indeed anonymous before verify the ticket being used is indeed anonymous before communicating
communicating with the cross-realm KDC or the server, otherwise the with the cross-realm KDC or the server, otherwise the client's
client's identity may be revealed to the server unintentionally. identity may be revealed to the server unintentionally.
In cases where specific server principals must not have access to the In cases where specific server principals must not have access to the
client's identity (for example, an anonymous poll service), the KDC client's identity (for example, an anonymous poll service), the KDC
can define server principal specific policy that insure any normal can define server principal specific policy that insure any normal
service ticket can NEVER be issued to any of these server principals. 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 7. Acknowledgements
The authors would like to thank the following individuals for their The authors would like to thank the following individuals for their
insightful comments and fruitful discussions: Sam Hartman, Martin insightful comments and fruitful discussions: Sam Hartman, Clifford
Rex, Nicolas Williams, Jeffery Altman, Tom Yu, Chaskiel M Grundman, Neuman, Martin Rex, Nicolas Williams, Jeffery Altman, Tom Yu,
Love Hoernquist Aestrand, Jeffery Hutzelman, and Clifford Neuman. Chaskiel M Grundman, Love Hoernquist Aestrand, and Jeffery Hutzelman.
8. IANA Considerations 8. IANA Considerations
No IANA actions are required for this document. No IANA actions are required for this document.
9. Normative References 9. Normative References
[KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints", [KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints",
draft-ietf-krb-wg-naming, work in progress. draft-ietf-krb-wg-naming, work in progress.
skipping to change at page 9, line 5 skipping to change at page 9, line 21
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999. RFC 2246, January 1999.
[RFC2743] Linn, J., "Generic Security Service Application Program [RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000. Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120, Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005. July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
Authors' Addresses Authors' Addresses
Larry Zhu Larry Zhu
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
US US
Email: lzhu@microsoft.com Email: lzhu@microsoft.com
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