draft-ietf-krb-wg-anon-05.txt   draft-ietf-krb-wg-anon-06.txt 
NETWORK WORKING GROUP L. Zhu NETWORK WORKING GROUP L. Zhu
Internet-Draft P. Leach Internet-Draft P. Leach
Updates: 4120 (if approved) Microsoft Corporation Updates: 4120 (if approved) Microsoft Corporation
Intended status: Standards Track January 27, 2008 Intended status: Standards Track May 15, 2008
Expires: July 30, 2008 Expires: November 16, 2008
Anonymity Support for Kerberos Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-05 draft-ietf-krb-wg-anon-06
Status of this Memo Status of this Memo
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This Internet-Draft will expire on July 30, 2008. This Internet-Draft will expire on November 16, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract Abstract
This document defines extensions to the Kerberos protocol for the This document defines extensions to the Kerberos protocol for the
Kerberos client to authenticate the Kerberos Key Distribution Center Kerberos client to authenticate the Kerberos Key Distribution Center
and the Kerberos server, without revealing the client's identity. It and the Kerberos server, without revealing the client's identity. It
updates RFC 4120. These extensions can be used to secure updates RFC 4120. These extensions can be used to secure
communication between the anonymous client and the server. communication between the anonymous client and 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 . . . . . . . . . . . . . . . . . . . . . 4 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 4
5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 7 5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Normative References . . . . . . . . . . . . . . . . . . . . . 9 9. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 11
1. Introduction 1. Introduction
In certain situations, the Kerberos [RFC4120] client may wish to In certain situations, the Kerberos [RFC4120] client may wish to
authenticate a server and/or protect communications without revealing authenticate a server and/or protect communications without revealing
its own identity. For example, consider an application which its own identity. For example, consider an application which
provides read access to a research database, and which permits provides read access to a research database, and which permits
queries by arbitrary requestors. A client of such a service might queries by arbitrary requestors. A client of such a service might
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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
The anonymous Kerberos realm name is defined as a well-known realm The anonymous Kerberos realm name is defined as a well-known realm
name based on [KRBNAM]. The value is the literal "WELLKNOWN: name based on [KRBNAM]. The value is the literal "WELLKNOWN:
ANONYMOUS". An anonymous Kerberos realm name MUST NOT be present in ANONYMOUS". An anonymous Kerberos realm name MUST NOT be present in
the transited field of a ticket. the transited field of a ticket. However, as specified in Section 4,
the true name of the realm that issued the anonymous ticket MAY be
present in the transited field of a ticket.
The anonymous Kerberos principal name is defined as a well-known The anonymous Kerberos principal name is defined as a well-known
Kerberos principal name based on [KRBNAM]. The value of the name- Kerberos principal name based on [KRBNAM]. The value of the name-
type field is KRB_NT_WELLKNOWN [KRBNAM], and 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: string field is a sequence of two KerberosString components:
"WELLKNOWN", "ANONYMOUS". "WELLKNOWN", "ANONYMOUS".
Note that in this specification, the anonymous principal name and Note that in this specification, the anonymous principal name and
realm are only applicable to the client in Kerberos messages, the realm are only applicable to the client in Kerberos messages, the
server MUST NOT be anonymous in any Kerberos message. server MUST NOT be anonymous in any Kerberos message.
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TicketFlags ::= KerberosFlags TicketFlags ::= KerberosFlags
-- anonymous(14) -- anonymous(14)
-- TicketFlags and KerberosFlags are defined in [RFC4120] -- TicketFlags and KerberosFlags are defined in [RFC4120]
An anonymous ticket is a ticket that has all of the following An anonymous ticket is a ticket that has all of the following
properties: properties:
o The cname field contains the anonymous Kerberos principal name. o The cname field contains the anonymous Kerberos principal name.
o The crealm field contains the client's realm name, or the name of 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, the realm that issued the initial ticket for the client principal
or the anonymous realm name. (when the anonymous ticket is obtained using anonymous PKINIT from
a Kerberos realm other than that of the client as described in
Section 4), or the anonymous realm name.
o The anonymous ticket contains no information that can reveal the o The anonymous ticket contains no information that can reveal the
client's identity. However the ticket may contain the client client's identity. However the ticket may contain the client
realm, intermediate realms on the client's authentication path, realm, intermediate realms on the client's authentication path,
and authorization data that may provide information related to the and authorization data that may provide information related to the
client's identity. For example, an anonymous principal that is client's identity. For example, an anonymous principal that is
identifiable only within a particular group of users can be identifiable only within a particular group of users can be
implemented using authorization data and such authorization data, implemented using authorization data and such authorization data,
if included in the anonymous ticket, shall disclose the client's if included in the anonymous ticket, shall disclose the client's
membership of that group. membership of that group.
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4. Protocol Description 4. Protocol Description
In order to request an anonymous ticket, the client sets the In order to request an anonymous ticket, the client sets the
anonymous KDC option in an Authentication Exchange (AS) or Ticket anonymous KDC option in an Authentication Exchange (AS) or Ticket
Granting Service (TGS) request. The client can request an anonymous Granting Service (TGS) request. The client can request an anonymous
Ticket Granting Ticket (TGT) based on a normal TGT. Unless otherwise Ticket Granting Ticket (TGT) based on a normal TGT. Unless otherwise
specified, the client can obtain an anonymous ticket with the specified, the client can obtain an anonymous ticket with the
anonymous realm name only by requesting an anonymous ticket in an AS anonymous realm name only by requesting an anonymous ticket in an AS
exchange with the client realm set as anonymous in the request. exchange with the client realm set as anonymous in the request.
If the client wishes to authenticate the KDC anonymously, it sets the If the client wishes to authenticate to the KDC anonymously, it sets
client name as anonymous in the AS exchange and provides a the client name as anonymous in the AS exchange and provides a
PA_PK_AS_REQ pre-authentication data [RFC4556] where both the PA_PK_AS_REQ pre-authentication data [RFC4556] where both the
signerInfos field and the certificates field of the SignedData signerInfos field and the certificates field of the SignedData
[RFC3852] of the PA_PK_AS_REQ are empty. Because the anonymous [RFC3852] of the PA_PK_AS_REQ are empty. Because the anonymous
client does not have an associated asymmetric key pair, the client client does not have an associated asymmetric key pair, the client
MUST choose the Diffie-Hellman key agreement method by filling in the MUST choose the Diffie-Hellman key agreement method by filling in the
Diffie-Hellman domain parameters in the clientPublicValue [RFC4556]. Diffie-Hellman domain parameters in the clientPublicValue [RFC4556].
If the ticket in the PA-TGS-REQ of the TGS request is anonymous, or 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 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 option MUST be set in the request. Otherwise, the KDC MUST return a
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[RFC4556]. If all goes well, an anonymous ticket is generated [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] 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 pre-authentication data is included in the KDC reply according to
[RFC4556]. If the KDC does not have an asymmetric key pair, it MAY [RFC4556]. If the KDC does not have an asymmetric key pair, it MAY
reply anonymously or reject the authentication attempt. If the KDC reply anonymously or reject the authentication attempt. If the KDC
replies anonymously, both the signerInfos field and the certificates replies anonymously, both the signerInfos field and the certificates
field of the SignedData [RFC3852] of PA_PK_AS_REP in the reply are 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 empty. The server name in the anonymous KDC reply contains the name
of the TGS. of the TGS.
The KDC conforming to this specification MUST indicate the support of
anonymous PKINIT as described above in this section according to
Section 3.4 of [RFC4556].
Upon receipt of the KDC reply that contains an anonymous ticket and a 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 PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
authenticate the KDC based on the KDC's signature in the 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 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 (the reply is anonymous), the client MUST reject the returned ticket
if it cannot authenticate the KDC otherwise. if it cannot authenticate the KDC otherwise.
The client can use the client keys to mutually authenticate with the 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, 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 the reply key is selected as normal according to Section 3.1.3 of
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optional authorization data. optional authorization data.
o If the authorization data element is unknown, the TGS MAY remove o If the authorization data element is unknown, the TGS MAY remove
it, or transfer it into the returned anonymous ticket, or reject it, or transfer it into the returned anonymous ticket, or reject
the authentication attempt, based on local policy for that the authentication attempt, based on local policy for that
authorization data type unless otherwise specified. If there is authorization data type unless otherwise specified. If there is
no policy defined for a given unknown authorization data type, the no policy defined for a given unknown authorization data type, the
authentication MUST be rejected. The error code is KDC_ERR_POLICY authentication MUST be rejected. The error code is KDC_ERR_POLICY
when the authentication is rejected. when the authentication is rejected.
The AD-INITIAL-VERIFIED-CAS authorization data [RFC4556] MAY be 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. 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 The TGS encodes the name of the previous realm into the transited
3.3.3.2 of [RFC4120]. If the client's realm is the anonymous realm, field according to Section 3.3.3.2 of [RFC4120]. Based on local
the abbreviation forms that build on the preceding name cannot be policy, the TGS MAY omit the previous realm if the cross realm TGT is
used at the start of the transited encoding. The null-subfield form an anonymous one to hide the authentication path of the client. The
(e.g., encoding ending with ",") could not be used next to the unordered set of realms in the transited field, if present, can
anonymous realm that can potentially be at the beginning where the reveal which realm may potentially be the realm of the client or the
client realm is filled in. realm that issued the anonymous TGT.
If the client is anonymous and the KDC does not have a key to encrypt 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 the reply (this can happen when, for example, the KDC does not
support PKINIT [RFC4556]), the KDC MUST return an error message with support PKINIT [RFC4556]), the KDC MUST return an error message with
the code KDC_ERR_NULL_KEY [RFC4120]. the code KDC_ERR_NULL_KEY [RFC4120].
If a client requires anonymous communication then the client MUST If a client requires anonymous communication then the client MUST
check to make sure that the ticket in the reply is actually anonymous check to make sure that the ticket in the reply is actually anonymous
by checking the presence of the anonymous ticket flag. This is by checking the presence of the anonymous ticket flag in the flags
because KDCs ignore unknown KDC options. A KDC that does not field of the EncKDCRepPart. This is because KDCs ignore unknown KDC
understand the anonymous KDC option will not return an error, but options. A KDC that does not understand the anonymous KDC option
will instead return a normal ticket. will not return an error, but will 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]. described in [RFC4120].
A server accepting 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.
5. GSS-API Implementation Notes 5. GSS-API Implementation Notes
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Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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