draft-ietf-krb-wg-anon-08.txt   draft-ietf-krb-wg-anon-09.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 August 20, 2008 Intended status: Standards Track September 10, 2008
Expires: February 21, 2009 Expires: March 14, 2009
Anonymity Support for Kerberos Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-08 draft-ietf-krb-wg-anon-09
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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The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on February 21, 2009. This Internet-Draft will expire on March 14, 2009.
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
(KDC) and the Kerberos server, without revealing the client's (KDC) and the Kerberos server, without revealing the client's
identity or the client's realm to the server or to the KDC. It identity or the client's realm to the server or to the KDC. 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 . . . . . . . . . . . . . . . . . . . . . 5 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5
4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . . 5 4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . . 5
4.1.1. Anonymous PKINIT . . . . . . . . . . . . . . . . . . . 6
4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 7 4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 7
4.3. Subsequent Exchanges and Protocol Actions Common to AS 4.3. Subsequent Exchanges and Protocol Actions Common to AS
and TGS for Anonymity Support . . . . . . . . . . . . . . 9 and TGS for Anonymity Support . . . . . . . . . . . . . . 9
5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 9 5. Interoperability Requirements . . . . . . . . . . . . . . . . 10
6. The Choice of the Ticket Session Key of a Ticket Obtained 6. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 10
using PKINIT . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. PKINIT Client Contribution to the Ticket Session Key . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7.1. Combinging Two protocol Keys . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
10. Normative References . . . . . . . . . . . . . . . . . . . . . 13 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15 11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
11.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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
the client's own identity. For example, consider an application the client's own identity. For example, consider an application
which provides read access to a research database, and which permits which 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
wish to authenticate the service, to establish trust in the wish to authenticate the service, to establish trust in the
information received from it, but might not wish to disclose the information received from it, but might not wish to disclose the
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o The crealm field contains the client's realm name or the anonymous o The crealm field contains the client's realm name or the anonymous
realm name. 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, would disclose the client's
membership of that group. membership of that group.
o The anonymous ticket flag is set. o The anonymous ticket flag is set.
The anonymous KDC option is defined as bit 14 (with the first bit The anonymous KDC option is defined as bit 14 (with the first bit
being bit 0) in the KDCOptions: being bit 0) in the KDCOptions:
KDCOptions ::= KerberosFlags KDCOptions ::= KerberosFlags
-- anonymous(14) -- anonymous(14)
-- KDCOptions and KerberosFlags are defined in [RFC4120] -- KDCOptions and KerberosFlags are defined in [RFC4120]
As described in Section 4, the anonymous KDC option is set to request As described in Section 4, the anonymous KDC option is set to request
an anonymous ticket. an anonymous ticket in an Authentication Service (AS) request or an
Ticket Granting Service (TGS) request.
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 Service exchange (AS) or anonymous KDC option in an AS request or an TGS request.
Ticket Granting Service (TGS) exchange.
The rest of this section is organized as follows: it first describes The rest of this section is organized as follows: it first describes
protocol actions specific to AS exchanges, then it describes those of protocol actions specific to AS exchanges, then it describes those of
TGS exchange. These are then followed by the decription of protocol TGS exchange. These are then followed by the decription of protocol
actions common to both AS and TGS and those in subsequent exchanges. actions common to both AS and TGS and those in subsequent exchanges.
4.1. Anonymity Support in AS Exchange 4.1. Anonymity Support in AS Exchange
The client requests an anonymous ticket by setting the anonymous KDC The client requests an anonymous ticket by setting the anonymous KDC
option in an AS exchange. option in an AS exchange.
The Kerberos client can use the client's long term keys to The Kerberos client can use the client's long term keys, or the
authenticate with the KDC and requests an anonymous ticket in an AS client's X.509 certificates [RFC4556], or any other preauthenication
exchange where the client's identity is known to the KDC. data, to authenticate to the KDC and requests an anonymous ticket in
an AS exchange where the client's identity is known to the KDC.
Alternatively the Kerberos client can request an anonymous ticket
without revealing the client's identity to the KDC as follows: the
client sets the client name as the anonymous principal 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 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
KRB-ERROR message with the code KDC_ERR_BADOPTION. 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.
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].
When policy allows, the KDC issues an anonymous ticket. If the When policy allows, the KDC issues an anonymous ticket. If the
client name in the request is the anonymous principal, the client client name in the request is the anonymous principal, the client
realm (crealm) in the reply is the anonymous realm, otherwise the realm (crealm) in the reply is the anonymous realm, otherwise the
client realm is the realm of the AS. According to [RFC4120] the client realm is the realm of the AS. According to [RFC4120] the
client name and the client realm in the EncTicketPart of the reply client name and the client realm in the EncTicketPart of the reply
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The AD-INITIAL-VERIFIED-CAS authorization data as defined in The AD-INITIAL-VERIFIED-CAS authorization data as defined in
[RFC4556] contains the issuer name of the client certificate. This [RFC4556] contains the issuer name of the client certificate. This
authorization is not applicable and MUST NOT be present in the authorization is not applicable and MUST NOT be present in the
returned anonymous ticket when anonymous PKINIT is used. When the returned anonymous ticket when anonymous PKINIT is used. When the
client is authenticated (i.e. anonymous PKINIT is not used), if it is client is authenticated (i.e. anonymous PKINIT is not used), if it is
undesirable to disclose such information about the client's identity, undesirable to disclose such information about the client's identity,
the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be removed from the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be removed from
the returned anonymous ticket. the returned anonymous ticket.
Note that in order to obtain an anonymous ticket with the anonymous The client can use the client keys to mutually authenticate with the
realm name, the client MUST set the client name as the anonymous KDC, request an anonymous TGT in the AS request. And in that case,
principal in the request when requesting an anonymous ticket in an AS the reply key is selected as normal according to Section 3.1.3 of
exchange. Anonymity PKINIT is the only way via which an anonymous [RFC4120].
ticket with the anonymous realm as the client realm can be generated
in this specification. 4.1.1. Anonymous PKINIT
This sub-section defines anonymity PKINIT.
As described earlier in this section, the client can request an
anonymous ticket by authenticating to the KDC using the client's
identity; alternatively without revealing the client's identity to
the KDC, the Kerberos client can request an anonymous ticket as
follows: the client sets the client name as the anonymous principal
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.
Upon receiving the anonymous PKINIT request from the 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.
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
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].
A KDC that supports anonymous PKINIT MUST indicate the support of A KDC that supports anonymous PKINIT MUST indicate the support of
PKINIT according to Section 3.4 of [RFC4556]. PKINIT according to Section 3.4 of [RFC4556].
Note that in order to obtain an anonymous ticket with the anonymous
realm name, the client MUST set the client name as the anonymous
principal in the request when requesting an anonymous ticket in an AS
exchange. Anonymity PKINIT is the only way via which an anonymous
ticket with the anonymous realm as the client realm can be generated
in this specification.
4.2. Anonymity Support in TGS Exchange 4.2. Anonymity Support in TGS Exchange
In a TGS request, the client can request an anonymous Ticket Granting The client requests an anonymous ticket by setting the anonymous KDC
Ticket (TGT) or an anonymous cross realm TGT using a normal TGT, in option in a TGS exchange, and in that request the client can use a
which case the client's identity is known to the TGS. The client can normal Ticket Granting Ticket (TGT) with the client's identity, or an
also use an anonymous TGT to request an anonymous service ticket. anonymous TGT, or an anonymous cross realm TGT. If the client uses a
normal TGT, the client's identity is known to the TGS.
Note that the client can completely hide the client's identity in an Note that the client can completely hide the client's identity in an
AS exchange using anonymous PKINIT as described in the previous AS exchange using anonymous PKINIT as described in the previous
section. section.
If the ticket in the PA-TGS-REQ of the TGS request is an anonymous If the ticket in the PA-TGS-REQ of the TGS request is an anonymous
one, the anonymous KDC option MUST be set in the request. Otherwise, one, the anonymous KDC option MUST be set in the request. Otherwise,
the KDC MUST return a KRB-ERROR message with the code the KDC MUST return a KRB-ERROR message with the code
KDC_ERR_BADOPTION. KDC_ERR_BADOPTION.
When policy allows, the KDC issues an anonymous ticket. If the When policy allows, the KDC issues an anonymous ticket. If the
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identity. The specification of a new authorization data type MUST identity. The specification of a new authorization data type MUST
specify the processing rules of the authorization data when an specify the processing rules of the authorization data when an
anonymous ticket is returned. If there is no processing rule defined anonymous ticket is returned. If there is no processing rule defined
for an authorization data element or the authorization data element for an authorization data element or the authorization data element
is unknown, the TGS MUST process it when an anonymous ticket is is unknown, the TGS MUST process it when an anonymous ticket is
returned as follows: returned as follows:
o If the authorization data element may reveal the client's o If the authorization data element may reveal the client's
identity, it MUST be removed unless otherwise specified. identity, it MUST be removed unless otherwise specified.
o If the authorization data element is intended to restrict the use o If the authorization data element, that could reveal's the
of the ticket or limit the rights otherwise conveyed in the client's identity. is intended to restrict the use of the ticket
ticket, it cannot be removed in order to hide the client's or limit the rights otherwise conveyed in the ticket, it cannot be
identity. In this case, the authentication attempt MUST be removed in order to hide the client's identity. In this case, the
rejected, and the TGS MUST return an error message with the code authentication attempt MUST be rejected, and the TGS MUST return
KDC_ERR_POLICY. Note this is applicable to both critical and an error message with the code KDC_ERR_POLICY. Note this is
optional authorization data. applicable to both critical and 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 as defined in The AD-INITIAL-VERIFIED-CAS authorization data as defined in
[RFC4556] contains the issuer name of the client certificate. If it [RFC4556] contains the issuer name of the client certificate. If it
is undesirable to disclose such information about the client's is undesirable to disclose such information about the client's
identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be
removed from an anonymous ticket. removed from an anonymous ticket.
The TGS encodes the name of the previous realm into the transited 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 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 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 an anonymous one in order to hide the authentication path of the
unordered set of realms in the transited field, if present, can client. The unordered set of realms in the transited field, if
reveal which realm may potentially be the realm of the client or the present, can reveal which realm may potentially be the realm of the
realm that issued the anonymous TGT. The anonymous Kerberos realm client or the realm that issued the anonymous TGT. The anonymous
name MUST NOT be present in the transited field of a ticket. The Kerberos realm name MUST NOT be present in the transited field of a
true name of the realm that issued the anonymous ticket MAY be ticket. The true name of the realm that issued the anonymous ticket
present in the transited field of a ticket. MAY be present in the transited field of a ticket.
4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for 4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for
Anonymity Support Anonymity Support
In both AS and TGS exchanges, the realm field in the KDC request is In both AS and TGS exchanges, the realm field in the KDC request is
always the realm of the target KDC, not the anonymous realm when the always the realm of the target KDC, not the anonymous realm when the
client requests an anonymous ticket. client requests an anonymous ticket.
Absent other information the KDC MUST NOT include any identifier in
the returned anonymous ticket that could reveal the client's identity
to the server.
Unless anonymous PKINIT is used, if a client requires anonymous Unless anonymous PKINIT is used, if a client requires anonymous
communication then the client MUST check to make sure that the ticket communication then the client MUST check to make sure that the ticket
in the reply is actually anonymous by checking the presence of the in the reply is actually anonymous by checking the presence of the
anonymous ticket flag in the flags field of the EncKDCRepPart. This anonymous ticket flag in the flags field of the EncKDCRepPart. This
is because KDCs ignore unknown KDC options. A KDC that does not is because KDCs ignore unknown KDC options. A KDC that does not
understand the anonymous KDC option will not return an error, but understand the anonymous KDC option will not return an error, but
will instead return a normal ticket. 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].
Note that the anonymous principal name and realm are only applicable Note that the anonymous principal name and realm are only applicable
to the client in Kerberos messages, the server cannot be anonymous in to the client in Kerberos messages, the server cannot be anonymous in
any Kerberos message per this specification. any Kerberos message per this specification.
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.
If the client realm in a ticket is the anonymous realm, absent other Upon receipt of an anonymous ticket, the transited policy check is
information any realm in the authentication path is allowed by the preformed in the same way as that of a normal ticket if the client's
cross-realm policy check. realm is not the anonymous realm; if the client realm is the
anonymous realm, absent other information any realm in the
authentication path is allowed by the cross-realm policy check.
5. GSS-API Implementation Notes 5. Interoperability Requirements
Conforming implementations MUST support the anonymous principal with
a non-anonymous realm, and they MAY support the anonymous principal
with the anonymous realm using anonymous PKINIT.
6. GSS-API Implementation Notes
GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
represent the anonymous identity. In addition, Section 2.1.1 of represent the anonymous identity. In addition, Section 2.1.1 of
[RFC1964] defines the single string representation of a Kerberos [RFC1964] defines the single string representation of a Kerberos
principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The
anonymous principal with the anonymous realm corresponds to the GSS- anonymous principal with the anonymous realm corresponds to the GSS-
API anonymous principal. A principal with the anonymous principal API anonymous principal. A principal with the anonymous principal
name and a non-anonymous realm is an authenticated principal, hence name and a non-anonymous realm is an authenticated principal, hence
such a principal does not correspond to the anonymous principal in such a principal does not correspond to the anonymous principal in
GSS-API with the GSS_C_NT_ANONYMOUS name type, and this principal's GSS-API with the GSS_C_NT_ANONYMOUS name type. The [RFC1964] name
corresponding name component within the exportable name as defined in syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the
Section 2.1.3 of [RFC1964] MUST signify the realm name according to anonymous principal name with a non-anonymous realm name and for
Section 2.1.1 of [RFC1964]. displaying and exporting these names.
At the GSS-API [RFC2743] level, the use of an anonymous principal At the GSS-API [RFC2743] level, an initiator/client requests the use
with the anonymous realm by the initiator/client requires the of an anonymous principal with the anonymous realm by asserting the
initiator/client to assert the "anonymous" flag when calling "anonymous" flag when calling GSS_Init_Sec_Context(). The GSS-API
GSS_Init_Sec_Context(); while the use of an anonymous principal with implementation MAY provide implementation-specific means for
a non-anonymous realm by the initiator/client is based on requesting the use of an anonymous principal with a non-anonymous
implementation specific local policy. realm.
GSS-API does not know or define "anonymous credentials", so the GSS-API does not know or define "anonymous credentials", so the
(printable) name of the anonymous principal will rarely be used by or (printable) name of the anonymous principal will rarely be used by or
relevant for the initiator/client. The printable name is relevant relevant for the initiator/client. The printable name is relevant
for the acceptor/server when performing an authorization decision for the acceptor/server when performing an authorization decision
based on the initiator name that is returned from the acceptor side based on the initiator name that is returned from the acceptor side
upon the successful security context establishment. upon the successful security context establishment.
A GSS-API initiator MUST carefully check the resulting context A GSS-API initiator MUST carefully check the resulting context
attributes from the initial call to GSS_Init_Sec_Context() when attributes from the initial call to GSS_Init_Sec_Context() when
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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".
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 [RFC2743] to GSS_Init_Sec_Context(). anon_req_flag [RFC2743] to GSS_Init_Sec_Context().
6. The Choice of the Ticket Session Key of a Ticket Obtained using 7. PKINIT Client Contribution to the Ticket Session Key
PKINIT
The definition in this section was motivated by protocol analysis of The definition in this section was motivated by protocol analysis of
anonymous PKINIT (defined in this document) in building Kerberos FAST anonymous PKINIT (defined in this document) in building tunneling
[FAST] tunneling channels and subsequent channel bindings. In order channels [FAST] and subsequent channel bindings. In order to enable
to enable FAST and other applications of anonymous PKINIT to form applications of anonymous PKINIT to form channels, all
channels, all implementations of anonymous PKINIT need to meet the implementations of anonymous PKINIT need to meet the requirements of
requirements of this section. this section. There is otherwise no connection to the rest of this
document.
PKINIT is useful in conjunction with Kerberos FAST to construct PKINIT is useful for constructing tunneling channels. To ensure that
tunneling channels. To ensure that an attacker cannot create a an attacker cannot create a channel with a given name, it is
channel with a given name, it is desirable that neither the KDC nor desirable that neither the KDC nor the client can unilaterally
the client can unilaterally determine the ticket session key. To determine the ticket session key. To achieve that end, a KDC
achieve that end, a KDC conforming to this definition MUST encrypt a conforming to this definition MUST encrypt a randomly generated key,
randomly generated key, called the KDC contribution key, in the called the KDC contribution key, in the PA_PKINIT_KX padata (defined
PA_PKINIT_KX padata. The KDC contribution key is then combined with next in this section). The KDC contribution key is then combined
the reply key to form the ticket session key of the returned ticket. with the reply key to form the ticket session key of the returned
These two keys are then combined using KRB-FX-CF2 as defined in ticket. These two keys are then combined using the KRB-FX-CF2
[FAST], where K1 is the KDC contribution key, K2 is the reply key, operation defined in Section 7.1, where K1 is the KDC contribution
the input pepper1 is American Standard Code for Information key, K2 is the reply key, the input pepper1 is American Standard Code
Interchange (ASCII) [ASAX34] string "PKINIT", and the input pepper2 for Information Interchange (ASCII) [ASAX34] string "PKINIT", and the
is ASCII string "KeyExchange". input pepper2 is ASCII string "KeyExchange".
PA_PKINIT_KX 135 PA_PKINIT_KX 135
-- padata for PKINIT that contains an encrypted -- padata for PKINIT that contains an encrypted
-- KDC contribution key. -- KDC contribution key.
PA-PKINIT-KX ::= EncryptedData -- EncryptionKey PA-PKINIT-KX ::= EncryptedData -- EncryptionKey
-- Contains an encrypted key randomly -- Contains an encrypted key randomly
-- generated by the KDC (known as the KDC contribution key). -- generated by the KDC (known as the KDC contribution key).
-- Both EncryptedData and EncryptionKey are defined in [RFC4120] -- Both EncryptedData and EncryptionKey are defined in [RFC4120]
The PA_PKINIT_KX padata MUST be included in the KDC reply when The PA_PKINIT_KX padata MUST be included in the KDC reply when
anonymous PKINIT is used; it should be included if PKINIT is used anonymous PKINIT is used; it SHOULD be included if PKINIT is used
with the Diffie-Helleman key exchange but the client is not with the Diffie-Helleman key exchange but the client is not
anonymous; it MUST NOT be included otherwise (e.g. when PKINIT is anonymous; it MUST NOT be included otherwise (e.g. when PKINIT is
used with the public key encryption as the key exchange). used with the public key encryption as the key exchange).
The padata-value field of the PA-PKINIT-KX type padata contains the The padata-value field of the PA-PKINIT-KX type padata contains the
DER [X680] [X690] encoding of the Abstract Syntax Notation One DER [X680] [X690] encoding of the Abstract Syntax Notation One
(ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is a (ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is a
EncryptedData. The clear text data being encrypted is the DER EncryptedData. The clear text data being encrypted is the DER
encoded Kerberos session key randomly generated by the KDC. The encoded Kerberos session key randomly generated by the KDC. The
encryption key is the reply key and the key usage number is encryption key is the reply key and the key usage number is
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The client then decrypts the KDC contribution key and verifies the The client then decrypts the KDC contribution key and verifies the
ticket session key in the returned ticket is the combined key of the ticket session key in the returned ticket is the combined key of the
KDC contribution key and the reply key as described above. A KDC contribution key and the reply key as described above. A
conforming client MUST reject anonymous PKINIT authentication if the conforming client MUST reject anonymous PKINIT authentication if the
PA_PKINIT_KX padata is not present in the KDC reply or if the ticket PA_PKINIT_KX padata is not present in the KDC reply or if the ticket
session key of the returned ticket is not the combined key of the KDC session key of the returned ticket is not the combined key of the KDC
contribution key and the reply key when PA-PKINIT-KX is present in contribution key and the reply key when PA-PKINIT-KX is present in
the KDC reply. the KDC reply.
7. Security Considerations 7.1. Combinging Two protocol Keys
KRB-FX-CF2() combines two protocol keys based on the pseudo-random()
function defined in [RFC3961].
Given two input keys, K1 and K2, where K1 and K2 can be of two
different enctypes, the output key of KRB-FX-CF2(), K3, is derived as
follows:
KRB-FX-CF2(protocol key, protocol key, octet string,
octet string) -> (protocol key)
PRF+(K1, pepper1) -> octet-string-1
PRF+(K2, pepper2) -> octet-string-2
KRB-FX-CF2(K1, K2, pepper1, pepper2) ->
random-to-key(octet-string-1 ^ octet-string-2)
Where ^ denotes the exclusive-OR operation. PRF+() is defined as
follows:
PRF+(protocol key, octet string) -> (octet string)
PRF+(key, shared-info) -> pseudo-random( key, 1 || shared-info ) ||
pseudo-random( key, 2 || shared-info ) ||
pseudo-random( key, 3 || shared-info ) || ...
Here the counter value 1, 2, 3 and so on are encoded as a one-octet
integer. The pseudo-random() operation is specified by the enctype
of the protocol key. PRF+() uses the counter to generate enough bits
as needed by the random-to-key() [RFC3961] function for the
encryption type specified for the resulting key; unneeded bits are
removed from the tail.
8. Security Considerations
Since KDCs ignore unknown options, a client requiring anonymous Since KDCs ignore unknown options, a client requiring anonymous
communication needs to make sure that the returned ticket is actually communication needs to make sure that the returned ticket is actually
anonymous. This is because a KDC that that does not understand the anonymous. This is because 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 the client's identity to the server but the client not reveal the client's identity to the server but the client
identity may be revealed to the KDC of the server principal (when the 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), server principal is in a different realm than that of the client),
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If the KDC that issued an anonymous ticket were to maintain records If the KDC that issued an anonymous ticket were to maintain records
of the association of identities to an anonymous ticket, then someone of the association of identities to an anonymous ticket, then someone
obtaining such records could breach the anonymity. Additionally, the obtaining such records could breach the anonymity. Additionally, the
implementations of most (for now all) KDC's respond to requests at implementations of most (for now all) KDC's respond to requests at
the time that they are received. Traffic analysis on the connection the time that they are received. Traffic analysis on the connection
to the KDC will allow an attacker to match client identities to to the KDC will allow an attacker to match client identities to
anonymous tickets issued. Because there are plaintext parts of the anonymous tickets issued. Because there are plaintext parts of the
tickets that are exposed on the wire, such matching by a third party tickets that are exposed on the wire, such matching by a third party
observer is relatively straightforward. A service that is observer is relatively straightforward. A service that is
authenticated by this mechanism may be able to infer the identity of authenticated by the anonymous principals may be able to infer the
the party by examining and linking quasi-static protocol information identity of the client by examining and linking quasi-static protocol
such as the IP address from which a request is received. information such as the IP address from which a request is received.
The client's real identity is not revealed when the client is The client's real identity is not revealed when the client is
authenticated as the anonymous principal. Application servers MAY authenticated as the anonymous principal. Application servers MAY
reject the authentication in order to, for example, prevent reject the authentication in order to, for example, prevent
information disclosure or as part of Denial of Service (DOS) information disclosure or as part of Denial of Service (DOS)
prevention. Application servers MUST avoid accepting anonymous prevention. Application servers MUST avoid accepting anonymous
credentials in situations where they must record the client's credentials in situations where they must record the client's
identity; for example, when there must be an audit trail. identity; for example, when there must be an audit trail.
8. Acknowledgements 9. Acknowledgements
JK Jaganathan helped editing early revisions of this document. JK Jaganathan helped editing early revisions of this document.
Clifford Neuman contributed the core notions of this document. Clifford Neuman contributed the core notions of this document.
Ken Raeburn reviewed the document and provided suggestions for Ken Raeburn reviewed the document and provided suggestions for
improvements. improvements.
Martin Rex wrote the text for GSS-API considerations. Martin Rex wrote the text for GSS-API considerations.
skipping to change at page 13, line 12 skipping to change at page 14, line 22
Sam Hartman and Nicolas Williams were great champions of this work. Sam Hartman and Nicolas Williams were great champions of this work.
Miguel Garcia and Phillip Hallam-Baker reviewed the document and Miguel Garcia and Phillip Hallam-Baker reviewed the document and
provided helpful suggestions. provided helpful suggestions.
In addition, the following individuals made significant In addition, the following individuals made significant
contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love
Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia. Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.
9. IANA Considerations 10. IANA Considerations
This document defines a new 'anonymous' Kerberos well-known name and This document defines a new 'anonymous' Kerberos well-known name and
a new 'anonymous' Kerberos well-known realm based on [KRBNAM]. IANA a new 'anonymous' Kerberos well-known realm based on [KRBNAM]. IANA
is requested to add these two values to the Kerberos naming is requested to add these two values to the Kerberos naming
registries that are created in [KRBNAM]. registries that are created in [KRBNAM].
10. Normative References 11. References
11.1. Normative References
[ASAX34] American Standard Code for Information Interchange, [ASAX34] American Standard Code for Information Interchange,
ASA X3.4-1963, American Standards Association, June 17, ASA X3.4-1963, American Standards Association, June 17,
1963. 1963.
[FAST] Zhu, L. and S. Hartman, "A Generalized Framework for
Kerberos Pre-Authentication",
draft-ietf-krb-wg-preauth-framework (work in progress),
2008.
[KRBNAM] Zhu, L., "Additional Kerberos Naming Constraints", [KRBNAM] Zhu, L., "Additional Kerberos Naming Constraints",
draft-ietf-krb-wg-naming (work in progress), 2007. draft-ietf-krb-wg-naming (work in progress), 2008.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996. RFC 1964, June 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004. RFC 3852, July 2004.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Kerberos 5", RFC 3961, February 2005.
[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 [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002, [X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation. (ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002, [X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules: Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules Encoding Rules (CER) and Distinguished Encoding Rules
(DER). (DER).
11.2. Informative References
[FAST] Zhu, L. and S. Hartman, "A Generalized Framework for
Kerberos Pre-Authentication",
draft-ietf-krb-wg-preauth-framework (work in progress),
2008.
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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