draft-ietf-krb-wg-anon-07.txt   draft-ietf-krb-wg-anon-08.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 July 27, 2008 Intended status: Standards Track August 20, 2008
Expires: January 28, 2009 Expires: February 21, 2009
Anonymity Support for Kerberos Anonymity Support for Kerberos
draft-ietf-krb-wg-anon-07 draft-ietf-krb-wg-anon-08
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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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 January 28, 2009. This Internet-Draft will expire on February 21, 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 to the server or to the KDC. It updates RFC 4120. These identity or the client's realm to the server or to the KDC. It
extensions can be used to secure communication between the anonymous updates RFC 4120. These extensions can be used to secure
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 . . . . . . . . . . . . . . . . . . . . . 5
5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 8 4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 4.3. Subsequent Exchanges and Protocol Actions Common to AS
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 and TGS for Anonymity Support . . . . . . . . . . . . . . 9
9. Normative References . . . . . . . . . . . . . . . . . . . . . 10 5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 6. The Choice of the Ticket Session Key of a Ticket Obtained
Intellectual Property and Copyright Statements . . . . . . . . . . 12 using PKINIT . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
10. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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 the client's own identity. For example, consider an application
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 its information received from it, but might not wish to disclose the
identity to the service for privacy reasons. client's identity to the service for privacy reasons.
Extensions to Kerberos are specified in this document by which a Extensions to Kerberos are specified in this document by which a
client can authenticate the Key Distribution Center (KDC) and request client can authenticate the Key Distribution Center (KDC) and request
an anonymous ticket. The client can use the anonymous ticket to an anonymous ticket. The client can use the anonymous ticket to
authenticate the server and protect subsequent client-server authenticate the server and protect subsequent client-server
communications. communications.
By using the extensions defined in this specification, the client may By using the extensions defined in this specification, the client can
reveal its identity in its initial request to its own KDC, but it can request an anonymous ticket where the client may reveal the client's
remain anonymous thereafter to KDCs on the cross-realm authentication identity to the client's own KDC, or the client can hide the client's
path, and to the server with which it communicates. identity completely by using anonymous Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT) as defined in
Section 4.1. Using the returned anonymous ticket, the client remains
anonymous in subsequent Kerberos exchanges thereafter to KDCs on the
cross-realm authentication path, and to the server with which it
communicates.
In this specification, the client realm in the anonymous ticket is
the anonymous realm name when anonymous PKINIT is used to obtain the
ticket. The client realm is the client's real realm name if the
client is authenticated using the client's long term keys. Note that
the membership of a realm can imply a member of the community
represented by the realm.
The interaction with Generic Security Service Application Program
Interface (GSS-API) is described after the protocol description.
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
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], and the value of this well-known realm name
ANONYMOUS". is the literal "WELLKNOWN:ANONYMOUS".
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".
The anonymous ticket flag is defined as bit 14 (with the first bit The anonymous ticket flag is defined as bit 14 (with the first bit
being bit 0) in the TicketFlags: being bit 0) in the TicketFlags:
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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".
The anonymous ticket flag is defined as bit 14 (with the first bit The anonymous ticket flag is defined as bit 14 (with the first bit
being bit 0) in the TicketFlags: being bit 0) in the TicketFlags:
TicketFlags ::= KerberosFlags TicketFlags ::= KerberosFlags
-- anonymous(14) -- anonymous(14)
-- TicketFlags and KerberosFlags are defined in [RFC4120] -- TicketFlags and KerberosFlags are defined in [RFC4120]
This is a new ticket flag that is used to indicate a ticket is an This is a new ticket flag that is used to indicate a ticket is an
anonymous one. anonymous one.
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 anonymous
the realm that issued the initial ticket for the client principal realm name.
(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 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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-- 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.
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 Authentication Service exchange (AS) or
Ticket Granting Service (TGS) exchange. The client can request an Ticket Granting Service (TGS) exchange.
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 The rest of this section is organized as follows: it first describes
the client name as anonymous in the AS exchange and provides a protocol actions specific to AS exchanges, then it describes those of
PA_PK_AS_REQ pre-authentication data [RFC4556] where both the TGS exchange. These are then followed by the decription of protocol
signerInfos field and the certificates field of the SignedData actions common to both AS and TGS and those in subsequent exchanges.
[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 4.1. Anonymity Support in AS Exchange
if the client in the AS request is anonymous, the anonymous KDC
The client requests an anonymous ticket by setting the anonymous KDC
option in an AS exchange.
The Kerberos client can use the client's long term keys to
authenticate with 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
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 Upon receiving the AS request with a PA_PK_AS_REQ [RFC4556] from the
anonymous client, the KDC processes the request according to Section anonymous client, the KDC processes the request according to Section
3.1.2 of [RFC4120]. The KDC skips the checks for the client's 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 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 the binding between the client's public key and the client name), but
performs otherwise-applicable checks, and proceeds as normal performs otherwise-applicable checks, and proceeds as normal
according to [RFC4556]. For example, the AS MUST check if the according to [RFC4556]. For example, the AS MUST check if the
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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.
A KDC that supports anonymous PKINIT MUST indicate the support of If the client is anonymous and the KDC does not have a key to encrypt
PKINIT according to Section 3.4 of [RFC4556]. 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].
When policy allows, the KDC issues an anonymous ticket. If the
client name in the request is the anonymous principal, the client
realm (crealm) in the reply is the anonymous realm, otherwise 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
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.
Care MUST be taken by the KDC not to reveal the client's identity in
the authorization data of the returned ticket when populating the
authorization data in a returned anonymous ticket.
The AD-INITIAL-VERIFIED-CAS authorization data as defined in
[RFC4556] contains the issuer name of the client certificate. This
authorization is not applicable and MUST NOT be present in the
returned anonymous ticket when anonymous PKINIT is used. When the
client is authenticated (i.e. anonymous PKINIT is not used), if it is
undesirable to disclose such information about the client's identity,
the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be removed from
the returned anonymous ticket.
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.
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
[RFC4120]. [RFC4120].
For the TGS exchange, the reply key is selected as normal according A KDC that supports anonymous PKINIT MUST indicate the support of
to Section 3.3.3 of [RFC4120]. PKINIT according to Section 3.4 of [RFC4556].
When policy allows, the KDC issues an anonymous ticket. Based on 4.2. Anonymity Support in TGS Exchange
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, In a TGS request, the client can request an anonymous Ticket Granting
the client name and the client realm in the EncTicketPart of the Ticket (TGT) or an anonymous cross realm TGT using a normal TGT, in
reply MUST match with the corresponding client name and the client which case the client's identity is known to the TGS. The client can
realm of the anonymous ticket in the reply. The client MUST use the also use an anonymous TGT to request an anonymous service ticket.
client name and the client realm returned in the KDC-REP in Note that the client can completely hide the client's identity in an
AS exchange using anonymous PKINIT as described in the previous
section.
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,
the KDC MUST return a KRB-ERROR message with the code
KDC_ERR_BADOPTION.
When policy allows, the KDC issues an anonymous ticket. If the
ticket in the TGS request is an anonymous one, the client name and
the client realm are copied from that ticket; otherwise the ticket in
the TGS request is a normal ticket, the returned anonymous ticket
contains the client name as the anonymous principal and the client
realm as the true realm of the client. In all cases, according to
[RFC4120] 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 subsequent message exchanges when using the obtained anonymous
ticket. ticket.
When propagating authorization data in the ticket or in the enc- Care MUST be taken by the TGS not to reveal the client's identity in
authorization-data field of the request, the TGS MUST ensure that the the authorization data of the returned ticket. When propagating
client confidentiality is not violated in the returned anonymous authorization data in the ticket or in the enc-authorization-data
ticket. The TGS MUST process the authorization data recursively field of the request, the TGS MUST ensure that the client
according to Section 5.2.6 of [RFC4120] beyond the container levels confidentiality is not violated in the returned anonymous ticket.
such that all embedded authorization elements are interpreted. AS or The TGS MUST process the authorization data recursively according to
TGS SHOULD NOT populate identity-based authorization data into an Section 5.2.6 of [RFC4120] beyond the container levels such that all
anonymous ticket in that such authorization data typically reveals embedded authorization elements are interpreted. The TGS SHOULD NOT
the client's identity. The specification of a new authorization data populate identity-based authorization data into an anonymous ticket
type MUST specify the processing rules of the authorization data when in that such authorization data typically reveals the client's
an anonymous ticket is returned. If there is no processing rule identity. The specification of a new authorization data type MUST
defined for an authorization data element or the authorization data specify the processing rules of the authorization data when an
element is unknown, the TGS MUST process it when an anonymous ticket anonymous ticket is returned. If there is no processing rule defined
is returned as follows: 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 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 is intended to restrict the use
of the ticket or limit the rights otherwise conveyed in the of the ticket or limit the rights otherwise conveyed in the
ticket, it cannot be removed in order to hide the client's ticket, it cannot be removed in order to hide the client's
identity. In this case, the authentication attempt MUST be identity. In this case, the authentication attempt MUST be
rejected, and the KDC MUST return an error message with the code rejected, and the TGS MUST return an error message with the code
KDC_ERR_POLICY. Note this is applicable to both critical and KDC_ERR_POLICY. Note this is applicable to both critical and
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 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 based on local policy of the TGS. 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 to hide the authentication path of the client. The
unordered set of realms in the transited field, if present, can unordered set of realms in the transited field, if present, can
reveal which realm may potentially be the realm of the client or the reveal which realm may potentially be the realm of the client or the
realm that issued the anonymous TGT. The anonymous Kerberos realm realm that issued the anonymous TGT. The anonymous Kerberos realm
name MUST NOT be present in the transited field of a ticket. The 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 true name of the realm that issued the anonymous ticket MAY be
present in the transited field of a ticket. present in the transited field of a ticket.
If the client is anonymous and the KDC does not have a key to encrypt 4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for
the reply (this can happen when, for example, the KDC does not Anonymity Support
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 In both AS and TGS exchanges, the realm field in the KDC request is
check to make sure that the ticket in the reply is actually anonymous always the realm of the target KDC, not the anonymous realm when the
by checking the presence of the anonymous ticket flag in the flags client requests an anonymous ticket.
field of the EncKDCRepPart. This is because KDCs ignore unknown KDC
options. A KDC that does not understand the anonymous KDC option Unless anonymous PKINIT is used, if a client requires anonymous
will not return an error, but will instead return a normal 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
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 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
information any realm in the authentication path is allowed by the
cross-realm policy check.
5. GSS-API Implementation Notes 5. GSS-API Implementation Notes
At the GSS-API [RFC2743] level, the use of an anonymous principal by GSS-API defines the name_type GSS_C_NT_ANONYMOUS [RFC2743] to
the initiator/client requires the initiator/client to assert the represent the anonymous identity. In addition, Section 2.1.1 of
"anonymous" flag when calling GSS_Init_Sec_Context(). [RFC1964] defines the single string representation of a Kerberos
principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. The
anonymous principal with the anonymous realm corresponds to the GSS-
API anonymous principal. A principal with the anonymous principal
name and a non-anonymous realm is an authenticated principal, hence
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
corresponding 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].
At the GSS-API [RFC2743] level, the use of an anonymous principal
with the anonymous realm by the initiator/client requires the
initiator/client to assert the "anonymous" flag when calling
GSS_Init_Sec_Context(); while the use of an anonymous principal with
a non-anonymous realm by the initiator/client is based on
implementation specific local policy.
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
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 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 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. Security Considerations 6. The Choice of the Ticket Session Key of a Ticket Obtained using
PKINIT
The definition in this section was motivated by protocol analysis of
anonymous PKINIT (defined in this document) in building Kerberos FAST
[FAST] tunneling channels and subsequent channel bindings. In order
to enable FAST and other applications of anonymous PKINIT to form
channels, all implementations of anonymous PKINIT need to meet the
requirements of this section.
PKINIT is useful in conjunction with Kerberos FAST to construct
tunneling channels. To ensure that an attacker cannot create a
channel with a given name, it is desirable that neither the KDC nor
the client can unilaterally determine the ticket session key. To
achieve that end, a KDC conforming to this definition MUST encrypt a
randomly generated key, called the KDC contribution key, in the
PA_PKINIT_KX padata. The KDC contribution key is then combined with
the reply key to form the ticket session key of the returned ticket.
These two keys are then combined using KRB-FX-CF2 as defined in
[FAST], where K1 is the KDC contribution key, K2 is the reply key,
the input pepper1 is American Standard Code for Information
Interchange (ASCII) [ASAX34] string "PKINIT", and the input pepper2
is ASCII string "KeyExchange".
PA_PKINIT_KX 135
-- padata for PKINIT that contains an encrypted
-- KDC contribution key.
PA-PKINIT-KX ::= EncryptedData -- EncryptionKey
-- Contains an encrypted key randomly
-- generated by the KDC (known as the KDC contribution key).
-- Both EncryptedData and EncryptionKey are defined in [RFC4120]
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
with the Diffie-Helleman key exchange but the client is not
anonymous; it MUST NOT be included otherwise (e.g. when PKINIT is
used with the public key encryption as the key exchange).
The padata-value field of the PA-PKINIT-KX type padata contains the
DER [X680] [X690] encoding of the Abstract Syntax Notation One
(ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is a
EncryptedData. The clear text data being encrypted is the DER
encoded Kerberos session key randomly generated by the KDC. The
encryption key is the reply key and the key usage number is
KEY_USAGE_PA_PKINIT_KX (44).
The client then decrypts the KDC contribution key and verifies 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
conforming client MUST reject anonymous PKINIT authentication if the
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
contribution key and the reply key when PA-PKINIT-KX is present in
the KDC reply.
7. 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 its identity to the server but its identity may be not reveal the client's identity to the server but the client
revealed to the KDC of the server principal (when the server identity may be revealed to the KDC of the server principal (when the
principal is in a different realm than that of the client), and any server principal is in a different realm than that of the client),
KDC on the cross-realm authentication path. The Kerberos client MUST and any KDC on the cross-realm authentication path. The Kerberos
verify the ticket being used is indeed anonymous before communicating client MUST verify the ticket being used is indeed anonymous before
with the server, otherwise the client's identity may be revealed communicating with the server, otherwise the client's identity may be
unintentionally. revealed 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 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. observer is relatively straightforward. A service that is
authenticated by this mechanism may be able to infer the identity of
the party by examining and linking quasi-static protocol 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.
7. Acknowledgements 8. 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 10, line 9 skipping to change at page 13, line 12
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.
8. IANA Considerations 9. 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].
9. Normative References 10. Normative References
[KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints", [ASAX34] American Standard Code for Information Interchange,
draft-ietf-krb-wg-naming, work in progress. ASA X3.4-1963, American Standards Association, June 17,
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",
draft-ietf-krb-wg-naming (work in progress), 2007.
[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.
[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,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
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
Paul Leach Paul Leach
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
US US
Email: paulle@microsoft.com Email: paulle@microsoft.com
Full Copyright Statement Full Copyright Statement
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