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Updates:
Obsoleted by:
Keywords: [--------], kerberos realm
Internet Engineering Task Force (IETF) L. Zhu
Request for Comments: 6112 P. Leach
Updates: 4120, 4121, 4556 Microsoft Corporation
Category: Standards Track S. Hartman
ISSN: 2070-1721 Painless Security
April 2011
Anonymity Support for Kerberos
Abstract
This document defines extensions to the Kerberos protocol to allow a
Kerberos client to securely communicate with a Kerberos application
service without revealing its identity, or without revealing more
than its Kerberos realm. It also defines extensions that allow a
Kerberos client to obtain anonymous credentials without revealing its
identity to the Kerberos Key Distribution Center (KDC). This
document updates RFCs 4120, 4121, and 4556.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6112.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Zhu, et al. Standards Track [Page 1]
RFC 6112 Kerberos Anonymity Support April 2011
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5
4.1. Anonymity Support in AS Exchange . . . . . . . . . . . . . 5
4.1.1. Anonymous PKINIT . . . . . . . . . . . . . . . . . . . 6
4.2. Anonymity Support in TGS Exchange . . . . . . . . . . . . 7
4.3. Subsequent Exchanges and Protocol Actions Common to AS
and TGS for Anonymity Support . . . . . . . . . . . . . . 9
5. Interoperability Requirements . . . . . . . . . . . . . . . . 10
6. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 10
7. PKINIT Client Contribution to the Ticket Session Key . . . . . 11
7.1. Combining Two Protocol Keys . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . . 16
Zhu, et al. Standards Track [Page 2]
RFC 6112 Kerberos Anonymity Support April 2011
1. Introduction
In certain situations, the Kerberos [RFC4120] client may wish to
authenticate a server and/or protect communications without revealing
the client's own identity. For example, consider an application that
provides read access to a research database and that permits queries
by arbitrary requesters. A client of such a service might wish to
authenticate the service, to establish trust in the information
received from it, but might not wish to disclose the client's
identity to the service for privacy reasons.
Extensions to Kerberos are specified in this document by which a
client can authenticate the Key Distribution Center (KDC) and request
an anonymous ticket. The client can use the anonymous ticket to
authenticate the server and protect subsequent client-server
communications.
By using the extensions defined in this specification, the client can
request an anonymous ticket where the client may reveal the client's
identity to the client's own KDC, or the client can hide the client's
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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definitions
The anonymous Kerberos realm name is defined as a well-known realm
name based on [RFC6111], and the value of this well-known realm name
is the literal "WELLKNOWN:ANONYMOUS".
Zhu, et al. Standards Track [Page 3]
RFC 6112 Kerberos Anonymity Support April 2011
The anonymous Kerberos principal name is defined as a well-known
Kerberos principal name based on [RFC6111]. The value of the name-
type field is KRB_NT_WELLKNOWN [RFC6111], and the value of the name-
string field is a sequence of two KerberosString components:
"WELLKNOWN", "ANONYMOUS".
The anonymous ticket flag is defined as bit 16 (with the first bit
being bit 0) in the TicketFlags:
TicketFlags ::= KerberosFlags
-- anonymous(16)
-- TicketFlags and KerberosFlags are defined in [RFC4120]
This is a new ticket flag that is used to indicate that a ticket is
an anonymous one.
An anonymous ticket is a ticket that has all of the following
properties:
o The cname field contains the anonymous Kerberos principal name.
o The crealm field contains the client's realm name or the anonymous
realm name.
o The anonymous ticket contains no information that can reveal the
client's identity. However, the ticket may contain the client
realm, intermediate realms on the client's authentication path,
and authorization data that may provide information related to the
client's identity. For example, an anonymous principal that is
identifiable only within a particular group of users can be
implemented using authorization data and such authorization data,
if included in the anonymous ticket, would disclose the client's
membership of that group.
o The anonymous ticket flag is set.
The anonymous KDC option is defined as bit 16 (with the first bit
being bit 0) in the KDCOptions:
KDCOptions ::= KerberosFlags
-- anonymous(16)
-- KDCOptions and KerberosFlags are defined in [RFC4120]
Zhu, et al. Standards Track [Page 4]
RFC 6112 Kerberos Anonymity Support April 2011
As described in Section 4, the anonymous KDC option is set to request
an anonymous ticket in an Authentication Service (AS) request or a
Ticket Granting Service (TGS) request.
4. Protocol Description
In order to request an anonymous ticket, the client sets the
anonymous KDC option in an AS request or a TGS request.
The rest of this section is organized as follows: it first describes
protocol actions specific to AS exchanges, then it describes those of
TGS exchanges. These are then followed by the description of
protocol actions common to both AS and TGS and those in subsequent
exchanges.
4.1. Anonymity Support in AS Exchange
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, the client's
X.509 certificates [RFC4556], or any other pre-authentication 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.
If the client in the AS request is anonymous, the anonymous KDC
option MUST be set in the request. Otherwise, the KDC MUST return a
KRB-ERROR message with the code KDC_ERR_BADOPTION.
If the client is anonymous and the KDC does not have a key to encrypt
the reply (this can happen when, for example, the KDC does not
support PKINIT [RFC4556]), the KDC MUST return an error message with
the code KDC_ERR_NULL_KEY [RFC4120].
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 KDC 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.
Zhu, et al. Standards Track [Page 5]
RFC 6112 Kerberos Anonymity Support April 2011
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.
The client can use the client keys to mutually authenticate with the
KDC and request an anonymous Ticket Granting Ticket (TGT) in the AS
request. In that case, the reply key is selected as normal,
according to Section 3.1.3 of [RFC4120].
4.1.1. Anonymous PKINIT
This sub-section defines anonymous 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 PA_PK_AS_REQ pre-authentication data
[RFC4556] where the signerInfos field of the SignedData [RFC5652] of
the PA_PK_AS_REQ is empty, and the certificates field is absent.
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 PA_PK_AS_REP [RFC4556] pre-authentication data is
Zhu, et al. Standards Track [Page 6]
RFC 6112 Kerberos Anonymity Support April 2011
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, the
signerInfos field of the SignedData [RFC5652] of PA_PK_AS_REP in the
reply is empty, and the certificates field is absent. 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
PA_PK_AS_REP [RFC4556] pre-authentication data, the client can then
authenticate the KDC based on the KDC's signature in the
PA_PK_AS_REP. If the KDC's signature is missing in the KDC reply
(the reply is anonymous), the client MUST reject the returned ticket
if it cannot authenticate the KDC otherwise.
A KDC that supports anonymous PKINIT MUST indicate the support of
PKINIT, according to Section 3.4 of [RFC4556]. In addition, such a
KDC MUST indicate support for anonymous PKINIT by including a padata
element of padata-type PA_PKINIT_KX and empty padata-value when
including PA-PK-AS-REQ in an error reply.
When included in a KDC error, PA_PKINIT_KX indicates support for
anonymous PKINIT. As discussed in Section 7, when included in an AS-
REP, PA_PKINIT_KX proves that the KDC and client both contributed to
the session key for any use of Diffie-Hellman key agreement with
PKINIT.
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
The client requests an anonymous ticket by setting the anonymous KDC
option in a TGS exchange, and in that request the client can use a
normal Ticket Granting Ticket (TGT) with the client's identity, or an
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
AS exchange using anonymous PKINIT, as described in the previous
section.
Zhu, et al. Standards Track [Page 7]
RFC 6112 Kerberos Anonymity Support April 2011
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
ticket.
Care MUST be taken by the TGS not to reveal the client's identity in
the authorization data of the returned ticket. When propagating
authorization data in the ticket or in the enc-authorization-data
field of the request, the TGS MUST ensure that the client
confidentiality is not violated in the returned anonymous ticket.
The TGS MUST process the authorization data recursively, according to
Section 5.2.6 of [RFC4120], beyond the container levels such that all
embedded authorization elements are interpreted. The TGS SHOULD NOT
populate identity-based authorization data into an anonymous ticket
in that such authorization data typically reveals the client's
identity. The specification of a new authorization data type MUST
specify the processing rules of the authorization data when an
anonymous ticket is returned. If there is no processing rule defined
for an authorization data element or the authorization data element
is unknown, the TGS MUST process it when an anonymous ticket is
returned as follows:
o If the authorization data element may reveal the client's
identity, it MUST be removed unless otherwise specified.
o If the authorization data element, that could reveal the client's
identity, is intended to restrict the use of the ticket or limit
the rights otherwise conveyed in the ticket, it cannot be removed
in order to hide the client's identity. In this case, the
authentication attempt MUST be rejected, and the TGS MUST return
an error message with the code KDC_ERR_POLICY. Note this is
applicable to both critical and optional authorization data.
Zhu, et al. Standards Track [Page 8]
RFC 6112 Kerberos Anonymity Support April 2011
o If the authorization data element is unknown, the TGS MAY remove
it, or transfer it into the returned anonymous ticket, or reject
the authentication attempt, based on local policy for that
authorization data type unless otherwise specified. If there is
no policy defined for a given unknown authorization data type, the
authentication MUST be rejected. The error code is KDC_ERR_POLICY
when the authentication is rejected.
The AD-INITIAL-VERIFIED-CAS authorization data, as defined in
[RFC4556], contains the issuer name of the client certificate. If it
is undesirable to disclose such information about the client's
identity, the AD-INITIAL-VERIFIED-CAS authorization data SHOULD be
removed from an anonymous ticket.
The TGS encodes the name of the previous realm into the transited
field, according to Section 3.3.3.2 of [RFC4120]. Based on local
policy, the TGS MAY omit the previous realm, if the cross realm TGT
is an anonymous one, in order to hide the authentication path of the
client. The unordered set of realms in the transited field, if
present, can reveal which realm may potentially be the realm of the
client or the realm that issued the anonymous TGT. The anonymous
Kerberos realm name MUST NOT be present in the transited field of a
ticket. The true name of the realm that issued the anonymous ticket
MAY be present in the transited field of a ticket.
4.3. Subsequent Exchanges and Protocol Actions Common to AS and TGS for
Anonymity Support
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
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
communication, then the client MUST check to make sure that the
ticket in the reply is actually anonymous by checking the presence of
the anonymous ticket flag in the flags field of the EncKDCRepPart.
This is because KDCs ignore unknown KDC options. A KDC that does not
understand the anonymous KDC option will not return an error, but
will instead return a normal ticket.
The subsequent client and server communications then proceed as
described in [RFC4120].
Zhu, et al. Standards Track [Page 9]
RFC 6112 Kerberos Anonymity Support April 2011
Note that the anonymous principal name and realm are only applicable
to the client in Kerberos messages, the server cannot be anonymous in
any Kerberos message per this specification.
A server accepting an anonymous service ticket may assume that
subsequent requests using the same ticket originate from the same
client. Requests with different tickets are likely to originate from
different clients.
Upon receipt of an anonymous ticket, the transited policy check is
performed in the same way as that of a normal ticket if the client's
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. 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
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. 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. The [RFC1964] name
syntax for GSS_KRB5_NT_PRINCIPAL_NAME MUST be used for importing the
anonymous principal name with a non-anonymous realm name and for
displaying and exporting these names. In addition, this syntax must
be used along with the name type GSS_C_NT_ANONYMOUS for displaying
and exporting the anonymous principal with the anonymous realm.
At the GSS-API [RFC2743] level, an initiator/client requests the use
of an anonymous principal with the anonymous realm by asserting the
"anonymous" flag when calling GSS_Init_Sec_Context(). The GSS-API
implementation MAY provide implementation-specific means for
requesting the use of an anonymous principal with a non-anonymous
realm.
GSS-API does not know or define "anonymous credentials", so the
(printable) name of the anonymous principal will rarely be used by or
relevant for the initiator/client. The printable name is relevant
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RFC 6112 Kerberos Anonymity Support April 2011
for the acceptor/server when performing an authorization decision
based on the initiator name that is returned from the acceptor side
upon the successful security context establishment.
A GSS-API initiator MUST carefully check the resulting context
attributes from the initial call to GSS_Init_Sec_Context() when
requesting anonymity, because (as in the GSS-API tradition and for
backwards compatibility) anonymity is just another optional context
attribute. It could be that the mechanism doesn't recognize the
attribute at all or that anonymity is not available for some other
reasons -- and in that case the initiator MUST NOT send the initial
security context token to the acceptor, because it will likely reveal
the initiators identity to the acceptor, something that can rarely be
"un-done".
Portable initiators are RECOMMENDED to use default credentials
whenever possible, and request anonymity only through the input
anon_req_flag [RFC2743] to GSS_Init_Sec_Context().
7. PKINIT Client Contribution to the Ticket Session Key
The definition in this section was motivated by protocol analysis of
anonymous PKINIT (defined in this document) in building tunneling
channels [RFC6113] and subsequent channel bindings. In order to
enable applications of anonymous PKINIT to form channels, all
implementations of anonymous PKINIT need to meet the requirements of
this section. There is otherwise no connection to the rest of this
document.
PKINIT is useful for constructing 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 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 (defined next in this
section). 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 the KRB-FX-CF2 operation defined in
Section 7.1, 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".
Zhu, et al. Standards Track [Page 11]
RFC 6112 Kerberos Anonymity Support April 2011
PA_PKINIT_KX 147
-- 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-Hellman 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 [X.680] [X.690] encoding of the Abstract Syntax Notation One
(ASN.1) type PA-PKINIT-KX. The PA-PKINIT-KX structure is an
EncryptedData. The cleartext data being encrypted is the DER-encoded
KDC contribution 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.
7.1. Combining 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)
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RFC 6112 Kerberos Anonymity Support April 2011
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
communication needs to make sure that the returned ticket is actually
anonymous. This is because a KDC that does not understand the
anonymous option would not return an anonymous ticket.
By using the mechanism defined in this specification, the client does
not reveal the client's identity to the server but the client
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),
and any KDC on the cross-realm authentication path. The Kerberos
client MUST verify the ticket being used is indeed anonymous before
communicating with the server, otherwise, the client's identity may
be revealed unintentionally.
In cases where specific server principals must not have access to the
client's identity (for example, an anonymous poll service), the KDC
can define server-principal-specific policy that ensures any normal
service ticket can NEVER be issued to any of these server principals.
If the KDC that issued an anonymous ticket were to maintain records
of the association of identities to an anonymous ticket, then someone
obtaining such records could breach the anonymity. Additionally, the
implementations of most (for now all) KDC's respond to requests at
the time that they are received. Traffic analysis on the connection
to the KDC will allow an attacker to match client identities to
anonymous tickets issued. Because there are plaintext parts of the
tickets that are exposed on the wire, such matching by a third-party
observer is relatively straightforward. A service that is
authenticated by the anonymous principals may be able to infer the
Zhu, et al. Standards Track [Page 13]
RFC 6112 Kerberos Anonymity Support April 2011
identity of the client by examining and linking quasi-static protocol
information such as the IP address from which a request is received,
or by linking multiple uses of the same anonymous ticket.
Two mechanisms, the FAST facility with the hide-client-names option
in [RFC6113] and the Kerberos5 starttls option [STARTTLS], protect
the client identity so that an attacker would never be able to
observe the client identity sent to the KDC. Transport or network
layer security between the client and the server will help prevent
tracking of a particular ticket to link a ticket to a user. In
addition, clients can limit how often a ticket is reused to minimize
ticket linking.
The client's real identity is not revealed when the client is
authenticated as the anonymous principal. Application servers MAY
reject the authentication in order to, for example, prevent
information disclosure or as part of Denial of Service (DoS)
prevention. Application servers MUST avoid accepting anonymous
credentials in situations where they must record the client's
identity; for example, when there must be an audit trail.
9. Acknowledgements
JK Jaganathan helped editing early revisions of this document.
Clifford Neuman contributed the core notions of this document.
Ken Raeburn reviewed the document and provided suggestions for
improvements.
Martin Rex wrote the text for GSS-API considerations.
Nicolas Williams reviewed the GSS-API considerations section and
suggested ideas for improvements.
Sam Hartman and Nicolas Williams were great champions of this work.
Miguel Garcia and Phillip Hallam-Baker reviewed the document and
provided helpful suggestions.
In addition, the following individuals made significant
contributions: Jeffrey Altman, Tom Yu, Chaskiel M Grundman, Love
Hornquist Astrand, Jeffrey Hutzelman, and Olga Kornievskaia.
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RFC 6112 Kerberos Anonymity Support April 2011
10. IANA Considerations
This document defines a new 'anonymous' Kerberos well-known name and
a new 'anonymous' Kerberos well-known realm based on [RFC6111]. IANA
has added these two values to the Kerberos naming registries that are
created in [RFC6111].
11. References
11.1. Normative References
[ASAX34] American Standards Institute, "American Standard Code for
Information Interchange", ASA X3.4-1963, June 1963.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[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
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556,
June 2006.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)",
STD 70, RFC 5652, September 2009.
[RFC6111] Zhu, L., "Additional Kerberos Naming Constraints",
RFC 6111, April 2011.
[X.680] "Abstract Syntax Notation One (ASN.1): Specification of
Basic Notation", ITU-T Recommendation X.680: ISO/IEC
International Standard 8824-1:1998, 1997.
[X.690] "ASN.1 encoding rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)", ITU-T Recommendation
X.690 ISO/IEC International Standard 8825-1:1998, 1997.
Zhu, et al. Standards Track [Page 15]
RFC 6112 Kerberos Anonymity Support April 2011
11.2. Informative References
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113, April 2011.
[STARTTLS] Josefsson, S., "Using Kerberos V5 over the Transport
Layer Security (TLS) protocol", Work in Progress,
August 2010.
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: larry.zhu@microsoft.com
Paul Leach
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
EMail: paulle@microsoft.com
Sam Hartman
Painless Security
EMail: hartmans-ietf@mit.edu
Zhu, et al. Standards Track [Page 16]