NETWORK WORKING GROUP L. Zhu
Internet-Draft Microsoft Corporation
Intended status: Standards Track J. Altman
Expires: May 7, 2009 Secure Endpoints
N. Williams
Sun
November 3, 2008
Public Key Cryptography Based User-to-User Authentication - (PKU2U)
draft-zhu-pku2u-09
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Abstract
This document defines a Generic Security Services Application Program
Interface (GSS-API) mechanism based on Public Key Infrastructure
(PKI) - PKU2U. This mechanism is based on Kerberos V messages and the
Kerberos V GSS-API mechanism, but without requiring a Kerberos Key
Distribution Center (KDC).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. The PKU2U Realm Name . . . . . . . . . . . . . . . . . . . . . 3
4. The NULL Principal Name . . . . . . . . . . . . . . . . . . . 4
5. PKU2U Principal Naming . . . . . . . . . . . . . . . . . . . . 4
5.1. GSS_C_NT_DN . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. GSS_C_NT_HOSTNAME . . . . . . . . . . . . . . . . . . . . 6
5.3. GSS_C_NT_EMAIL_ADDR . . . . . . . . . . . . . . . . . . . 7
5.4. GSS_KRB5_NT_PRINCIPAL_NAME . . . . . . . . . . . . . . . . 7
5.5. GSS_C_NT_ANONYMOUS . . . . . . . . . . . . . . . . . . . . 9
5.6. GSS_C_NT_HOSTBASED_SERVICE - Matching Host-based
Principal Names to Acceptor Certificates . . . . . . . . . 9
6. The Protocol Description and the Context Establishment
Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Context Token Derived from KRB_AS_REQ . . . . . . . . . . 12
6.2. Context Token Derived from KRB_AS_REP . . . . . . . . . . 15
6.3. Context Tokens Imported from RFC4121 . . . . . . . . . . . 16
7. Guidelines for Credentials Selection . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Normative References . . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
The Generic Security Services Application Programming Interface (GSS-
API) is a generic protocol and API for providing authentication and
session protection to applications. It is generic in that it
supports multiple authentication mechanisms. Today there exists only
one workable, widely deployed, standards-track GSS-API mechanism: the
Kerberos V GSS-API mechanism [RFC1964] [RFC4121], which is based on
Kerberos V [RFC4120]. There is a need to provide a GSS-API mechanism
which does not require Kerberos V Key Distribution Center (KDC)
infrastructure, and which supports the use of public key
cryptography, particularly Public Key Infrastructure (PKI) [RFC5280],
including the use of public key certificates without a PKI.
This document specifies such a mechanism: the Public Key User to User
mechanism (PKU2U).
PKU2U is based on building blocks taken from Kerberos V [RFC4120],
PKINIT, [RFC4556] (which in turn uses PKI [RFC5280]) building
blocks), and the Kerberos V GSS-API mechanism [RFC1964] [RFC4121].
In spite of using Kerberos V building blocks, PKU2U does not require
any Kerberos V KDC infrastructure. And though PKU2U also uses PKI
building blocks, PKU2U can be used without a PKI by pre-sharing
certificates and/or pre-associating name/certificate bindings.
Therefore PKU2U can be used for true peer-to-peer authentication, as
well as for PKI-based authentication.
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].
In this document, the GSS-API initiator or acceptor is referred to as
the peer when the description is applicable to both the initiator and
the acceptor.
3. The PKU2U Realm Name
The PKU2U realm name is defined as a reserved Kerberos realm name per
[KRB-NAMING], and it has the value of "WELLKNOWN:PKU2U".
The PKU2U realm name has no meaning, but is intended to be used in
the Kebreros V Protocol Data Units (PDUs) that are re-used by PKU2U
wherever realm names are needed. Unless otherwise specified, the
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realm name in any Kerberos message used by PKU2U is the PKU2U realm
name.
4. The NULL Principal Name
The NULL Kerberos principal name is defined as a well-known Kerberos
principal name based on [KRB-NAMING]. The value of the name-type
field is KRB_NT_WELLKNOWN [KRB-NAMING], and the value of the name-
string field is a sequence of two KerberosString components:
"WELLKNOWN", "NULL".
The NULL Kerberos principal name is used in the Kerberos messages
where there is no Kerberos representation of the principal name, for
example, when the client name is a Distinguished Name. When the NULL
principal name is used in the Kerberos messages, the principal name
is either not used or provided separately (for example in the
PA_PKU2U_NAME padata defined in Section 6.1).
5. PKU2U Principal Naming
The GSS-API targ_name supplied for the initiator MUST NOT be
GSS_C_NO_NAME in PKU2U.
PKU2U principal names can be Kerberos principal names, and they can
also be distiguished names, or subject alternative names [RFC5280] as
they appear in the certificate of any PKU2U peer, as well as any
names agreed to out of band that do not appear in the peer
certificates.
Certificates may be associated with multiple principal names. This
presents problems for the GSS-API bindings of a PKI-based mechanism
because, for example, for any given, established GSS-API security
mechanism there can be only one initiator name, and one acceptor
name, and credential handles may be associated with only one name.
We resolve these problems as follows:
o We define multiple GSS-API name types corresponding to several
GeneralName choices [RFC5280], along with syntaxes, display forms,
and exported name token formats for each. For most of the name-
types listed below the exported name token formats consists of a
GeneralName with the usual exported name token header as per-
RFC2743. Two name-types are shared with the Kerberos V mechanism
and use the Kerberos V mechanism's query and display syntaxes,
canonicalization rules, and exported name token format.
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o The cred_name of a credential handle acquired with
GSS_C_NT_NO_NAME as the desired_name SHOULD be the Distinguished
Name (DN) of the certificate underlying the credential. If there
are multiple certificates and private keys, then either one MUST
be selected by local, implementation-specific means, or credential
acquisition with GSS_C_NT_NO_NAME MUST fail (implementers may
choose which of these two behaviors to provide).
o When using desired_name values other than GSS_C_NT_NO_NAME for
credential handle acquisition then the implementation MUST use
exact matching of the given desired_name to a certificate's DN or
Subject Alternative Names (SANs) for all name-types given below,
except for GSS_C_NT_DN, where matching rules are fuzzier and given
below. The names of a X.509 certificate will be compared to the
given desired_name in this order: certificate DN first, then SANs
in the order in which they appear in the certificate. When
multiple certificates and private keys are available the order in
which the various certificates are searched is significant; no
canonical certificate collation order is defined herein.
o The cred_name of a credential object acquired with a desired_name
other than GSS_C_NT_NO_NAME MUST be equal to the certificate DN or
SAN matched by the given desired_name.
o We provide a method (see below) by which initiators can assert, in
their context tokens, one of these names of the initiator. We
also provide a method of asserting names that do not appear in a
X.509 certificate, in which case the binding of X.509 certificate
to the asserted name is done out-of band. The name to be
asserted, of course, is the cred_name of the cred_handle passed to
GSS_Init_sec_context().
o The initiator's context tokens may also indicate what is the
expected name of the acceptor -- the targ_name passed in to
GSS_Init_sec_context().
o No attempt is made to map Kerberos V realm names to trust anchor
certificate authority (CA) names.
o We provide a method of matching host-based service principal names
to acceptor certificates, so that: a) initiators need not know the
particulars of an acceptor's certificates' names a priori, b)
acceptors can select a credential to accept a security context
with that the initiator will accept, c) existing certificates for
web servers, may be used as host-based service principal names as
though the service name were "HTTP".
Thus GSS-API initiator applications that use the GSS_C_NO_NAME as the
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desired_name arguments of GSS_Acquire_cred() and GSS_Add_cred(), or
GSS_C_NO_CREDENTIAL as the cred argument of GSS_Init_sec_context()
will assert the selected X.509 certificate's subject DN, and that
X.509 certificate's subject DN will be the name returned by
GSS_Inquire_cred() and GSS_Inquire_cred_by_mech().
And portable GSS-API initiator applications using
GSS_C_NT_HOSTBASED_SERVICE for naming acceptors (i.e., for importing
a name to use as the targ_name input argument of
GSS_Init_sec_context()) will have a reasonable chance of success in
authenticating peers with X.509 certificates predating this
specification.
5.1. GSS_C_NT_DN
The name type GSS_C_NT_DN, with Object Identifier (OID) <TBD> (see
Section 10), is defined. This corresponds to the 'directoryName'
choice of the 'GeneralName' Abstract Syntax Notation One (ASN.1)
[CCITT.X680.2002] type defined in [RFC5280].
The query syntax and display form for names of this type SHALL be as
described in [RFC4514].
As RFC4514 says, "[c]omparison of DNs for equality is to be performed
in accordance with the distinguishedNameMatch matching rule
[RFC4517]".
There is no reasonable way to canonicalize names of this type without
providing certificates to match query forms of GSS_C_NT_DN against,
such as in the form of a directory. Therefore
GSS_Canonicalize_name() as applied to names imported with the
GSS_C_NT_DN name-type MUST search available certificate databases, or
directories, or MUST fail. No method of locating and searching
directories for matching certificate DNs is specified herein. Note
though that GSS_Inquire_cred_by_mech() and GSS_Inquire_sec_context()
can and, indeed, MUST return "mechanism names" (MN) (see [RFC2743]).
The exported name token format for names of this type SHALL be the
Distinguished Encoding Rules (DER) [CCITT.X680.2002]
[CCITT.X690.2002] encoding of a GeneralName with directoryName as the
choice.
Implementation support for this name type is REQUIRED.
5.2. GSS_C_NT_HOSTNAME
The name type GSS_C_NT_HOSTNAME, with OID <TBD>, is defined. This
corresponds to the 'dNSName' choice of the 'GeneralName' ASN.1 type
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defined in [RFC5280].
The query syntax for names of this type SHALL be a DNS name [RFC1034]
in either ACE or Unicode form [RFC3490].
The display and canonical form of names of this type SHALL be a DNS
domain name in ACE form, with character case folded down.
Canonicalization consists merely of applying the ToASCII() function
and case-folding the result.
The exported name token format for names of this type SHALL be the
DER encoding of a GeneralName with dNSName as the choice and the DNS
domain name in ACE form and case folded down.
Implementation support for this name type is OPTIONAL.
5.3. GSS_C_NT_EMAIL_ADDR
The name type GSS_C_NT_EMAIL_ADDR, with OID <TBD>, is defined. This
corresponds to the 'rfc822Name' choice of the 'GeneralName' ASN.1
type defined in [RFC5280].
The query syntax and display form for names of this type SHALL be the
text representation of an 'addr-spec' as defined in [RFC0822].
The canonical form of names of this type SHALL be the query form with
case folded down. Note that the domain name part of an addr-spec is
a "domain name slot" and so the canonicalization rules for
GSS_C_NT_HOSTNAME given above apply here as well.
The exported name token form for this name type SHALL be the DER-
encoding of a GeneralName with the rfc822Name choice.
Implementation support for this name type is OPTIONAL.
5.4. GSS_KRB5_NT_PRINCIPAL_NAME
PKU2U supports the use of GSS_KRB5_NT_PRINCIPAL_NAME names [RFC1964].
The query, display, canonical and exported name token forms of names
of this type SHALL be as specified in RFC4121. The realm name part
of GSS_KRB5_NT_PRINCIPAL_NAME names is optional for the query syntax;
when canonicalized, names of this type lacking a realm name will have
the well-known PKU2U realm name affixed.
When the realm name of a GSS_KRB5_NT_PRINCIPAL_NAME NAME is defaulted
or otherwise is the well-known PKU2U realm name, then the "cname"' or
sname fields of the Kerberos V PDUs used to construct PKU2U security
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context tokens MUST be set to the principal name part of the given
NAME. Otherwise the PA_PKU2U_NAME pre-authentication data MUST be
used to indicate a name of id-pkinit-san type [RFC4556] corresponding
to the given NAME. See Section 5.4.
No attempt is made to map Kerberos V realm names to trust anchor
certificate authority (CA) names.
Note that having more than one mechanism share name-types has
implications for multi-mechanism, pluggable GSS-API implementations
(commonly referred to as "mechglue"). Specifically:
o It must be the responsibility of the mechanism, not of the
mechglue, to ensure that the standard exported name token header
(which includes a mechanism OID), is included in exported name
tokens. The exported name token for a GSS_KRB5_NT_PRINCIPAL_NAME
MN produced by PKU2U would have PKU2U's mechanism OID in the
header.
o A pluggable mechglue must be able to find a mechanism that can
import an exported name token if an available mechanism can
produce that exported name token. For example, a pluggable
mechglue where PKU2U is available but where the Kerberos V
mechanism [RFC1964] is not should still be able to import exported
Kerberos V name tokens since PKU2U can create such tokens. One
way to do this would be for the mechglue to try the mechanism
named in the exported name token header, if it is available, else
try all other available mechanisms until one succeeds or all fail.
It would be reasonable for a mechglue implementer to require that
the Kerberos V mechanism be available if PKU2U is too.
o It must be possible for GSS_Acquire_cred(), GSS_Add_cred() to use
a Kerberos V "mechanism name" (MN; see [RFC2743]) as desired_name
argument value to acquire a PKU2U credential. Similarly, it must
be possible to use a Kerberos V MN as the target_name in a call to
GSS_Init_sec_context with PKU2U as the mech OID. A multi-
mechanism mechglue implementer would likely have a mechglue-layer
NAME object that internally keeps a reference to a NAME object
produced by the underlying mechanism, but a pluggable mechglue
could not expect two different mechanisms to be able to share
their internal NAME objects. A clever implementer can work around
this by exporting the one mechanism's MN and then re-importing
using the target mechanism's GSS_Import_name() service function.
o It must be possible for the credential inquiry functions (e.g.,
GSS_Inquire_cred() and GSS_Inquire_cred_by_mech()) to return a
cred_name that is an MN of a different mechanism than the
credential element being inquired.
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Implementation support for this name type, with defaulted realm name
or with the PKU2U realm name is REQUIRED, but it is OPTIONAL for use
with any other realm names.
5.5. GSS_C_NT_ANONYMOUS
This is a generic GSS-API name-type. Implementation support for this
name type is OPTIONAL. See Section 6.1 for more information.
See [RFC2743] and [RFC2744] for more information about this name
type.
The PKU2U mechanism only supports anonymous initiators, not
acceptors.
Implementation support for this name type is RECOMMENDED.
5.6. GSS_C_NT_HOSTBASED_SERVICE - Matching Host-based Principal Names
to Acceptor Certificates
Support for GSS_C_NT_HOSTBASED_SERVICE names is REQUIRED as described
herein.
The query form of this name type is as per-RFC2743. The canonical
and exported name token forms are as per-RFC1964. The display form
of this name type is left unspecified, but should either be as per-
RFC2743 or the same as the display form for
GSS_KRB5_NT_PRINCIPAL_NAME [RFC1964].
Initiators using names of type GSS_C_NT_HOSTBASED_SERVICE to identify
target acceptors represent these names as Kerberos V principal names
as per [RFC1964] but with a well-known realm name of "WELLKNOWN:
PKU2U" (see Section 5.4).
Acceptors match such names to acceptor certificates as follows.
Initiators then match the certificate chosen by the acceptor in the
same manner.
Initiators can also assert host-based service names as the initiator
name. In this case acceptors MUST also apply the matching rules
below, in order, to validate the initiator's assertion.
1. If there is an out-of-band binding of the peer's host-based
service name to its certificate, then the certificate matches.
2. If the peer has a certificate with an id-pkinit-san subject
alternative name matching the initiator-provided acceptor name,
then the X.509 certificate matches.
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3. If a X.509 certificate has a dNSName SAN that matches the
hostname part of the host-based service principal name, and
either the anyExtendedKeyUsage extended key usage (EKU), or no
EKU is present, or an EKU is present which corresponds to the
service part of the host-based service principal name, then the
X.509 certificate matches. The id-kp-serverAuth EKU SHALL be
considered to match the 'HTTP' service name. (See Section 10,
IANA considerations, where the GSS-API service name registry is
extended to include an EKU for each service name.)
4. Implementations SHOULD, subject to local configuration, allow
matches where the single-component cn of the DN of a X.509
certificate matches the hostname part of the host-based service
name, for some or all service names. This feature is needed to
allow the use of existing X.509 web certificates.
Implementation support for this name type as an acceptor name is
REQUIRED. Implementation support for this name type as an initiator
name is OPTIONAL.
6. The Protocol Description and the Context Establishment Tokens
The PKU2U mechanism is a GSS-API mechanism based on [RFC4120],
[RFC4556] and [RFC4121].
The per-message tokens of the PKU2U mechanism are the same as those
of the Kerberos V GSS-API mechanism [RFC4121].
The GSS_Pseudo_random() function [RFC4401] of the PKU2U is the same
as that of the Kerberos V GSS-API mechanism [RFC4402].
The PKU2U security context token exchange consists of KRB_AS_REQ and
KRB_AS_REP (and KRB_ERROR) Kerberos KDC PDUs (with no changes to
their ASN.1 description, but with other minor changes/requirements
described below) as context tokens, with the acceptor as the KDC,
followed by context tokens from [RFC4121] using the Kerberos V Ticket
PDU issued by the acceptor-as-KDC. PKINIT [RFC4556] is the only
acceptable pre-authentication method in this document. Caching the
ticket issued by the acceptor allows subsequent security context
exchanges between the same to peers to use a single context token
round-trip -- a "fast reconnect" feature.
PKU2U differs from Kerberos V with PKINIT in several minor ways as
follows (this is not a complete list):
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o KDC PDUs are not exchanged as usual in Kerberos, but wrapped as
[the first two] GSS-API context tokens.
o PKU2U does not use KDC certificates.
o PKU2U adds pa-data types for carrying the initiator's assertion of
its name and the targ_name passed to GSS_Init_sec_context().
PKU2U differs from the Kerberos V GSS-API mechanism in several ways:
o KDC PDUs are not exchanged as described in [RFC4120], but wrapped
as GSS-API context tokens.
o PKU2U allows the use of principal names matching PKI naming (see
Section 5). PKU2U does support the use of Kerberos V naming, but
requires only support of Kerberos V naming to a limited extent
(full support is optional).
o PKU2U adds an extension [GSS-EXTS] to the RFC4121 initial context
token for binding the AP-REQ to the AS exchange that precedes is
(that is, when the initiator has to request a ticket from the
acceptor).
o The number of round-trips can vary. If the initiator already has
a ticket for the acceptor then the context token exchange will be
half a round-trip or one round-trip, as per RFC4121. Otherwise
one or two round-trips are added for the AS exchanges needed to
acquire a ticket. Note that two AS exchanges may be required when
the initiator's initial choice of X.509 certificate does not match
the acceptor's trust anchors, in which case the acceptor SHOULD
reply with a KRB-ERROR with TD-TRUSTED-CERTIFIERS indicating what
the acceptor's trust anchors are, and then the initiator can
engage in a second AS exchange within the same GSS-API context.
To recapitulate, the acceptor and the initiator communicate by
tunneling the authentication service exchange messages through the
use of the GSS-API tokens and application traffic. In the event of
security context token loss, message duplication, or out of order
message delivery, the security context MUST fail to establish.
All security context establishment tokens MUST follow the
InitialContextToken syntax defined in Section 3.1 of [RFC2743].
PKU2U is identified by the Objection Identifier (OID) id-kerberos-
pku2u.
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The PKU2U OID is:
id-kerberos-pku2u ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
pku2u(7) }
All context establishment tokens consist of some Kerberos V PDU or
another, prefixed with a two-octet token type ID, and the
InitialContextToken header (see above).
The innerToken described in section 3.1 of [RFC2743] and subsequent
GSS-API mechanism tokens have the following formats: it starts with a
two-octet token-identifier (TOK_ID), followed by a Kerberos message.
The TOK_ID values for the AS-REQ message and the AS-REP message are
defined in the table blow:
Token TOK_ID Value in Hex
-----------------------------------------------
KRB_AS_REQ 05 00
KRB_AS_REP 06 00
The TOK_ID values for all other Kerberos messages are the same as
defined in [RFC4121].
It should be noted that by using anonymous PKINIT [KRB-ANON], PKU2U
can authenticate the acceptor without revealing the initiator's
identity
6.1. Context Token Derived from KRB_AS_REQ
When the initiator does not have a service ticket to the acceptor, it
requests a ticket from the acceptor instead of from the KDC by
constructing a KRB_AS_REQ PDU [RFC4120] and using it as the context
token, with a token type ID prefixed. This will be the initiator's
initial context token, therefore it MUST also have the standard
header bearing the OID of the mechanism being used (in this case,
PKU2U's OID).
The initiator MUST NOT set any KDC options in the 'kdc-options' field
of the AS-REQ.
The 'realm' field of the AS-REQ MUST be set to the PKU2U well-known
PKU2U realm name ("WELLKNOWN:PKU2U" [KRB-NAMING].
If the initiator wishes to assert a name of type
GSS_KRB5_NT_PRINCIPAL_NAME or GSS_C_NT_HOSTBASED_SERVICE, then it
MUST set the 'cname' field of the AS-REQ accordingly if and only if
the realm name part of the given name object is defaulted or the
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well-known PKU2U realm name. Otherwise the initiator MUST add a pa-
data element (see below) stating the name that the initiator wishes
to assert, it MUST set the cname field to the NULL principal name as
defined in Section 4.
If the targ_name passed to GSS_Init_sec_context() is of type
GSS_C_NT_HOSTBASED_NAME then the initiator sets the 'sname' field of
the AS-REQ to match the parsed name as per [RFC4121]. If the target
name does not have a representation as a Kerberos principal name per
[RFC1964], then the initiator MUST add a pa-data element (see below)
stating the given targ_name and the initiator MUST set the 'sname'
field of the AS-REQ to the NULL principal name as defined in
Section 4.
The padata used to convey initiator and target names is of type
PA_PKU2U_NAME <136> and it's value consists of the DER
[CCITT.X680.2002] [CCITT.X690.2002] encoding of the ASN.1 type
InitiatorNameAssertion (with explicit tagging).
InitiatorName ::= CHOICE {
-- -1 -> certificate DN
-- 0..16384 -> subjectAltName named by
-- this index
sanIndex INTEGER (-1..16384), -- DN or SAN
nameNotInCert GeneralName, -- name not present in cert
-- (see RFC5280 for definition
of GeneralName)
...
}
TargetName ::= CHOICE {
exportedTargName OTCET STRING, -- exported krb5 name
generalName [0] GeneralName, -- all other PKI names
-- (tagged to distinguish
-- from nameNotInCert
-- choice of InitiatorName)
...
}
InitiatorNameAssertion ::= SEQUENCE {
initiatorName InitiatorName OPTIONAL,
targetName TargetName OPTIONAL,
...
}
The initiatorName, if present, contains the initiator's name. The
initiator can fill out either the sanIndex field or the nameNotInCert
field to indicate the name of the initiator.
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The sanIndex field, if present, is used to refer to either the
Distinguished Name or the SubjectAltName in the initiator's X.509
certificate. A sanIndex value of -1 value refers to the initiator's
certificate's DN. All other legal values of sanIndex refer to the
corresponding element of the SubjectAltName sequence. A value of 0
means the first instance of GeneralName in the SubjectAltName
sequence, and 1 means the second, and so on. If the sanIndex value
is equal or biger than the number of GeneralName elements in the
SubjectAltName, the security context establishment attempt MUST fail.
The nameNotInCert field, if present, contains the initiator's
GeneralName.
If an initiator name assertion is included, the acceptor MUST verify
that this asserted name is either present in the initiator's
certificate or otherwise bound to the initiator's certificate by out-
of-band provisioning (e.g., by a table lookup). Failure to validate
the asserted initiator's name MUST cause GSS_Accept_sec_context() to
return an error and, optionally, to output a KRB_ERROR context token
as per-RFC4121.
The initiatorName field MUST NOT be present if the initiator is
anonymous or if the 'cname' field of the AS-REQ is not the NULL name
(see Section 4).
Target names passed to GSS_Init_sec_context() that can be represented
as Kerberos V principal names, namely, names of
GSS_KRB5_NT_PRINCIPAL_NAME and GSS_C_NT_HOSTBASED_SERVICE, MUST be
represented as the 'sname' field of the AS-REQ or as the
exportedTargName choice of TargetName (if the realm part is not the
PKU2U realm name). The contents of the exportedTargName octet string
MUST be an exported name token for the Kerberos V mechanism
containing a Kerberos V principal name.
Other target names are represented as a generalName choice of
TargetName. These may be present in an acceptor certificate, or
agreed out of band.
The acceptor MUST select an appropriate acceptor credential that
matches the AS-REQ's 'sname' (if not NULL) or the targetName provided
in the InitiatorNameAssertion, when present.
The targetName field MUST NOT be present if the 'sname' field of the
AS-REQ is not the NULL name. The targetName field MUST be present if
the 'sname' field of the AS-REQ is the NULL name.
The PA_PKU2U_NAME padata SHOULD NOT be present when the initiatorName
and targetName both shouldn't be present.
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Implementation note: the encrypted part of a PKU2U Ticket can be
anything at all since the only entity that will consumer a given
PKU2U Ticket is the same entity that issued it. Implementers may
choose to use the traditional EncTicketPart ASN.1 type [RFC4120] and
DER encoding.
6.2. Context Token Derived from KRB_AS_REP
When the initiator's initial context token is a AS-REQ then the
acceptor MUST reply with either a KRB-ERROR token as per [RFC4121] or
a token derived from a KRB_AS_REP PDU [RFC4120] constructed to
respond to the initiator's KRB_AS_REQ.
The initiator MUST use PKINIT pre-authentication and the acceptor
MUST require it. If the initiator does not use PKINIT pre-
authentication then the acceptor MUST respond with a KRB-ERROR and
indicate that PKINIT is required.
If the initiator's KRB_AS_REQ token is valid, and the asserted
initiator's name, if present, is bound with the initiator's
certificate, and the acceptor can select a certificate based on the
initiator's asserted targ_name, the acceptor then constructs a
KRB_AS_REP using PKINIT as described in [RFC4556], except that the
acceptor's certificate is used in the place of the KDC certificate.
If and only if the initiator's X.509 certficate is validated using
PKI, the acceptor SHOULD include an authorization element
AD_INITIAL_VERIFIED_CAS [RFC4556] in the returned ticket. If an
InitiatorName is included in the PA_PKU2U_NAME padata in the request,
an authorization element of the type ad-pku2u-client-name <143> MUST
be included in the returned ticet and this authorization element
contains the DER encoded InitiatorName in the request.
The initiator then validates the KRB-AS-REP reply context token
according to Section 3.1.5 of [RFC4120] and Section 3.2.4 of
[RFC4556]. The inclusion of the EKU KeyPurposeId [RFC5280] id-
pkinit-KPKdc in the X.509 certificate in the response is not
applicable when PKU2U is used because there is no KDC involved in
this protocol. The initiator MUST verify that the acceptor's
certificate is bound with the targ_name passed in to
GSS_Init_sec_context(), by verifying either the targ_name matches
with either the subject DN or one instance of the SubjectAltName name
in the acceptor's certificate, or otherwise the targ_name is bound
with the acceptor's certificate by out-of-band provisioning (e.g., by
a table lookup). Failure to validate this name binding MUST cause
the authentication to be rejected.
The 'flags' field of the AS-REP MUST have only the 'initial' and
'pre-authent' flags set.
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The 'authtime' field of the AS-REP MUST be set to the acceptor's
current time as it is when it formats the AS-REP.
Otherwise all other aspects of the AS-REP are as described in
[RFC4120].
The values of the tkt-vno, realm and 'sname' fields of the Ticket
issued by the acceptor are unspecified. The initiator MUST NOT
examine them for correctness. Cut-n-paste attacks are prevented by
the fact that PKU2U provides integrity protection for all cleartext
in Kerberos V PDUs used by PKU2U (and for the mechanism OID).
6.3. Context Tokens Imported from RFC4121
Once the initiator has a Kerberos V Ticket for the acceptor the
security context token exchange will continue with those of the
Kerberos V GSS-API mechanism [RFC4121] with the following
modifications:
o The mechanism OID of PKU2U SHALL be used instead of that of the
Kerberos V GSS-API mechanism;
o The 'crealm' field of the initiator's Authenticator MUST be set to
the PKU2U realm name and if the 'cname" field is the NULL
principal name, an authorization element of the type ad-pku2u-
client-name <143> MUST be included in the authenticator and this
authorization element contains the DER encoded InitiatorName in
the AS-REQ based on which the ticket was obtained;
o The sub-session key MUST be used in the initiator's Authenticator;
o The contents of the encrypted part of the Ticket can be
implementation specific since the only entity consuming it will be
the same entity that issues it;
o If the initiator's initial context token is a KRB_AS_REQ token
(i.e., not KRB_AP_REQ token), then the Exts field in the
Authenticator of the KRB_AP_REQ-derived token MUST contain an
extension [GSS-EXTS] of the type GSS_EXTS_FINISHED <2> as defined
next in this section.
The 'cusec', 'ctime', 'seq-number' and 'authorization-data' fields of
the Authenticator are set as per [RFC4121] and [RFC4120].
The 'cksum' field of the Authenticator MUST be set as per [RFC4121].
The extension data of the GSS_EXTS_FINISHED extension type [GSS-EXTS]
contains the DER encoding of the ASN.1 structure KRB-FINISHED.
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GSS_EXTS_FINISHED 2
--- The type for the checksum extension.
KRB-FINISHED ::= SEQUENCE {
gss-mic [1] Checksum,
-- Contains the checksum (RFC3961) of the GSS-API tokens
-- that have been exchanged between the initiator and the
-- acceptor and prior to the containing AP-REQ GSS-API token.
-- The checksum is performed over the GSS-API
-- context tokens in the order that the tokens were sent.
...
}
The gss-mic field contains a Kerberos checksum [RFC3961] that is
computed over all the preceding context tokens of this GSS-API
context (including the InitialContextToken header), concatenated in
chronological order (note that GSS-API context token exchanges are
synchronous). The checksum type is the required checksum type of the
enctype of the subkey in the authenticator, the protocol key for the
checksum operation is the authenticator subkey, and the key usage
number is KEY_USAGE_FINISHED <41>.
The acceptor MUST process the KRB_AP_REQ token as usual for RFC4121,
except that if the context token exchange included an AS exchange,
then the acceptor MUST also validate the GSS_EXTS_FINISHED and return
an error if it is not valid or not present. But if a KRB_AP_REQ
context token is the initial context token then the acceptor MUST
return an error if GSS_EXTS_FINISHED is present.
The GSS_EXTS_FINISHED (along with the ticket) binds the second part
of the context token exchange to the first, and it binds the pa-data
used in the request as well (this needs to be done because PKINIT
does not bind pa-data other than PKINIT pa-data from the request).
GSS_EXTS_FINISHED also protects all otherwise unauthenticated
plaintext in Kerberos V PDUs. Note that GSS_EXTS_FINISHED also
protects the mechanism OID in the InitialContextToken header.
The acceptor MUST verify that the ad-pku2u-client-name authorization
element is present in the authenticator if and only there is an
authorization element of the same type in the ticket and the values
of these two elements MUST match exactly based on bit-wise
comparison.
7. Guidelines for Credentials Selection
If a peer, either the initiator or the acceptor, has multiple pairs
of public-key private keys and certificates, a choice is to be made
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in choosing the best fit. The trustedCertifiers field in the PA-PK-
AS-REQ structure [RFC4556] SHOULD be filled by the initiator, to
provide hints for guiding the selection of an appropriate certificate
chain by the acceptor.
If the initiator's X.509 certificate cannot be validated according to
[RFC5280], the acceptor SHOULD send back the TD-TRUSTED-CERTIFIERS
structure [RFC4556] that provides hints for guiding the selection of
an appropriate certificate by the initiator. In this case
GSS_Accept_sec_context() returns GSS_S_CONTINUE_NEEDED, and the
initiator gets to try again in its subsequent AS-REQ token.
The GSS-API does not provide a programming interface to make this
credential selection interactive, though implementers may provide
methods for user interaction related to credential selection and
acquisition (e.g., name and password/PIN prompts). Whenever the
execution context allows for direct interaction of the mechanism with
the user then it is RECOMMENDED that implementations interact with
the user to select a credential whenever multiple credentials are
equally usable and no other mechanism is available to inform the
credential selection.
If the certificates cannot be selected interactively, multiple
certificates are equally usable, and there is no other mechanism
available for credential selection, then it is RECOMMENDED that
initiators fail the context. Users should be able to retry using a
specific credential (this requires that distinct credentials have
distinct names that can be used to acquire each credential
separately).
8. Security Considerations
The security considerations in [RFC4120], [RFC4121], [RFC4556] and
[RFC5280] apply here. This mechanism relaxes some requirements of
PKINIT and adds a device for protecting otherwise unauthenticated
plaintext in the protocol (see Section 6.3) -- it is crucial that
this device be faithfully implemented. It is also crucial that both
the initiator and the acceptor MUST be able to verify the binding
between the signing key and the asserted identity.
Note that PKU2U is just as susceptible to replays of AP-REQs as the
traditional Kerberos V GSS-API mechanism [RFC4121], though only when
using an AP-REQ as the initial security context token. It is
important, therefore, to use a replay cache to detect replays.
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9. Acknowledgements
The authors would like to thank Jeffrey Hutzelman for his insightful
comments on the earlier revisions of this document.
In addition, the following individuals have provided review comments
for this document: Sam Hartman, Leif Johansson, Olga Kornievskaia,
Martin Rex, and Sunil Gottumukkala.
Ari Medvinsky provided help in editing the initial revisions of this
document.
The text for the DN mapping is compiled from the email discussions
among the following individuals: Howard Chu, Martin Rex, Jeffrey
Hutzelman, Kevin Coffman, Henry B. Hotz, Leif Johansson, and Olga
Kornievskaia. Howard and Jeffery clearly illustrated the challenges
in creating a unique mapping, while Nicolas and Martin demonstrated
the relevance and interactions to GSS-API and Kerberos.
10. IANA Considerations
This document defines the PKU2U realm and the place-holder well-known
principal name. The IANA registry for the reserved names should be
updated to reference this document. Two entries are added: one entry
for the well-known realm "WELLKNOWN:PKU2U", and another for the well-
known principal name "WELLKNOWN/NULL".
This document defines GSS_EXTS_FINISHED extension type. The
corresponding IANA registry [GSS-EXTS] need to be updated to
reference this document. The following single registration should be
added in the registry for "Kerberos V GSS-API mechanism extension
types": 2, "GSS-API token checksum", "Extension to provide a checksum
for GSS-API tokens", the RFC # of this document.
This document also instructs the IANA to extend the "SMI Security for
Name System Designators Codes (nametypes)" registry to include an OID
for each registration, and to allocate OIDs for the following GSS-API
name-types in that registry:
o gss-distinguished-name (GSS_C_NT_DN)
o gss-hostname (GSS_C_NT_HOSTNAME)
o gss-IP-address (GSS_C_NT_IP_ADDR)
o gss-e-mail-address (GSS_C_NT_EMAIL_ADDR)
11. Normative References
[CCITT.X680.2002]
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International International Telephone and Telegraph
Consultative Committee, "Abstract Syntax Notation One
(ASN.1): Specification of basic notation",
CCITT Recommendation X.680, July 2002.
[CCITT.X690.2002]
International International Telephone and Telegraph
Consultative Committee, "ASN.1 encoding rules:
Specification of basic encoding Rules (BER), Canonical
encoding rules (CER) and Distinguished encoding rules
(DER)", CCITT Recommendation X.690, July 2002.
[GSS-EXTS]
Emery, S., "Kerberos Version 5 GSS-API Channel Binding
Hash Agility", draft-ietf-krb-wg-gss-cb-hash-agility (work
in progress), 2007.
[KRB-ANON]
Zhu, L. and P. Leach, "Kerberos Anonymity Support",
draft-ietf-krb-wg-anon (work in progress), 2007.
[KRB-NAMING]
Zhu, L., "Additional Kerberos Naming Constraints",
draft-ietf-krb-wg-naming (work in progress), 2007.
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[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.
[RFC2744] Wray, J., "Generic Security Service API Version 2 :
C-bindings", RFC 2744, January 2000.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3961] Raeburn, K., "Encryption and Checksum Specifications for
Zhu, et al. Expires May 7, 2009 [Page 20]
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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.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC4402] Williams, N., "A Pseudo-Random Function (PRF) for the
Kerberos V Generic Security Service Application Program
Interface (GSS-API) Mechanism", RFC 4402, February 2006.
[RFC4514] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, June 2006.
[RFC4517] Legg, S., "Lightweight Directory Access Protocol (LDAP):
Syntaxes and Matching Rules", RFC 4517, June 2006.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
Authors' Addresses
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
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Jeffery Altman
Secure Endpoints
255 W 94th St
New York, NY 10025
US
Email: jaltman@secure-endpoints.com
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
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