NETWORK WORKING GROUP L. Zhu
Internet-Draft Microsoft Corporation
Intended status: Standards Track J. Altman
Expires: January 15, 2009 Secure Endpoints
N. Williams
Sun
July 14, 2008
Public Key Cryptography Based User-to-User Authentication - (PKU2U)
draft-zhu-pku2u-05
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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. PKU2U Principal Naming . . . . . . . . . . . . . . . . . . . . 4
4.1. GSS_C_NT_DN . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. GSS_C_NT_HOSTNAME . . . . . . . . . . . . . . . . . . . . 5
4.3. GSS_C_NT_IP_ADDR . . . . . . . . . . . . . . . . . . . . . 5
4.4. GSS_C_NT_EMAIL_ADDR . . . . . . . . . . . . . . . . . . . 5
4.5. GSS_KRB5_NT_PRINCIPAL_NAME . . . . . . . . . . . . . . . . 6
4.6. Matching host-based service principal names to
acceptor certificates . . . . . . . . . . . . . . . . . . 6
5. The Protocol Description and the Context Establishment
Tokens . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Context token derived from KRB_AS_REQ . . . . . . . . . . 8
5.2. Context token derived from KRB_AS_REP . . . . . . . . . . 9
5.3. Context tokens imported from RFC4121 . . . . . . . . . . . 10
6. Guidelines for Credentials Selection . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Normative References . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
Peer-to-peer systems are increasingly popular today. In a peer-to-
peer system, all clients provide resources that contribute positively
to the total capacity of the overall system and there is no single
point of failure. This distributed nature makes such systems highly
scalable and robust.
A true peer-to-peer system is self-organized, typically there is no
trusted third party in such environments. Consequently the Kerberos
protocol as defined in [RFC4120] and [RFC4556] is inadequate to
provide security services. Currently there is no interoperable GSS-
API mechanism for establishing trust in the information received from
the peer. The inability to authenticate the messages exchanged among
peers enables many attacks such as poisoning (e.g. providing data
contents are different from the description) and polluting (e.g.
inserting "bad" packets).
To remedy this, the PKU2U protocol extends [RFC4120] and [RFC4556] to
support peer-to-peer authentication without the help of a Key
Distribution Center (KDC) [RFC4120]. This mechanism can act as a
bridge between Public Key Infrastructure (PKI) and GSS-API for such
environments.
In addition, the binding of PKU2U for GSS-API is defined based on
[RFC4121].
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".
Unless otherwise specified, the realm name in any Kerberos message
used by PKU2U is the PKU2U realm name.
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4. PKU2U Principal Naming
PKU2U principal names are certificate names and subject alternative
names [RFC3280] as they appear in the certificate of any PKU2U peer,
as well as any names agreed to out of band where a certificate
authenticates a name that does not appear in the certificate.
Thus certificates may be associated with multiple principal names.
This presents problems for the GSS-API bindings of a PKI-based
mechanism. We resolve these problems as follows:
o We define multiple GSS-API name types corresponding to several
GeneralName choices [RFC3280], along with syntaxes, display forms,
and exported name token formats for each.
o The cred_name of credential object acquired with a desired_name
other than GSS_C_NT_NO_NAME MUST be equal to the name used as the
desired_name.
o We provide that the cred_name of a default credential, or of a
credential acquired with the GSS_C_NT_NO_NAME desired_name MUST be
the first id-pkinit-san subject alternative name of the
certificate, if there is one, or else it MUST be the DN of its
certificate.
o We provide a method by which initiators can assert one of these
names to the other, and we provide a default of asserting the
certificate DN. We also provide a method of asserting names that
do not appear in the caller's certificate. This assertion
consists of a traditional Kerberos V principal name appearing in
the expected 'cname' fields of various Kerberos V PDUs used to
construct PKU2U security context tokens, and an optional
authorization-data type consisting of an integer which identifies
one of the several names of the initiator's certificate.
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.
Thus GSS-API initiators that use the GSS_C_NO_NAME as the
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 certificate's DN, and that certificate's 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
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authenticating peers with certificates predating this specification.
4.1. GSS_C_NT_DN
We introduce a new name type, GSS_C_NT_DN, with OID <TBD>,
corresponding to the 'Name' ASN.1 type defined in [RFC3280].
The query syntax and display form for names of this type SHALL be as
described in [RFC4514].
The exported name token format for names of this type SHALL be the
'Name' value as it appears in a certificate DN (or directoryName
subject alternative name), but re-encoded in DER.
Support for name type is REQUIRED.
4.2. GSS_C_NT_HOSTNAME
We introduce a new name type, GSS_C_NT_HOSTNAME, with OID <TBD>,
corresponding to the 'dNSName' choice of the 'GeneralName' ASN.1 type
defined in [RFC3280].
The query syntax, display and exported name token forms for names of
this type SHALL be a DNS name [RFC1034] in ACE form [RFC3490].
Support for name type is OPTIONAL.
4.3. GSS_C_NT_IP_ADDR
We introduce a new name type, GSS_C_NT_IP_ADDR, with OID <TBD>,
corresponding to the 'iPAddress' choice of the 'GeneralName' ASN.1
type defined in [RFC3280].
The query syntax, display and exported name token forms for names of
this type SHALL be the text representation of an IPv4 or IPv6 address
(XXX add references).
Support for name type is OPTIONAL.
4.4. GSS_C_NT_EMAIL_ADDR
We introduce a new name type, GSS_C_NT_EMAIL_ADDR, with OID <TBD>,
corresponding to the 'rfc822Name' choice of the 'GeneralName' ASN.1
type defined in [RFC3280].
The query syntax, display and exported name token forms for names of
this type SHALL be the text representation of an 'addr-spec' as
defined in [RFC0822].
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Support for name type is OPTIONAL.
4.5. GSS_KRB5_NT_PRINCIPAL_NAME
PKU2U supports the use of GSS_KRB5_NT_PRINCIPAL_NAME names [RFC1964].
These appear as expected in various 'cname', 'crealm', 'sname' and
'srealm' fields of the Kerberos V PDUs used to construct PKU2U
security context tokens. For PKU2U names of this type correspond to
subject alternative names of type 'id-pkinit-san' [RFC4556]. When no
such SAN appears in a certificate, then names of this type correspond
to certificates as agreed out of band (e.g., there may be a table
mapping certificates to Kerberos V principal names).
Support for name type is REQUIRED.
4.6. Matching host-based service principal names to acceptor
certificates
Support for GSS_C_NT_HOSTBASED_SERVICE names is REQUIRED as described
herein.
Initiators using names of type GSS_C_NT_HOSTBASED_SERVICE to identify
target acceptors represent these names as Kerberos V principal names
as per [RFC4121] but with a well-known realm name of "PKU2U" (see
Section 5.1).
Acceptors match such names to acceptor certificates as follows.
Initiators then match the certificate chosen by the acceptor in the
same manner.
1. If the acceptor has a certificate with an id-pkinit-san subject
alternative name matching the initiator-provided acceptor name,
then the certificate matches.
2. If the acceptor has a certificate with 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
certificate matches. The id-kp-serverAuth EKU SHALL be
considered to match the 'HTTP' service name. (See Section 9,
IANA considerations, where the GSS-API service name registry is
extended to include an EKU for each service name.)
3. Implementations SHOULD, subject to local configuration, allowing
matches where the CN of the DN of a cert 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 web
certificates.
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5. 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 PKU2U security context token exchange consists of KRB-AS-REQ and
KRB-AS-REP (and KRB-ERROR) Kerberos KDC PDUs (with 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. Caching that 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 restart" feature.
PKU2U differs from Kerberos V with PKINIT in several minor ways:
o KDC PDUs are not exchanged as usual in Kerberos, but wrapped as
GSS-API context tokens
o PKU2U allows the use of out-of-band binding of certificates to
principal names
o PKU2U does not require the use of 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 usual in Kerberos, but wrapped as
GSS-API context tokens
o PKU2U allows the use of principal names matching PKI naming
o PKU2U adds an extension 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)
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. The reliable
delivery of the authentication service exchange messages at the GSS-
API token level is mandatory. In the event of message loss, message
duplication, or out of order message delivery, the security context
MUST fail to establish.
All 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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id-kerberos-pku2u ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
pku2u(7) }
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 KRB_AS_REQ message and the KRB_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].
By using anonymous PKINIT [KRB-ANON], PKU2U can provide server-
authentication without revealing the client's identity.
5.1. 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 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).
If the initiator wishes to assert a name of type GSS_C_NT_ANONYMOUS
then it MUST set the cname field to WELLKNOWN/WELLKNOWN with name
type KRB_NT_WELLKNOWN [KRB-ANON]. If the initiator wishes to assert
a name of type GSS_KRB5_NT_PRINCIPAL_NAME, then it MUST set the cname
and realm fields to match. 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 anonymous principal
name, and it MUST set the realm field to "WELLKNOWN:PKU2U" with type
"other" [KRB-NAMING].
If the targ_name passed to GSS_Init_sec_context() is of
GSS_C_NT_ANONYMOUS type, then the sname field of the AS-REQ SHALL be
set to WELLKNOWN/WELLKNOWN with name type KRB_NT_WELLKNOWN
[KRB-ANON]. 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
to match the parsed name as per [RFC4121], and there is no notion of
server realm name in this case. In all other cases the initiator
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MUST produce the exported name token for the given targ_name and MUST
include it as the pa-value of an pa-data element whose pa-type SHALL
be <TBD>, and the initiator MUST set the sname field of the AS-REQ to
WELLKNOWN/WELLKNOWN with name type KRB_NT_WELLKNOWN [KRB-ANON] -- the
presence of the targ_name pa-data element means the targ_name is not
of type GSS_C_NT_ANONYMOUS.
The token type ID for this token SHALL be 05 00.
The pa-data element for the initiator's name assertion SHALL be of
ad-type <TBD> and its ad-value SHALL consist of the BER encoding of
the following:
InitiatorNameAssertion ::= CHOICE {
INTEGER (-1..16384),
GeneralName -- see [RFC3280].
}
The acceptor MUST validate this token as a Kerberos V KDC would
validate a PKINIT AS-REQ, and also MUST check that the initiator's
asserted name is present in the initiator's certificate or otherwise
bound to the initiator's certificate by out-of-band negotiation
(e.g., by a table lookup). Failure to validate this token MUST cause
GSS_Accept_sec_context() to return an error and, optionally, to
output a KRB-ERROR context token as per-RFC4121.
5.2. Context token derived from KRB_AS_REP
When the initiator's initial context token is a KRB_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 acceptor MUST only accept PKINIT pre-authentication. If the
initiator's KRB_AS_REQ token is valid then the acceptor MUST select a
certificate (if it has more than one) which matches the initiator's
targ_name, and then the acceptor MUST construct a KRB_AS_REP as usual
for PKINIT, except that there is no requirement to use a KDC
certificate.
The initiator then validates this token according to Section 3.1.5 of
[RFC4120] and Section 3.2.4 of [RFC4556]. The inclusion of the EKU
KeyPurposeId [RFC3280] 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 matches the targ_name passed in to
GSS_Init_sec_context().
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5.3. 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 a sub-session key MUST be included in the initiator's
Authenticator;
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 [GSS-EXTS] MUST
contain an extension of the type GSS_EXTS_FINISHED (extension type
ID <TBA>) and the extension data contains a DER-encoded value of
the Kerberos V 'Checksum' type, where the checksum (actually, a
MAC, see [RFC3961]) is taken over all the preceding context tokens
in this exchange (including the InitialContextToken header),
concatenated in chronological order (remember, GSS-API context
token exchanges are synchronous). The key usage number for this
checksum is KEY_USAGE_FINISHED (number 41).
The acceptor MUST process this token as usual for RFC4121, except
that if the context token exchange included an AS eschange, then the
acceptor MUST also validate the GSS_EXTS_FINISHED and return an error
if it is not valid or not present. But if this 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.
KEY_USAGE_FINISHED 41
6. Guidelines for Credentials Selection
If a peer, either the initiator or the acceptor, has multiple pairs
of public-key private keys, a choice is to be made 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
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acceptor.
If the initiator's X.509 certificate cannot be validated according to
[RFC3280], 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 way to make this credential selection
interactive, however, whenever the context allows for direct
interaction of the mechanism with the user then it is RECOMMENDED
that implementations do so.
If the certificates cannot be selected interactively, and multiple
certificates can be used, it is RECOMMENDED that initiators fail the
context establishment thus avoid confusions caused by an unexpected
programmatic selection. 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).
7. Security Considerations
The security considerations in [RFC4556] apply here. This mechanism
relaxes some requirements of PKINIT and adds a device for protecting
otherwise unauthenticated plaintext in the protocol -- 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 associated identity.
8. 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: Nicolas Williams, 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 directly from the email
discussions among the following individuals: Howard Chu, Martin Rex,
Nicolas Williams, Jeffrey Hutzelman, Kevin Coffman, Henry B. Hotz,
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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.
9. IANA Considerations
Section 3 defines the PKU2U realm. The IANA registry for the
reserved names should be updated to reference this document.
This document defines GSS_EXTS_FINISHED extension type. The
corresponding IANA registry 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":
GSS_EXTS_FINISHED, "GSS-API token checksum", "Extension to provide a
checksum for GSS-API tokens", the RFC # of this document.
This document also expands the existing GSS-API service name
registry, that IANA maintains, to have a an assignment of one or more
OIDs for each service name. The id-kp-serverAuth OID [RFC3280] is
immediately registered as the OID for the "HTTP" service name.
10. Normative References
[GSS-EXTS]
Emery, S., "Kerberos Version 5 GSS-API Channel Binding
Hash Agility",
draft-ietf-krb-wg-gss-cb-hash-agility-03.txt (work in
progress), 2007.
[KRB-ANON]
Zhu, L. and P. Leach, "Kerberos Anonymity Support",
draft-ietf-krb-wg-anon-04.txt (work in progress), 2007.
[KRB-NAMING]
Zhu, L., "Additional Kerberos Naming Constraints",
draft-ietf-krb-wg-naming-04 (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.
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[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.
[RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[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
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.
[RFC4514] Zeilenga, K., "Lightweight Directory Access Protocol
(LDAP): String Representation of Distinguished Names",
RFC 4514, June 2006.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
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
Zhu, et al. Expires January 15, 2009 [Page 14]
Internet-Draft PKU2U July 2008
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