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Versions: 02 03 04                                                      
INTERNET-DRAFT                                    Ari Medvinsky
draft-ietf-cat-kerberos-pk-tapp-04.txt            Keen.com, Inc.
Expires May 9, 2001                               Matthew Hur
Informational                                     Cisco Systems
                                                  Sasha Medvinsky
                                                  Motorola
                                                  Clifford Neuman
                                                  USC/ISI

Public Key Utilizing Tickets for Application Servers (PKTAPP)


0. Status Of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.  Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF),
its areas, and its working groups.  Note that other groups may also
distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time.  It is inappropriate to use Internet-Drafts
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progress."

The list of current Internet-Drafts can be accessed at
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To learn the current status of any Internet-Draft, please check
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The distribution of this memo is unlimited.  It is filed as
draft-ietf-cat-kerberos-pk-tapp-04.txt, and expires May 9,
2001.  Please send comments to the authors.

1. Abstract

Public key based Kerberos for Distributed Authentication[1], (PKDA)
proposed by Sirbu & Chuang, describes PK based authentication that
eliminates the use of a centralized key distribution center while
retaining the advantages of Kerberos tickets.  This draft describes how,
without any modification, the PKINIT specification[2] may be used to
implement the ideas introduced in PKDA.  The benefit is that only a
single PK Kerberos extension is needed to address the goals of PKINIT &
PKDA.



2. Introduction

With the proliferation of public key cryptography, a number of public
key extensions to Kerberos have been proposed to provide
interoperability with the PK infrastructure and to improve the Kerberos
authentication system [4].  Among these are PKINIT[2] (under development
in the CAT working group) and more recently PKDA [1] proposed by Sirbu &
Chuang of CMU.  One of the principal goals of PKINIT is to provide for
interoperability between a PK infrastructure and Kerberos.  Using
PKINIT, a user can authenticate to the KDC via a public key certificate.
A ticket granting ticket (TGT), returned by the KDC, enables a PK user
to obtain tickets and authenticate to kerberized services.  The PKDA
proposal goes a step further.  It supports direct client to server
authentication, eliminating the need for an online key distribution
center.  In this draft, we describe how, without any modification, the
PKINIT protocol may be applied to achieve the goals of PKDA. For direct
client to server authentication, the client will use PKINIT to
authenticate to the end server (instead of a central KDC), which then,
will issue a ticket for itself.  The benefit of this proposal, is that a
single PK extension to Kerberos can addresses the goals of PKINIT and
PKDA.


3. PKDA background

The PKDA proposal provides direct client to server authentication, thus
eliminating the need for an online key distribution center.  A client
and server take part in an initial PK based authentication exchange,
with an added caveat that the server acts as a Kerberos ticket granting
service and issues a traditional Kerberos ticket for itself.  In
subsequent communication, the client makes use of the Kerberos ticket,
thus eliminating the need for public key operations on the server.  This
approach has an advantage over SSL in that the server does not need to
save state (cache session keys).  Furthermore, an additional benefit, is
that Kerberos tickets can facilitate delegation (see Neuman[3]).

Below is a brief overview of the PKDA protocol.  For a more detailed
description see [1].

SCERT_REQ: Client to Server
The client requests a certificate from the server.  If the serverÆs
certificate is cached locally, SCERT_REQ and SCERT_REP are omitted.

SCERT_REP:  Server to Client
The server returns its certificate to the client.

PKTGS_REQ: Client to Server
The client sends a request for a service ticket to the server.  To
authenticate the request, the client signs, among other fields, a time
stamp and a newly generated symmetric key .  The time stamp is used to
foil replay attacks;  the symmetric key is used by the server to secure
the PKTGS_REP message.
The client provides a certificate in the request (the certificate
enables the server to verify the validity of the clientÆs signature) and
seals it along with the signed information using the serverÆs public
key.


PKTGS_REP:  Server to Client
The server returns a service ticket (which it issued for itself) along
with the session key for the ticket.  The session key is protected by
the client-generated key from the PKTGS_REQ message.

AP_REQ:  Client to Server
After the above exchange, the client can proceed in a normal fashion,
using the conventional Kerberos ticket in an AP_REQ message.


4. PKINIT background

One of the principal goals of PKINIT is to provide for interoperability
between a public key infrastructure and Kerberos.  Using a public key
certificate, a client can authenticate to the KDC and receive a TGT
which enables the client to obtain service tickets to kerberized
services..  In PKINIT, the AS-REQ and AS-REP messages remain the same;
new preauthentication data types are used to conduct the PK exchange.
Client and server certificates are exchanged via the preauthentication
data.  Thus, the exchange of certificates , PK authentication, and
delivery of a TGT can occur in two messages.

Below is a brief overview of the PKINIT protocol.  For a more detailed
description see [2].

PreAuthentication data of AS-REQ:  Client to Server
The client sends a list of trusted certifiers, a signed PK
authenticator, and its certificate.  The PK authenticator, based on the
Kerberos authenticator, contains the name of the KDC, a timestamp, and a
nonce.

PreAuthentication data of AS-REP:  Server to Client
The server responds with its certificate and the key used for decrypting
the encrypted part of the AS-REQ.  This key is encrypted with the
clientÆs public key.

AP_REQ:  Client to Server
After the above exchange, the client can proceed in a normal fashion,
using the conventional Kerberos ticket in an AP_REQ message.


5. Application of PKINIT to achieve equivalence to PKDA

While PKINIT is normally used to retrieve a ticket granting ticket
(TGT), it may also be used to request an end service ticket.  When used
in this fashion, PKINIT is functionally equivalent to PKDA.  We
introduce the concept of a local ticket granting server (LTGS) to
illustrate how PKINIT may be used for issuing end service tickets based
on public key authentication.  It is important to note that the LTGS may
be built into an application server, or it may be a stand-alone server
used for issuing tickets within a well-defined realm, such as a single
machine.  We will discuss both of these options.


5.1. The LTGS

The LTGS processes the Kerberos AS-REQ and AS-REP messages with PKINIT
preauthentication data.  When a client submits an AS-REQ to the LTGS, it
specifies an application server, in order to receive an end service
ticket instead of a TGT.


5.1.1. The LTGS as a standalone server

The LTGS may run as a separate process that serves applications which
reside on the same machine. This serves to consolidate administrative
functions and provide an easier migration path for a heterogeneous
environment consisting of both public key and Kerberos.  The LTGS would
use one well-known port (port #88 - same as the KDC) for all message
traffic and would share a symmetric with each service.  After the client
receives a service ticket, it then contacts the application server
directly.  This approach is similar to the one suggested by Sirbu , et
al [1].

5.1.1.1. Ticket Policy for PKTAPP Clients

It is desirable for the LTGS to have access to a PKTAPP client ticket
policy. This policy will contain information for each client, such as
the maximum lifetime of a ticket, whether or not a ticket can be
forwardable, etc. PKTAPP clients, however, use the PKINIT protocol for
authentication and are not required to be registered as Kerberos
principals.

As one possible solution, each public key Certification Authority could
be registered in a secure database, along with the ticket policy
information for all PKTAPP clients that are certified by this
Certification Authority.

5.1.1.2. LTGS as a Kerberos Principal

Since the LTGS serves only PKTAPP clients and returns only end service
tickets for other services, it does not require a Kerberos service key
or a Kerberos principal identity. It is therefore not necessary for the
LTGS to even be registered as a Kerberos principal.

The LTGS still requires public key credentials for the PKINIT exchange,
and it may be desired to have some global restrictions on the Kerberos
tickets that it can issue. It is recommended (but not required) that
this information be associated with a Kerberos principal entry for the
LTGS.


5.1.1.3. Kerberos Principal Database

Since the LTGS issues tickets for Kerberos services, it will require
access to a Kerberos principal database containing entries for at least
the end services. Each entry must contain a service key and may also
contain restrictions on the service tickets that are issued to clients.
It is recommended that (for ease of administration) this principal
database be centrally administered and distributed (replicated) to all
hosts where an LTGS may be running.

In the case that there are other clients that do not support PKINIT
protocol, but still need access to the same Kerberos services, this
principal database will also require entries for Kerberos clients and
for the TGS entries.

5.1.2. The LTGS as part of an application server

The LTGS may be combined with an application server.  This accomplishes
direct client to application server authentication; however, it requires
that applications be modified to process AS-REQ and AS-REP messages.
The LTGS would communicate over the port assigned to the application
server or over the well known Kerberos port for that particular
application.

5.1.2.2. Ticket Policy for PKTAPP Clients

Application servers normally do not have access to a distributed
principal database. Therefore, they will have to find another means of
keeping track of the ticket policy information for PKTAPP clients. It is
recommended that this ticket policy be kept in a directory service (such
as LDAP).

It is critical, however, that both read and write access to this ticket
policy is restricted with strong authentication and encryption to only
the correct application server.  An unauthorized party should not have
the authority to modify the ticket policy. Disclosing the ticket policy
to a 3rd party may aid an adversary in determining the best way to
compromise the network.

It is just as critical for the application server to authenticate the
directory service. Otherwise an adversary could use a man-in-the-middle
attack to substitute a false ticket policy with a false directory
service.

5.1.2.3. LTGS Credentials

Each LTGS (combined with an application service) will require public key
credentials in order to use the PKINIT protocol. These credentials can
be stored in a single file that is both encrypted with a password-
derived symmetric key and also secured by an operating system. This
symmetric key may be stashed somewhere on the machine for convenience,
although such practice potentially weakens the overall system security
and is strongly discouraged.

For added security, it is recommended that the LTGS private keys are
stored inside a temper-resistant hardware module that requires a pin
code for access.


5.1.2.4. Compatibility With Standard Kerberos

Even though an application server is combined with the LTGS, for
backward compatibility it should still accept service tickets that have
been issued by the KDC. This will allow Kerberos clients that do not
support PKTAPP to authenticate to the same application server (with the
help of a KDC).

5.1.3. Cross-Realm Authentication

According to the PKINIT draft, the client's realm is the X.500 name of
the Certification Authority that issued the client certificate. A
Kerberos application service will be in a standard Kerberos realm, which
implies that the LTGS will need to issue cross-realm end service
tickets. This is the only case, where cross-realm end service tickets
are issued. In a standard Kerberos model, a client first acquires a
cross-realm TGT, and then gets an end service ticket from the KDC that
is in the same realm as the application service.

6. Protocol differences between PKINIT and PKDA

Both PKINIT and PKDA will accomplish the same goal of issuing end
service tickets, based on initial public key authentication.  A PKINIT-
based implementation and a PKDA implementation would be functionally
equivalent.  The primary differences are that 1)PKDA requires the client
to create the symmetric key while PKINIT requires the server to create
the key and 2)PKINIT accomplishes in two messages what PKDA accomplishes
in four messages.

7. Summary

The PKINIT protocol can be used, without modification to facilitate
client to server authentication without the use of a central KDC.  The
approach described in this draft  (and originally proposed in PKDA[1])
is essentially a public key authentication protocol that retains the
advantages of Kerberos tickets.

Given that PKINIT has progressed through the CAT working group of the
IETF, with plans for non-commercial distribution (via MITÆs v5 Kerberos)
as well as commercial support, it is worthwhile to provide PKDA
functionality, under the PKINIT umbrella.

8. Security Considerations

PKTAPP is based on the PKINIT protocol and all security considerations
already listed in [2] apply here.

When the LTGS is implemented as part of each application server, the
secure storage of its public key credentials and of its ticket policy
are both a concern.  The respective security considerations are already
covered in sections 5.1.2.3 and 5.1.2.2 of this document.


9. Bibliography

[1] M. Sirbu, J. Chuang.  Distributed Authentication in Kerberos Using
Public Key Cryptography.  Symposium On Network and Distributed System
Security, 1997.

[2] B. Tung, C. Neuman, M. Hur, A. Medvinsky, S. Medvinsky, J. Wray,
J. Trostle.  Public Key Cryptography for Initial Authentication in
Kerberos.  Internet Draft, July 2000.
(ftp://ietf.org/internet-drafts/draft-ietf-cat-kerberos-pk-init-12.txt)

[3] C. Neuman, Proxy-Based Authorization and Accounting for
Distributed Systems.  In Proceedings of the 13th International
Conference on Distributed Computing Systems, May 1993.

[4] J. Kohl, C. Neuman.  The Kerberos Network Authentication Service
(V5).  Request for Comments 1510.

10.  Expiration Date

This draft expires May 9, 2001.

11. Authors

Ari Medvinsky
Keen.com, Inc.
150 Independence Dr.
Menlo Park, CA 94025
Phone +1 650 289 3134
E-mail: ari@keen.com

Matthew Hur
Cisco Systems
500 108th Ave. NE, Suite 500
Bellevue, WA 98004
Phone:
EMail: mhur@cisco.com

Sasha Medvinsky
Motorola
6450 Sequence Dr.
San Diego, CA 92121
Phone: +1 858 404 2367
E-mail: smedvinsky@gi.com

Clifford Neuman
USC Information Sciences Institute
4676 Admiralty Way Suite 1001
Marina del Rey CA 90292-6695
Phone: +1 310 822 1511
E-mail: bcn@isi.edu