One document matched: draft-ietf-cat-pktapp-00.txt

                                                  B. Clifford Neuman
                                                  USC/ISI


                                                  Jan. 97 (Expires July, 97)


Public Key Utilizing Tickets for Application Servers (PKTAPP)


0. Status Of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
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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 à Server
The client requests a certificate from the server.  If the server’s 
certificate is cached locally, SCERT_REQ and SCERT_REP are omitted.

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

PKTGS_REQ: Client à 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 client’s signature) and 
seals it along with the signed information using the server’s public 
key.  
 
PKTGS_REP:  Server à 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 à 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 à 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 ß 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 
client’s public key.

AP_REQ:  Client à 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].


5.1.2. 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.


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. Discussion

The concept of Kerberos names (i.e. domain based names) is orthogonal to 
the concept of public key names, in that X.500 distinguished names do 
not reside within a particular realm.  The assumption is that all user 
principals will use X.500 distinguished names and that PKINIT will 
support both X.500 and Kerberos names for application servers.  However, 
it is our recommendation that X.500 be the default naming convention.


8. 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 MIT’s v5 Kerberos)  
as well as commercial support, it is worthwhile to provide PKDA 
functionality, under the PKINIT umbrella.


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.C. Neuman, B. Tung, J. Wray, . Trostle.  Public Key Cryptography 
for Initial Authentication in Kerberos.  Internet Draft, October 1996. 
(ftp://ietf.org/internet-drafts/draft-ietf-cat-kerberos-pk-init-02.txt)

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

[4] B.C. Neuman, Theodore Ts'o. Kerberos: An Authentication Service
for Computer Networks, IEEE Communications, 32(9):33-38.
September 1994.


Authors' Addresses

Ari Medvinsky <ari.medvinsky@cybersafe.com>
Matthew Hur   <matt.hur@cybersafe.com>

CyberSafe Corporation 
1605 NW Sammamish Raod
Suite 310
Issaquah, WA 98027-5378
Phone: (206) 391-6000
Fax:   (206) 391-0508
http:/www.cybersafe.com


B. 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