KX509: Leveraging Kerberos to Obtain Digital Certificates for

Web Client Authentication

University of MichiganKevin Coffman <kwc@umich.edu>

Bill Doster <billdo@umich.edu>

April 11, 2000 CIC TechForum 2000

Why X.509?

• An accepted international standard

• Application support out of the box– Web servers, web browsers, directory servers,

IMAP servers, etc

• Allows the possibility for inter-institution authentication

• No need for N²-1 cross-realm trusts

April 11, 2000 CIC TechForum 2000

Why Kerberos?

• We have been using Kerberos on campus since 1990

• We have 200K+ principals defined in our Kerberos database

• It’s an integral part of our infrastructure

• It is currently used for authenticating to many services (AFS, dial-in, e-mail, login servers, web pages.)

April 11, 2000 CIC TechForum 2000

Project History(Where We Started From)

• Started with MIT code for issuing certificates

• Shortcomings in the MIT code– Passwords passed to web server

– User interaction required• Obtain certificate

• Maintain and protect private key(s)

– Long-term certificates, ignoring revocation

– Only supported for Netscape Communicator

April 11, 2000 CIC TechForum 2000

Project Goals(What We Are Doing)

• Eliminate password prompts for web access (actually use Kerberos)

• Transparent web authentication– Make certificate generation automatic at Kerberos login

– Make certificate installation invisible to the user

• Browser-neutral, cross-platform

• Position for inter-institution authentication

April 11, 2000 CIC TechForum 2000

Project Non-goals(What We Are NOT Doing)

• Not a complete PKI

• Not to be used for e-mail or document encryption

• Not to be used for e-mail or document signing (not yet, anyway)

• Not a complete replacement of the current cookie method of authentication (not yet, anyway)

April 11, 2000 CIC TechForum 2000

KX509 Description

• Uses short-term (~1 day) certificates -- “junk keys”

• Obtains certificates securely from a kerberized certificate authority (KCA) server

• Used for authentication ONLY!

• Columbia PKCS#11 code

April 11, 2000 CIC TechForum 2000

Why “Junk Keys” ?

• Revocation becomes a non-issue

• Private key storage is less an issue

• The directory isn’t the center of the universe (?)– Certificate management is less critical– Certificate publication for sharing is not

necessary

April 11, 2000 CIC TechForum 2000

The Cookie Trail

April 11, 2000 CIC TechForum 2000

UnmodifiedKerberos “Login”

(kinit , klog ,Kerb95,…)

StandardKerberos

TGT Request

StandardKerberos

Service TicketRequest

Standard HTTPS(with X.509 Client

Authentication)

KX509 Overview

Kerberos Authenticated RequestWith public-key to be certified

X.509 v3 Certificategood for one day

UnmodifiedInternetExplorer

Kerberos Ticket File(plus registry onWindows)

UnmodifiedNetscapeBrowser

TGT

Use TGT to getservice ticket

Store GeneratedRSA key-pair &One-day certificate

Use RSA Key-pair& certificate

ClientWorkstation

KerberizedCertificateAuthority

(KCA)

UnmodifiedKerberos Server

(KDC)

UnmodifiedKerberos Server

(TGS)

Enterprise-WideKerberos Servers

UnmodifiedWeb Servers

Copy of KCA’sPublished Certificate

Enterprise & External Web Servers

login

password

PKCS#11module

kx509

April 11, 2000 CIC TechForum 2000

Demonstration...