Open-source Single Sign-On with CAS (Central Authentication Service)

Pascal Aubry, Vincent Mathieu & Julien Marchal

Copyright © 2004-2005 – ESUP-Portail consortium

Open-source Single Sign-On with CAS

• Single Sign-On– Why SSO?– The main principles of web SSO– The choice of CAS

• CAS (Central Authentication Service)– How does it work?– How to CAS-ify applications

• Web applications

• Non-web applications

• Limits• The effort of the ESUP-Portail consortium around CAS• Perspectives

Let’s fight generally accepted ideas!

• “Facing the growing number of passwords, Single Sign-On increases the security policy of firms and users’ comfort.”

• Well, yes, but…

Let’s fight generally accepted ideas!

• “SSO to federate authentication”

Let’s fight generally accepted ideas!

• “SSO is a suite of tools that memorizes passwords for users and provide them to applications”

• No, not at all!

Let’s fight generally accepted ideas!

• “SSO is generally deployed as a rest, after a centralized user directory and a unique entry point for the IS have been set up. SSO is then a modest project, easily developed in-house.”

• We must have missed something ;-)

Why Single Sign-On?

• Unique accounts but several authentications– Each time users access an application

• Security (password stealing)– Protect password transmission– Do not transmit passwords to applications

• Simplify applications

• Delegate developments without delegating authentication

• Abstract authentication– LDAP, NIS, database, NT, Active Directory, X509 certificates, …

SSO: the user’s point of view

web browser

app. #1 app. #2 app. #3

without SSO

service

web browser

app. #1 app. #2 app. #3

with SSO

service

SSO: principles on the web

• Authentication is centralized– One (redundant) authentication server

• Transparent HTTP redirections– From applications to the authentication server (when not authenticated)– From the authentication server to applications (when authenticated)

• Tokens propagate identities– Cookies, CGI parameters

CAS: why did we choose it?

• Security– Password is never transmitted to applications– Opaque tickets are used

• N-tier installations– Without transmitting any password!

• Portability (client libraries)– Java, Perl, JSP, ASP, PHP, PL/SQL, Apache and PAM modules

• Permanence– Developed by Yale University– World-wide used (mainly Universities)– Adopted by all the French educational community

• J2EE platform– Very light code (about 1000 lines)

• Open source• Integrated into uPortal

web browser

app. #1 app. #2 app. #3

with CAS

service

CAS: why did we choose it?

web browser

app. #1 app. #2 app. #3authentication

server

without SSO

userdatabase user

databaseservice

netId

passw

ord

net

Idp

assw

ord

web browser

app. #1 app. #2 app. #3

with CAS

service

CAS: why did we choose it?

web browser

app. #1 app. #2 app. #3authentication

server

without SSO

userdatabase user

databaseservice

netId

passw

ord

net

Idp

assw

ord

User authentication

CASserver

HT

TP

S

web browser

1ère authentification d’un utilisateur

CASserver

authentication form:

HT

TP

S

web browser

User authentication

• TGC: Ticket Granting Cookie– User’s passport to the CAS server– Private and protected cookie

(the only one used by CAS, optional)– Opaque re-playable ticket

CASserver

netIdpassword

HT

TP

Suser

database

web browser

TGC

TGC

1ère authentification d’un utilisateur

CASserver

netIdpassword

HT

TP

Suser

database

web browser

TGC

TGC

Accessing an applicationafter authentication

web browser

CASserver

TGC

HT

TP

S

application

TGC ST

ST

ST

• ST: Service Ticket– Browser’s passport to the CAS client (application)

– Opaque and non re-playable ticket

– Very limited validity (a few seconds)

ID

Accessing an applicationafter authentication

CASserver

HT

TP

S

TGC ST

ST

ST

ID

web browserTGC

• Redirections are transparent to users

application

• ST: Service Ticket– Browser’s passport to the CAS client (application)

– Opaque and non re-playable ticket

– Very limited validity (a few seconds)

Accessing an applicationwithout authentication

web browser

CASserver

HT

TP

S

Authentication form

application

Accessing an applicationwithout authentication

web browser

CASserver

TGC

HT

TP

S ST

ST

ID

netIdpassword ST

TGC

• No need to be previously authenticated to access an application

application

Remarks

• Once a TGC acquired, authentication is transparent for the access to any CAS-ified application of the workspace

• Once authenticated by an application, a session should be used between the browser and the application

Authenticating users with CAS

• CAS authentication left to administrators

• ESUP-Portail CAS Generic Handler– Mixed authentication– XML configuration

LDAPdirectory

database NISdomain

X509certificates

Kerberosdomain

Windows NTdomain

flat files

CASserver

Using the ESUP-Portail CAS GH<authentication debug="on"> <handler> <classname> org.esupportail.cas.server.handlers.ldap.FastBindLdapHandler </classname> <config> <filter>uid=%u,ou=people,dc=esup-portail,dc=org</filter> <server> <url>ldap://ldap.esup-portail.org</url> </server> </config> </handler> <handler> <classname> org.esupportail.cas.server.handlers.nis.NisHandler </classname> <config> <domain>ESUP-PORTAIL</domain> <encryption>pammd5</encryption> <server> <host>nismaster.esup-portail.org</host> <host>nisslave.esup-portail.org</host> </server> </config> </handler></authentication>

CAS-ifying a web application

• Use provided libraries

• Add a few lines of code

• Note: you can also protect static resources– With mod_cas, an Apache module

CAS-ifying a web application

• An example using phpCAS (ESUP-Portail)

<?php // include phpCAS library include_once('CAS/CAS.php');

// declare our script as a CAS client phpCAS::client(CAS_VERSION_2_0,'auth.univ.fr',443,'');

// redirect to the CAS server if needed phpCAS::authenticateIfNeeded();

// at this point, the user is authenticated?><h1>Successfull Authentication!</h1><p>User's login: <?php echo phpCAS::getUser(); ?>.</p>

N-tier installations

• PGT: Proxy Granting Ticket– Application’s passport for a user to the

CAS server– Opaque and re-playable ticket

web browser

CASserver

TGC

application(CAS proxy)

ST

ST

service

IDPGT

PGT

application(CAS proxy)

N-tier installations

web browser

CASserver

TGC

ST

service

PGT

PT

PTID

PGT

• PGT: Proxy Granting Ticket– Application’s passport for a user to the

CAS server– Opaque and re-playable ticket

• PT : Proxy Ticket– Application’s passport for a user to a tier

service– Opaque and non re-playable ticket– Very limited validity

PT

CAS-ifying a non web application

• One of the strongest points of CAS

• Use the pam_cas PAM module

• Example of PAM configuration:

auth sufficient /lib/security/pam_ldap.soauth sufficient /lib/security/pam_pwdb.so shadow nullokauth required /lib/security/pam_cas.so

pam_cas

The pam_cas PAM module

• Pam_cas authenticates users with a CAS ticket

pam_pwdb

clientapplication

pam_ldap

LDAP directory

login/password

login/password

/etc/passwd

login/password

clientapplication

CASserver

ticket

serverapplication

login/ticket

CAS-ifying an IMAP server

• Objectives

– Access an IMAP server from a web application that does not know the password of the user connected

– Let traditional mail clients authenticate “normally” (with a password)

– Do not modify the IMAP server

• The solution: pam_cas :-)

pam_cas

CAS-ifying an IMAP server

pam_pwdb

traditionalmail client

pam_ldap

LDAP directory

login / password

login / password

/etc/passwd

login / password

CAS-ified webmail(CAS proxy)

login / PT

CASserver

PT

web browser

ST

IMAPserver

pam_cas

pam_pwdb

pam_ldap

CAS-ifying Cyrus IMAPdtraditionalmail client

LDAPdirectory

login / password

login / password

/etc/passwd

CAS-ified webmail(CAS proxy)

CASserver

PT

web browserST

sasl

Cyrusimapd

login / PT

pam_cas

pam_pwdb

pam_ldap

sasl

CAS-ifying Cyrus IMAPdtraditionalmail client

Cyrusimapd

LDAPdirectory

login / password

login / password

/etc/passwd

CAS-ified webmail(CAS proxy)

login / PT

CASserver

PT

web browserST

sasl_authd

cache

Unix socket

Cyrus IMAP daemon

sasl_authd daemon

Cache

efficiency:

95%

Limits

• CAS deals with authentication, not authorization

• No redundancy– No native load-balancing (but low load)– No fault-tolerance (but very good reliability)

• No Single Sign-Off

• A very poor documentation

The effort of the ESUP-Portail consortium

• Writing documentation

• Adding libraries (phpCAS, esup-mod_cas, esup-pam_cas)

• Adding features to the CAS server– Authentication handlers (LDAP, NIS, files, databases, NT domains, …)– Mixed authentication– Authentication debug mode– Rendering customization (appearance, internationalization)– CAS quick start (Jakarta Tomcat + Yale CAS server + CAS GH)

• Supporting the French CAS community– Through forums and mailing lists

Now, what to do first (requirements)?

• Data organization– The Information System should be well-formed– Small technical problems, big political issues

• Data storage– A standard way to store users (an Excel sheet is no standard ;-)

• Competences– Web technologies– PKI (CRU)

• Voluntary policy– Is security a real concern?

So, what next?

• Add new authentication modes

• Make establishments cooperate– Propagate user attributes (namely or anonymously)– Shibboleth, Liberty Alliance

• CAS is now a JASIG project

• Share your experience!

Note this!

• CRU is starting a circle of trust (federation project) among the French educational community– Comité Réseau des Universités (http://www.cru.fr)– Authenticate at university level– Authorize at resource level by relying on propagated attributes

• Many questions, a few answers only at this time– Compare Shibboleth and Liberty Alliance– Test existing solutions– Study if CAS can be used with Shibboleth, and how– Can LASSO implement a WAYF service?– Would LASSO replace CAS?

• A goal– Show how it is possible for establishments to cooperate securely

Enjoy CAS!