OpenID is an open, decentralized, free framework for user-centric digital identity. Users represent themselves using URIs. For more information see the http://www.openid.net. As of CAS 3.5, CAS supports both the "dumb" and "smart" modes of the OpenID protocol. Dumb mode acts in a similar fashion to the existing CAS protocol. The smart mode differs in that it establishes an association between the client and the openId provider (OP) at the begining. Thanks to that association and the key exchange done during association, information exchanged between the client and the provider are signed and verified using this key. There is no need for the final request (which is equivalent in CAS protocol to the ticket validation).

A demo of the OpenId support in CAS server is available at : https://github.com/leleuj/cas-openid-demo.

Giving your users URIs

Configuring your users to have URIs.

OpenId identifiers are URIs. The default mechanism in CAS support is an uri ending with the actual user login (ie. http://my.cas.server/openid/fesnault where the actual user login is fesnault). This is not recommended and you should think of a more elaborated way of providing URIs to your users.

Add OpenId support module to CAS server

The first thing, with a CAS server webapp, is to add the OpenId support module dependency. This is done by adding this in the cas server webapp pom.xml.

<dependency>
    <groupId>org.jasig.cas</groupId>
    <artifactId>cas-server-support-openid</artifactId>
    <version>${project.version}</version>
</dependency>

Now let's dive into CAS configuration itself.

Declare the OpenID endpoint (since CAS 4.0)

Since CAS 4.0, the OpenID endpoint for discovery is no more enabled by default in the CAS server (in fact, it is no more available in the cas-server-support-webapp module : it has been moved to the cas-server-support-openid module).

The OpenID discovery endpoint should be enabled during the configuration process. In the web.xml file, the following mapping must be added :

  <servlet-mapping>
    <servlet-name>cas</servlet-name>
    <url-pattern>/openid/*</url-pattern>
  </servlet-mapping>

In the cas-servlet.xml file, the following mapping and bean must be added :

  <bean id="handlerMappingC" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
    <property name="mappings">
      <props>
        <prop key="/logout">logoutController</prop>
        ...
        <prop key="/openid/*">openIdProviderController</prop>
        ...


 
  <bean
      id="openIdProviderController"
      class="org.jasig.cas.support.openid.web.OpenIdProviderController"
      p:loginUrl="${server.prefix}/login"/>

Update webflow

CAS uses a spring webflow to describe the the authentication process. We need to change it a little bit to allow CAS to switch to OpenId authentication if it recognizes one. This is done in the login-webflow.xml fie. After the on-start element just add these two blocks :

  <!-- If the request contains a parameter called openid.mode and is not an association request, switch to openId. Otherwise, continue normal webflow. -->
  <decision-state id="selectFirstAction">
      <if
         test="externalContext.requestParameterMap['openid.mode'] neq ''
          &amp;&amp; externalContext.requestParameterMap['openid.mode'] neq null
          &amp;&amp; externalContext.requestParameterMap['openid.mode'] neq 'associate'"
         then="openIdSingleSignOnAction" else="ticketGrantingTicketExistsCheck" />
  </decision-state>
		
  <!-- The OpenID authentication action. If authentication is successful, send the ticket granting ticker. Otherwise, redirect to the login form. -->
  <action-state id="openIdSingleSignOnAction">
      <evaluate expression="openIdSingleSignOnAction" />
      <transition on="success" to="sendTicketGrantingTicket" />
      <transition on="error" to="viewLoginForm" />
      <transition on="warn" to="warn" />
  </action-state>

Enable OpenId in the AuthenticationManager

The authentication manager is the place where authentication takes place. We must provide it two elements needed for a successful OpenId authentication. The first thing to do is to detect the user name from the OpenId identifier. When your CAS server will work as an OP, users will authenticate with an OpenId identifier, looking like this : http://localhost:8080/cas/openid/fesnault. Actually, in your users database, this users login is probably fesnault. We must provide the CAS server with a way to extract the user principal from the credentials he provides us. This is the first thing we'll do in this section : add an OpenIdCredentialsToPrincipalResolver to the authentication manager. The next thing to give CAS is a specialized authentication handler.

Open the deployerConfigContext.xml file, and locate the authenticationManager bean definition. It has two properties containing beans. The credentials to principal property, add this bean definition :

     <!-- The openid credentials to principal resolver -->
     <bean class="org.jasig.cas.support.openid.authentication.principal.OpenIdCredentialsToPrincipalResolver" />

Then, in the authentication handler property, add this bean definition :

     <!-- The open id authentication handler -->
     <bean class="org.jasig.cas.support.openid.authentication.handler.support.OpenIdCredentialsAuthenticationHandler" p:ticketRegistry-ref="ticketRegistry" />

Adapt the Spring CAS servlet configuration

We now have to make CAS handle nicely the OpenId request he will be presented with. First, we'll add a handler for the /login url, when called to validate a ticket (CAS is implementing the dumb OpenId mode, which means it does not create an association at the beginning of the authentication process. It must then check the received authentication success notification, which is done by one extra HTTP request at the end of the process). Anywhere in the cas-servlet.xml file, add this bean definition :

    <bean id="handlerMappingOpendId"
          class="org.jasig.cas.support.openid.web.support.OpenIdPostUrlHandlerMapping">
        <!-- Notice we set the order value to 2, which is the order of the flow handler mapping. We'll fix that just next.
        The OpenIDPostUrlHandlerMapping MUST be called before the login webflow action is called, otherwise we will never be able to validate the authentication success. -->
        <property name="order" value="2"/>
        <property name="mappings">
            <props>
                <prop key="/login">delegatingController</prop>
            </props>
        </property>
    </bean>

As we gave the order of 2 to the OpenIdPostUrlHandlerMapping, we must modify the FlowHandlerMapping order to give it the order of 3. Find the FlowHandlerMapping bean declaration and change the p:order="2"' to p:order="3".

<bean class="org.springframework.webflow.mvc.servlet.FlowHandlerMapping" p:flowRegistry-ref="flowRegistry"
   p:order="3">

In the handlerMappingOpenId, we referenced a bean called delegatingController. this bean is a special controller, using the Delegate pattern, which delegates the processing of a request to the first controller of its delegates which says it can handle it. So now we'll provide two delegate controllers. The first one is handling the Smart OpenId association, and the second process the authentication and ticket validation. Add this two beans in the file.

The Smart OpenId controller :

   <bean id="smartOpenIdAssociationController" class="org.jasig.cas.support.openid.web.mvc.SmartOpenIdController"
        p:serverManager-ref="serverManager"
        p:successView="casOpenIdAssociationSuccessView" p:failureView="casOpenIdAssociationFailureView" />

The OpenId validation controller :

   <bean id="openIdValidateController" class="org.jasig.cas.web.ServiceValidateController"
          p:validationSpecificationClass="org.jasig.cas.validation.Cas20WithoutProxyingValidationSpecification"
          p:centralAuthenticationService-ref="centralAuthenticationService"
          p:proxyHandler-ref="proxy20Handler" p:argumentExtractor-ref="openIdArgumentExtractor"
          p:successView="casOpenIdServiceSuccessView" p:failureView="casOpenIdServiceFailureView" />

We are done with the delegates. Now we must create the Delegating controller itself, and give it a list of delegates referencing the two delegates we just defined. So add this definition :

   <bean id="delegatingController" class="org.jasig.cas.web.DelegatingController"
    p:delegates-ref="delegateControllers"/>
  
   <util:list id="delegateControllers">
        <ref bean="smartOpenIdAssociationController"/>
        <ref bean="openIdValidateController"/>
    </util:list>

Also, add the indicated lines to the <beans> definition at the top of the file, if they're not already there:

<beans xmlns="http://www.springframework.org/schema/beans"
       ...
       xmlns:util="http://www.springframework.org/schema/util"
       xsi:schemaLocation="...
       http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-3.1.xsd
       ...">

Next, we'll give CAS a handler for the OpenIdSingleSignOnAction we added in the spring webflow definition file. So add this bean definition anywhere :

     <!-- Be Careful
         The OpenIdSingleSignOnAction has an additional parameter not configured here.
         Its the "extractor" property which accepts a "org.jasig.cas.support.openid.web.support.OpenIdUserNameExtractor".
         The default one merely accepts the value after the last "/".
         A more robust implementation should check the entire URL. Note, that means the default one SHOULD NOT be used in production.
     -->
 
    <bean id="openIdSingleSignOnAction" class="org.jasig.cas.support.openid.web.flow.OpenIdSingleSignOnAction"
          p:centralAuthenticationService-ref="centralAuthenticationService"/>

Add an argument extractor

Finally, we must tell cas how to extract the OpenId from the authentication request (openid.mode, openid.sig, openid.assoc_handle...). This is done in the argumentExtractorsConfiguration.xml file, located in the spring-configuration directory. Add this bean into the file :

     <bean id="openIdArgumentExtractor" class="org.jasig.cas.support.openid.web.support.OpenIdArgumentExtractor" />

Then add a reference to this bean into the arguments extractors list, just below in the file :

     <util:list id="argumentExtractors">
        <ref bean="casArgumentExtractor" />
        <!-- The OpenId arguments extractor -->
        <ref bean="openIdArgumentExtractor" />
        <ref bean="samlArgumentExtractor" />
     </util:list>

Next we must provide a ServerManager, which is a class from the openid4java library, which allows us to handle the Diffie-Hellman algorithm used by the association process. In the spring-configuration/applicationContext.xml file, add this bean definition :

<bean id="serverManager" class="org.openid4java.server.ServerManager"
          p:oPEndpointUrl="${cas.securityContext.casProcessingFilterEntryPoint.loginUrl}"
          p:enforceRpId="false"  />

And finally, we need an applicationContext provider , so add this bean into spring-configuration/applicationContext.xml :

<bean id="applicationContextProvider" class="org.jasig.cas.util.ApplicationContextProvider" />

You're done ! CAS is now configured to work as an OpenId Provider.