Google Apps for Education (or any of the Google Apps) utilizes SAML 2.0 to provide an integration point for external authentication services. CAS 3.1 includes an "ArgumentExtractor" and accompanying "Service" to provide process and understand SAML 2.0 requests from Google.
The integration with Google Accounts is based off the excellent documentation provided by Google:
Step 1 - Generate DSA/RSA Keys
The first step is to generate DSA/RSA public and private keys. These are used to sign and read the Assertions. After you've generated your keys, you will need to register the public key with Google.
The keys will also need to be available to the CAS application (but not publicly available over the Internet). We recommend you place the keys within your classpath (i.e. WEB-INF/classes) though any location accessible by the user running the web server instance and not served publicly to the Internet is acceptable. Thus, inside WEB-INF is nice because WEB-INF is scoped to the web application but not normally served. Inside the webapp but not in WEB-INF is fraught with peril. Somewhere like /etc/cas/keys/ is a fine idea as well and protects the key from being overwritten on deploy of a new CAS webapp version.
The public.key and private.p8 go into your classpath. The x509.pem file should be uploaded into Google Apps.
Step 2 - Configure CAS Server
Google Accounts integration within CAS is enabled by simply adding an additional "ArgumentExtractor" to the list of ArgumentExtractors. An ArgumentExtractor attempts to obtain a service from the provided Request. Each ArgumentExtractor is responsible for understanding one type of Service.
You'll need to modify the WEB-INF/spring-configuration/argumentExtractorsConfiguration.xml, and add the following:
Reference that from the list of ArgumentExtractors, so it would look something like this:
You'll need to configure the keys so they can be loaded from the files. You do that as follows:
Replace the public.key and private.key with the names of your key files. If they are not available on the classpath, change the location to point to the location of the keys. If you are using DSA instead of RSA, change the algorithm as appropriate.
Step 3 - Configure Google
You'll need to provide Google with the URL for your SAML-based SSO service, as well as the URL your users will be redirected to when they log out of a hosted Google application.
Use the following URLs when you are configuring for Google Apps
If you wish to use a Google email username that is different from your CAS userid, please read Google Apps from MS-AD using the 'mail' attribute. While this uses MS-AD, it applies to any LDAP directory.
Step 4 - Add a new service in CAS
Name : Google Apps
Service URL : https://www.google.com/a/YourGoogleDomain/acs