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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:

• httphttps://codedevelopers.google.com/apis/apps/sso/saml_reference_implementation.htmlhttp://code.google.com/apis/google-apps/sso/saml_reference_implementation_web.html

Step 1 - Generate DSA/RSA Keys

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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 is acceptable:

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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.

openssl genrsa -out private.key 1024
openssl rsa -pubout -in private.key -out public.key -inform PEM -outform DER
openssl pkcs8 -topk8 -inform PER -outform DER -nocrypt -in private.key -out private.p8
openssl req -new -x509 -key private.key -out x509.pem -days 365

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You'll need to modify the WEB-INF/spring-configuration/argumentExtractorsConfiguration.xml, and add the following:
(note: the section  p:httpClient-ref="httpClient"   is only for 3.4 and up, and should be removed for 3.3.x)

<bean         nameid="googleAccountsArgumentExtractor" 	class="org.jasig.cas.web.support.GoogleAccountsArgumentExtractor"
 	     p:privateKey-ref="privateKeyFactoryBean"
	
      p:publicKey-ref="publicKeyFactoryBean"
      p:httpClient-ref="httpClient" />

Reference that from the list of ArgumentExtractors, so it would look something like this:

<util:list id="argumentExtractors">
	<ref bean="casArgumentExtractor" />
	<ref bean="samlArgumentExtractor" />
	<ref bean="googleAccountsArgumentExtractor" />
</util:list>

You'll need to configure the keys so they can be loaded from the files. You do that as follows:

<bean 	id="privateKeyFactoryBean" 	class="org.jasig.cas.util.PrivateKeyFactoryBean"
      	p:location="classpath:private.p8"
	
      p:algorithm="RSA" />

<bean 	id="publicKeyFactoryBean"
	class="org.jasig.cas.util.PublicKeyFactoryBean"
 	     p:location="classpath:public.key"
	
      p:algorithm="RSA" />

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 RSA DSA instead of DSARSA, change the algorithm as appropriate.

Step 3 - Configure Google

The final step is to 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

Sign-in page URL: https://yourCasServer/login
Sign-out page URL: https://yourCasServer/logout
Change password URL: http://whateverServerYouWouldLike

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