From the KnowledgeBase
CAS: University Central Authentication Service for web-based applications
The University Central Authentication Service (CAS) is a protocol for providing secure authentication for web-based applications -- one that you can trust will protect your University identity and credentials.
CAS-enabling a web application permits users to log on once and to be automatically authenticated to any other CAS-enabled application. This is also called single sign-on technology. The CAS web pages are protected by an Extended Validation SSL certificate from Verisign. The EV SSL certificate is only issued to organizations that have gone through a more rigorous screening to prove their identity. CAS uses your Princeton University netID and password. A CAS-enabled application ensures that your security credentials are not being collected by web-based applications. Applications will automatically re-direct you to the CAS web page to enter your credentials, or you can navigate to the CAS login page by going to www.princeton.edu/cas.
How can I implement CAS-authentication on my website?
If you are a web application developer or programmer at Princeton University, and would like to use CAS for authentication, documentation is available at the Princeton CAS Implementation KB online (Princeton login required).
How can I identify a safe site?
You can easily identify websites that are protected with an EV SSL certificate in current versions of most browsers by "green" markers in the browser's URL bar. Below are screen shots of the CAS login page showing the "green bar" of the Extended Validation SSL Certificate in three different browsers: