title: Single Sign On Framework
body: <p>I want to implement a SSO Framework. My requirement is thus:</p>
  
  <p>Once a user log's into particular website and he clicks on an external link, he should not be asked to verify his credentials again.</p>
  
  <p>EDIT: Here, I have control over the 'external link' that I speak of. The first link that the person sign's into can provide me credentials or other information that I require, but I have no control over it.</p>
  
  <p>I researched a bit, and found CAS to be relevant for my requirement. But, I don't want the end user to login to CAS initially, I need a framework that receives the credentials from the currently logged in website and uses the same to login to the other external site. Security is, of course, an important factor. Can you please give me some pointers/ ideas as to how to go about designing such a framework?</p>