Imagine the following SSO transaction with the Psychic Friends Network (PFN)
User wants to SSO in to PFN
User->PFN: My IDP is ..
PFN->User: No, No, don't tell me! I know this.
PFN->User: AOL? Yahoo!? MyOpenID? No? perhaps you could give me a hint?
User->PFN: It starts with a 'G'.
User->PFN: Yes, it's Google.
PFN->User: See, told you I knew.
PFN->Google: Please authenticate this user.
Google<->User: Sign-on steps
Google->PFN: Here is the assertion, the user is ....
PFN->Google: Wait, wait, don't tell me, let me guess first
PFN->Google: I'm getting a strong sense of an identifier for this user. I see a 'j', then a '2', followed by a '@', no wait it's a 'O'.