Why is RP using browser as a mediator, Why can't RP redirect directly to IP before authentication and vice versa after authentication?
I came up with some reasons but couldn't convince myself.. so asking you :)
1) RP and IP cannot keep a line of connection (against: what if RP calls STS web service and gets the data in response)
2) To manage cookies/session(against: but can't RP finally return the cookie? And the browser returns it in each request thus maintaining a session),
3) It is the browsers responsibility to pass the credentials to IP because of data protection policy(good one)
4) IP needs to know who the caller is (against: why?)