If you have been looking at the facebook, GameCenter and Google+ examples, you may have noticed one thing: All of their code samples look quite the same. The reason for this is pretty obvious: They all make use of what we call “key authentication”.
Key authentication allows you to use a single ‘key’ string to identify a player. This form of authentication makes it possible to use any form of external/foreign authentication system.
Find all about key authentication over here.