5. OpenID (login via gmail)
6. OAuth2 (login via Facebook)
7. Writing on Facebook wall with Spring Social
OpenID is to form authentication, what DVCS is to centralized version control system.
Ok, but without the technical mumbo-jumbo: OpenID allows the user to use one account (like Gmail) to login to other services (websites) without having to remember anything and without worries about password security.
And the best part is: chances are, you already have an account which is a provider to OpenID. Are you registered on Gmail, Yahoo, or even Blogger? You can already use it to login to other sites.
But why would you? Isn’t login/password good enough?
Have you ever wondered whether the service you are logging to, uses one-way password hashing or keeps the password as open text? Most users do not create password per site because they cannot remember more than, let’s say, three passwords. If one site keeps their password as open text, they are practically screwed. And more often than not, it’s the site that has the worst (or non-existing) security. Welcome to hell: all your bases are belong to us.
Kudos for that, though it smells like overengineering a bit :)
OpenID is pretty simple, and the process description at Wikipedia is practically everything you need, but as they say, one picture is worth 1000 words, so let’s look at a picture. There is a loot you can find with google, but most of them are in big-arrows-pointing-some-boxes-notation, and I find easier to read a sequence diagram, so here is my take on it: