SSO with OpenID

I trying to find the way to implement the full blown SSO using the OpenID so that user does not need to enter anything on the new site within the SSO.

I removed all cookies and started experimenting.

1. Logged in on stackoverflow.com
2. Looked at Fiddler and seen that stackauth.com is being contacted
3. started searching to see what is stackauth.com and ended up stackapps.com
4. I was RECOGNIZED and logged in to the stackapps.com !

To repeat, I deleted all cookies at the start. Ca开发者_JAVA百科n someone explain to me SSO that stackoverflow uses because I want to build SSO for sites on different domains? Maybe link if it is already explained somewhere.

Just checked. I gone to SuperUser.com and was initially not logged but after a moment at the top was it was displayed that I am logged in and to refresh that page.

Global Network Auto-Login How does this technically work (if it is not secret :) ) ?

Update

It is mentioned that HTML5 local storage is the key.

But, let's say that I don't have a problem with redirecting all of my users to the some central Auth site. What should I store in the cookie of that site? How will I get user information, do I need to write some API? I would like to stay out of writing my API so I don't have to handle all the security problems. I would rather use something existing like OpenID.

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You may take a look at DotNetOpenAuth. It is what StackOverflow uses to implement the OpenID authentication.