Multi-site login ala Google

Question

Not sure if the title is quite right for the question but I can't think of any other way to put it..

Suppose you wanted to create multiple different web apps, but you wanted a user who was logged into one app to be able to go straight to your other app without re-logging in (assuming they have perms to look at the other app as well). If I'm not mistaken, if you're logged into gmail you can go straight to your iGoogle, googleReader, etc without re-logging in (if you set it up right).

How would you approach this? What would you use? Assume the apps already exist and you don't want to change the initial login page for the users.

Answer 1

What you're looking for is called Single Sign On. If you follow the link you'll find several implementations.

Open ID as others have mentioned is not such a scheme as it requires a seperate login for each site. Open ID is merely a shared authentication system.

Answer 2

You would issue a cookie against foo.com, which would then be visible on app1.foo.com, app2.foo.com.

Each application can then use the cookie to access a centralised authentication system.