Single Sign On across multiple domains

Question

Our company has multiple domains set up with one website hosted on each of the domains. At this time, each domain has its own authentication which is done via cookies.

When someone logged on to one domain needs to access anything from the other, the user needs to log in again using different credentials on the other website, located on the other domain.

I was thinking of moving towards single sign on (SSO), so that this hassle can be eliminated. I would appreciate any ideas on how this could be achieved, as I do not have any experience in this regard.

Thanks.

Edit: The websites are mix of internet (external) and intranet (internal-used within the company) sites.

Answer 1

The SSO solution that I've implemented here works as follows:

1. There is a master domain, login.mydomain.com with the script master_login.php that manages the logins.
2. Each client domain has the script client_login.php
3. All the domains have a shared user session database.
4. When the client domain requires the user to be logged in, it redirects to the master domain (login.mydomain.com/master_login.php). If the user has not signed in to the master it requests authentication from the user (ie. display login page). After the user is authenticated it creates a session in a database. If the user is already authenticated it looks up their session id in the database.
5. The master domain returns to the client domain (client.mydomain.com/client_login.php) passing the session id.
6. The client domain creates a cookie storing the session id from the master. The client can find out the logged in user by querying the shared database using the session id.

Notes:

• The session id is a unique global identifier generated with algorithm from RFC 4122
• The master_login.php will only redirect to domains in its whitelist
• The master and clients can be in different top level domains. Eg. client1.abc.com, client2.xyz.com, login.mydomain.com