Single Sign On or Bridge with in the WordPress WebSites?

Question

I have 5 different Multisite Installations . I want the registration of all the Multi-Site Installations(Not only sub-sites, the entire installations) as single-sign-on . So a user can login to all the sites if he/she register on one Single website.

I could use Some OAuth plugins or Single-sign-on plugins. But they still need two steps to register and login on each WordPress Site. So I would like to customize or use any plugin to do so. I know some PHP and will try to do if any body help me.

Could anybody let me know if I should do it by coding or using a plugin?

Note : I am using WordPress Installation of 3.1.1 and VPS Server of CentOS

Update :

I heard about a few options like LDAP, Cosign on etc. There are a few plugins like LDAP, CoSign SSO etc . I don't know how to make use of them . Could anybody let me know if they help me in this situation?

BTW, I have activated XAuth and I don't know how to get started with it

Thanks !

Answer 1

As far as i know you need to make sure the cookies domains are the same through out all sites if they are on separate domains and even this can be a bit buggy at times. WPMUDEV.ORG created a sync plugin to sync users across multi sites but as far as a single sign on i really have not yet tackled that before and when i tried it got a little tricky so i just stuck with syncing users across all and if they wanted to sign on another site they could use that same username