How should I share sessions transparently across domains? I work on a Drupal site which has multiple domains (not sub-domains) using Domain Access to set different themes, display different content, etc. on each of them.  These multiple domains are used for canonical HTTP URLs. Each domain also has a corresponding sub-domain of a common domain which is used with Secure Pages for HTTPS (there's a wildcard certificate). This all works fine.

My next step is to manage to unify the session between HTTP (on the canonical domain) and HTTPS (on the sub-domain of the shared domain). Many other sites accomplish this by redirecting back to their main domain to assign or discover the session cookie and redirect back to a "set cookie" script on the the current domain to set it there too.

A semi-serious search of D.O and with Google didn't turn up anything useful so I'll ask here: has anyone done this before in Drupal before? Is there a module or (more likely) a patch I can use and am I on my own here?

Thanks for any suggestions.
There are some related answers and solutions at &quot;Enterprise&quot; authentication, single sign-on and user provisioning for multiple sites. 

While you have a more specific question, the goal is similar to what many SSO solutions in Drupal try to achieve.