Accessing encrypted session data in an external script

[eluser]Unknown[/eluser]
In order to connect Vanilla (forum software) with CI, I need to pass the user's id, name and email from CI to Vanilla. Therefore, I put the id of the user in a session, which is encrypted and stored in the database.
But how can I, in an external script which is used by Vanilla for the authorization, access the data in the session? If I have the session_id I can look the data up in the database, but the session (and so the session_id) is encrypted...