[Zope] Login user programatically from the values of REQUEST['HTTP_COOKIE']

Hello, Sorry for all my questions right now but it isn't that easy to make a payment gateway written in PHP to talk to Zope ... I was wondering if it was possible to login a user programatically with the data from HTTP_COOKIE. It contains all the data required (__ac_user, __ac) but the __ac is encoded. Anyone who have done this before ?? TIA, Gitte