How do I implement cross authentication with ZenCart?

I have a custom app that I wrote and I want to be able to share authentication between the sites. In other words I want to be able to sign into zencart and be able to securely navigate to my custom site without having to log in.

I looked into using php sessions but it seems that there is a layer in the code that puts all the session information in the database.

Has anyone else out there tried this? Could someone point me in the right direction?