securing a custom app

Hello, I've got a custom app that interacts with a database. I want to use something stronger than .htaccess to protect it and ssl is not available as this is a shared host. There will be several user's accessing this app and updating the database through it. What i was thinking was giving each a unique username, password, and ID string, which would be somehow used to compute a hash and if that would match access could be granted. That's just a guess on my part, i'd appreciate any suggestions. Thanks. Dave.