I'm developing now an app for Apple TV. It is a client-server app. On registration, users get a free trial period. My goal is to avoid situations when user will just register new account or reinstall the app and get the trial period again. I need to identify if trial was already activated for each concrete device. So I would like to know if there is a legal or semi-legal way to identify each device? I will appreciate any ideas!
You can store some value in the Keychain (read more about it here) and check if this value was previously stored in the Keychain. If so, it means that user installed your application before.