Retrieving the Apple Id (or some kind of account id) in Objective C

I am writing a paid app for the store which gives you a 1 year subscription to some content.

This means I need to store on my web service db, when the user first launches the application some kind of unique user id (Apple 开发者_如何转开发ID?), along with the date. I can then do validation to check if the subscription is still valid.

I can see how to get a UDID but that's not really very good if they want to install it on another device with their existing iTunes account.

I would rather not make the user have to enter in an identifier themselves.

Keep in mind that the user may very easily change the apple id he or she is using on the device. He may even allow others (familiy members...) to use the same apple id. I wouldn't share mine, but there is no technical barrier for others to do so. I can easily imagine apple not approving an app making such use of the users apple id, for this and a number of other reasons.

Having said this, I know of no way to retrieve it anyway.

Either tie the subscription to the UDID (and allow the user to migrate it later) or create some sort of accounts the users may log in to. I don't see a way around this...