How can I uniquely identify a user on an iOS device?

Question

I've read many topics and every one tells me that this can't be done.

Apple don't expose the Apple ID or anything else to identify the user when he changes devices (eg iPhone to iPad).

Later I downloaded the app OfflineMap 2, buy one feature for 5$ on iPhone.

I also downloaded this app on the iPad and when I run and press re-get flat trance (name of the product I bought) it asks me for my iTunes account and then I see on iPad that it's all good.

I thought I would have to re-buy it, but the application tells me that I already have it and lets me redownload.

So how can I do this if the application can't identify the user uniquely?

Answer 1

There's a difference between you identifying a user uniquely, and Apple identifying a user uniquely.

Apple is clearly capable, but that doesn't mean that they expose interfaces for anyone else to do so.

In-app purchases like you described are usually evaluated against receipts so that buying it on one device gives it to you everywhere.