The solution to the timestamp problem is quite easy, and that’s to also include a ‘checksum’ in the file. The checksum is something derived from in the file - for instance you could add up 20 different combinations of the individual digits in the timestamp (plus other data in the file, like the in app purchase information), convert those 20 different sums to letters (eg 1 or 27 or 53 becomes a), and you have a checksum. Then your app checks that what’s in the file also matches the checksum. I think it would be next to impossible for someone to crack a checksum as random as that wouldn’t it?
Please correct me if I’m wrong, but I thought you can’t use unique device identifiers in this, because apps are allowed to be used on multiple devices owned by the same person, plus you can also restore in-app purchases to new devices.
I’ve seen websites dedicated to ‘crack files’ for individual apps - just overwrite this file and away you go. They even give you a program to download to easily overwrite the file. So I don’t think it’s a waste of time. But yeah I guess your app needs to be popular enough first!
regards