In-App Purchase Programming Guide suggests you can persist In-App purchase in NSUserDefaults here. However I found this article saying that it is insecure and data in it are easily accessed and modified:
NSUserDefaults are stored in plist in binary format, with no encryption, and is stored in your app’s directory. This means that any user, even the “noobiest” one, can tinker with your NSUserDefaults with 5 minutes of their time.
If it is true user can easily get for free anything provided as in-app purchase that is persisted using NSUserDefaults.
Is the article still correct for iOS 8,9? If so how do you persist your in-app purchases? I prefer some simple solution. I do not (nor want to) validate receipts etc.