Validating in-app purchases in your iOS app

There was actually a very good talk about this at WWDC 2014[0] session 305 Preventing Unauthorized Purchases with Receipts.

[0] https://developer.apple.com/videos/wwdc/2014/

This is an interesting article and everyone who uses in-app purchases should understand how they work.

However, a lot of work has already been done by other developers and it's wise to reuse something such as CargoBay https://github.com/mattt/CargoBay.