How can my server securely authenticate iPhone in-app purchase?

Question

Look at Apple\'s diagram for the server purchase model.

In step #9, how can the server know that it is really talking with an iPhone that is entitled to the purchase

Answer 1

I do not think that can bind the receipt to the device.

My understanding is that you are allowed to install an application on multiple devices without extra cost. Binding it to the device would mean that if you for example upgrade/change your phone you would need to purchase all the apps again.