Check In App Purchase with a web service

Question

I need to design an app which use In App Purchase. In this app I have to use StoreKit (obviously!) and also I want to store a list of people who bought app and check them with my own web service. This is because of some jailbreak feature which can delude app store

What I did comes in following:

1. If user wants to use special feature and haven't bought it:

1.1 First I check In User Defaults and Check If User purchased the feature.

1.2 If not I show him/her apple page store and ask him/her to buy.

1.3 After confirmation I send data to my web service and also save it in User Defaults.

2. If user wants to use special feature and have bought it:

2.1 First Check if anything has stored in User Defaults

2.2 If nothing has bought show him/her page store

2.3 After getting has already purchased message from store I check it with my web service

2.4 If web service confirmed already purchased message then I let user use feature and save it in User Defaults.

I have problem with choosing a unique identifier which works among all apple devices.

Answer 1

You don't need to identify the device, you need to validate the transaction. The user should be able to restore purchases across re-installations and across devices.

The In-App Purchase programming guide states -

Persisting Using Your Own Server Send a copy of the receipt to your server along with some kind of credentials or identifier so you can keep track of which receipts belong to a particular user. For example, let users identify themselves to your server with an email or user name, plus a password. Don’t use the identifierForVendor property of UIDevice—you can’t use it to identify and restore purchases made by the same user on a different device, because different devices have different values for this property.

So, once you have the receipt you can send it to a web service. You can use a shared secret or a public/private key pair to sign the receipt so that your server can validate it.