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One use of UDID is verifying in-app purchase receipts. It's demonstrated in the sample code associated with Apple's article titled "In-App Purchase Receipt Validation on iOS".

See: https://developer.apple.com/library/ios/#releasenotes/StoreK...

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Never noticed that as I don't have any in app purchases that actually cost me anything. This usage would seem to be more because the ID is available rather than because it is needed though.

The bit about non-public APIs on that page is interesting though - if there is a genuinely necessary use case for device IDs it may not be affected.

The UDID embedded in receipts prevents/deters people from sharing them with others using MITM techniques. Sharing does happen and Apple's article helps address that issue.

True, but that is only one solution and a flawed one, as device ID can legitimately change - no developer needs to know whether I am using the phone I bought today or the one I bought a year ago.

Non-consumable in-app purchases are restorable on any iOS device you can sign in with your iTunes account. When you buy a new iOS device and use StoreKit's restore transaction feature, Apple will generate a new receipt with UDID of that device. In-app purchases are tied to your iTunes account whereas embedded UDIDs are tied to devices you sign in with your iTunes credential.

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