Touch ID sensor is hardware-paired to iPhones

Allyson Kazmucha at iMore:

To test this theory, we took two iPhone 5s handsets that had never been opened before, and verified Touch ID was working on both. The front assemblies, which contain the Touch ID assembly itself, were then removed and swapped. After re-assembly, both Touch ID setups failed. When returned to the original device, Touch ID once again operated correctly.

This confirmed for us that the Touch ID component cable assembly itself is tied to each individual A7 chip.

In short? Great for security, but tricky for repairers. If you’re ever taking apart an iPhone 5s, do NOT break that component cable assembly, otherwise Touch ID on that device will never work again!