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These technologies are tied together in a chain of trust, such that each layer verifies the next. In iOS systems, this chain begins with the Boot ROM, which is a piece of unchangeable code that is the first code to run each time the system starts. Each of the later pieces of software that are loaded are signed in a way that can be verified by the previous layer. So the Boot ROM can verify the bootloader’s signature, which can then verify the OS kernel signature, which can verify extensions and device drivers, etc. The real way it works has further protections and complexities, but this is the basic scheme.

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Wouldn't it be great if smartphones became unassailable sources of truth?"