Secure boot and activating iOS

In a concept not unlike that of how Chrome OS ensures both the integrity of its firmware and that its kernel hasn't been tampered with, field upgrades can similarly proceed in a secured fashion with a feature called verified boot. When an iOS device starts up, it verifies the kernel and the rest of the read-only OS partition to confirm that it matches a particular signature. The process would be halted and the device would go back to Device Firmware Upgrade mode or DFU (which would also be accompanied by the 'Connect to iTunes' screen) if the main OS partition is found to be nonfunctional. This can also be initiated if a wipe and reinstall is interrupted when initiated by iTunes, Apple Configurator, or the user themselves by going into the General section of Settings and navigating to Reset | Erase All Content and Settings.

The process from the time you power on the device to when you land in user space is referred to as the secure boot chain...