Secure boot
A boot process in which each stage verifies the cryptographic signature of the next before running it, so only firmware signed by the manufacturer executes.
Secure boot starts from code in read-only memory that cannot be changed after manufacturing. That code checks the signature of the bootloader against a public key stored in the chip; the bootloader checks the application firmware the same way. If any signature fails, the device halts or falls back to a known-good image. The chain anchors in a hardware root of trust.
Without secure boot, anyone with physical access or a write path to flash can install modified firmware that behaves normally while exfiltrating data or joining a botnet. With it, an OTA update mechanism can be trusted, because unsigned images are rejected. The cost is key management discipline: losing the signing key or shipping with debug keys enabled undoes the protection.
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