Explain the secure boot process in embedded systems.

Secure boot ensures only authenticated code runs on a device, protecting against firmware tampering. Boot chain: ROM bootloader (immutable, first code to run) contains root of trust public key. ROM verifies first-stage bootloader signature using public key. First-stage bootloader verifies second-stage/OS. Each stage verifies next stage before execution. Key components: Hardware root of trust (ROM, OTP fuses, secure element). Cryptographic verification (RSA, ECDSA signatures, hash chains). Secure key storage (eFuses, secure enclave, TPM). Anti-rollback mechanisms (version counters in OTP). Implementation: Sign firmware during build with private key. Store public key or hash in device OTP. Bootloader verifies signature before jumping to code. Failed verification halts boot or enters recovery. Challenges: Key management and provisioning. Handling key revocation. Balancing security with update flexibility. Side-channel attack resistance.