Secure Boot

Secure Boot Implementation

Purpose

Establish a root of trust and verify firmware integrity/authenticity from reset through handoff, including anti-rollback and recovery.

When to Apply

• Bootloader design/updates; adding secure boot to devices; integrating OTA/update flows.

Requirements (testable)

1. Root of Trust: Immutable boot ROM or fuse-backed root keys; verify first mutable stage signature. Rationale: trust anchor.
2. Signature Verification: Verify each stage (bootloader, app) with strong signature (e.g., ECDSA P-256, Ed25519); reject on failure. Rationale: authenticity/integrity.
3. Anti-Rollback: Enforce monotonic version/counter in secure storage; block rollback to vulnerable images. Rationale: prevent downgrade attacks.
4. Measured Boot/Logs: Record hash/version of loaded image; store for audit; optionally attest. Rationale: traceability and attestation.
5. Secure Updates Tie-in: Only install signed images; reuse verification path; fail-safe if verification fails. Rationale: consistent chain of trust.
6. Key Protection: Store verification keys in secure hardware or read-only region; support key rotation with dual-key or certificate chain. Rationale: prevent key tampering.
7. Recovery Path: Provide authenticated recovery image path; safe state if both primary/recovery invalid. Rationale: availability with integrity.

Recommended Practices

• Use minimal bootloader; avoid network stacks in first stage.
• Keep signature verification constant-time; validate manifest (lengths, addresses).
• Separate signing keys (offline) from device-embedded verification keys.
• Log boot result and reason codes for field diagnostics.

Patterns

Manifest verification (pseudo):

// REQ-SBOOT-VERIFY-01; TEST-SBOOT-04
bool verify_image(const manifest_t *m) {
    if (m->len > MAX_IMG || m->entry < FLASH_BASE) return false;
    uint8_t hash[32];
    sha256(image_region(m), m->len, hash);
    return ecdsa_verify(m->sig, m->sig_len, hash, root_pubkey);
}

Anti-rollback check:

// REQ-SBOOT-ROLL-02; TEST-SBOOT-06
if (m->version < stored_version()) {
    return false; // reject rollback
}

Boot log:

// REQ-SBOOT-LOG-03; TEST-SBOOT-08
boot_log_t log = { .version = m->version, .status = BOOT_OK };
log_boot(&log);

Anti-Patterns (risks)

• Accepting unsigned or self-signed images -> risk: code injection.
• Storing verification keys in writable flash without protection -> risk: key swap.
• No anti-rollback -> risk: reintroducing known vulnerabilities.
• Skipping length/region checks -> risk: overwrite/exec unintended memory.

Verification Checklist

• Root key immutable or fuse-protected; first mutable stage verified.
• Each stage signature verified with approved algorithms; failures halt/enter safe recovery.
• Anti-rollback enforced with monotonic counter/version.
• Boot measurements logged; available for diagnostics/attestation.
• Update path uses same verification; unsigned images rejected.
• Verification keys protected; rotation process defined and tested.
• Recovery path authenticated; safe behavior if both images invalid.

Traceability

• Requirements REQ-SBOOT-### linked to tests TEST-SBOOT-###; map to threat model controls.

References

• NIST SP 800-193 (Platform Firmware Resiliency).
• MCU vendor secure boot app notes.
• FDA Cybersecurity Premarket Guidance (integrity of software).

Changelog

• 1.0.0 (2026-01-04): Initial secure boot skill with root of trust, signature, anti-rollback, and recovery guidance.

Audit History

• 2026-01-04: Audit performed. Verified:
  • NIST SP 800-193 (Platform Firmware Resiliency) correctly referenced
  • Signature algorithm recommendations (ECDSA P-256, Ed25519) are current
  • Anti-rollback mechanisms accurately described