Project 15: Embedded Security Hardening Pipeline (Secure Boot, Flash Encryption, Signed OTA)

Build a security baseline that enforces boot trust, image authenticity, encrypted storage, and threat-modeled firmware operations.

Quick Reference

Attribute Value
Difficulty Master
Time Estimate 3 weeks
Main Programming Language C
Alternative Programming Languages C++, Rust
Coolness Level Level 5
Business Potential Level 4
Prerequisites Cryptography basics, firmware lifecycle concepts
Key Topics Secure boot, flash encryption, signed OTA, secrets handling, TLS policy

1. Learning Objectives

  1. Implement and validate secure boot chain behavior.
  2. Enforce signed OTA acceptance and tamper rejection.
  3. Protect secrets at rest and in CI workflows.
  4. Build an embedded threat model tied to concrete controls.

2. Theory

2.1 Chain of Trust in Embedded Boot

Security begins before app code. Boot ROM, bootloader, signatures, and key material form the trust boundary.

2.2 Operational Security in Firmware Lifecycle

Security controls must survive updates, provisioning, and incident response.

3. Specification

  • Enable secure boot and flash encryption in controlled environment.
  • Signed OTA pipeline with anti-rollback policy.
  • Secret storage plan with no plaintext credential leakage.

Output:

I sec: secure_boot verify=PASS
I sec: flash_encryption mode=release
W sec: ota candidate rejected reason=invalid_signature
I sec: secret_store=nvs_encrypted status=ready

4. Architecture

[Provisioning] -> [Key Policy] -> [Boot Chain]
                             -> [OTA Verifier]
                             -> [Secret Storage]

5. Implementation Guide

Core question:

“How do I harden firmware without sacrificing recoverability and maintainability?”

Design questions:

  1. Which threats are highest-impact for your deployment?
  2. Which operations require offline key custody?
  3. How are emergency rotation and revocation handled?

6. Testing

  • Tampered image rejection tests.
  • Downgrade/anti-rollback tests.
  • Certificate validation and expiration behavior.

7. Pitfalls

  • Enabling irreversible controls before rehearsing recovery.
  • Mixing development and production keys.
  • Missing negative tests for signature failures.

8. Extensions

  • Add hardware-backed attestation reports.
  • Add signed policy bundles for runtime security toggles.

9. Completion Checklist

  • Threat model maps controls to threats.
  • Tampered and downgraded images are consistently rejected.
  • Secret handling policy is documented and enforced.