Signed OTA Update Pipeline (ESP32 + STM32)

September 3, 2025

⚠️ Reference implementation for product teams and training. Adaptations to your hardware/toolchain are planned.

• Security by design: Ed25519 signatures + SHA-256 integrity + anti-rollback.
• Robustness: A/B slots, watchdog-friendly OTA writes, rollback on errors.
• Transparency: Canonical manifest (verifiable, diffable); reproducible signing process.
• DevEx: Python signer, verifier, CI workflow (GitHub Actions) – ready to use immediately.
• Cross-target: ESP32 (ESP-IDF v5.x) & STM32 (bootloader skeleton) from a single toolkit.

• Canonical Manifest (JSON): device, version, slot, timestamp, image.sha256/size, meta.*
• Signatures: Ed25519 over the canonicalized manifest body (stable sorting/separators)
• Anti-rollback: Monotonic version check (NVS/Flash) + optional HW counter
• A/B schema: Safe switching, self-test, commit only when healthy
• CI-capable: Build → Sign → Artifact, key injection via secrets, release promotion
• Extensible: SBOM (CycloneDX), release signatures (cosign), staged rollouts

• Tooling ("signer/"): Keygen, signer, package builder (.ota ZIP with manifest.json + firmware.bin), verifier
• ESP32 App: OTA flow (download/stream), versioning, NVS hook, rollback support via IDF bootloader
• STM32 Bootloader: Slot selection, manifest/hash validation (Ed25519 verify as port/library), jump logic, rollback marker
• CI Workflow: GitHub Actions template (build, sign, artifact upload)
• Documentation: Threat model, test plan, integration guide

[CI Build]  ──(Artifacts)──>  [Signer]  ──( .ota Package )──>  [OTA Server/CDN]
                                                │
                                         (HTTPS, Auth)
                                                │
                                         [Device Fleet]
ESP32: Downloader → Verify(Signature+Hash) → Stream to OTA Partition → Reboot → Self-Test → Commit
STM32: Bootloader selects slot → Verify(Signature+Hash) → Jump → on errors Rollback/Alternate Slot

• Authenticity & Integrity: Ed25519 over manifest; SHA-256 over image
• "Verify-before-commit": No activation without fully verified data
• Rollback resilience: A/B schema + monotonic version
• Key hygiene: Offline root, short-lived CI keys, rotation & pinning of public keys
• Supply chain: Optional SBOM + release signatures + reproducible builds

{
  "body": {
    "device": "esp32|stm32",
    "version": "1.2.3",
    "slot": "ota_0|ota_1|app_a|app_b",
    "timestamp": "2025-09-03T12:00:00Z",
    "image": { "sha256": "…", "size": 524288 },
    "meta": { "min_bootloader": "1.0.0", "rollback_protected": true }
  },
  "signature": { "algo": "ed25519", "value": "BASE64_SIG" }
}

• Partitions: factory, ota_0, ota_1
• OTA stream via esp_https_ota
• Pre-commit checks: manifest signature, image hash, version
• Self-test after reboot; esp_ota_mark_app_valid_cancel_rollback()

• Slots A/B + manifest header in reserved area
• Verify (signature/hash) before jump
• Rollback marker in backup SRAM/option bytes
• Power-fail-safe switching

1. Build (ESP32/STM32) → binary artifact
2. Sign: signer.py generates .ota with manifest + signature
3. Publish: Upload to OTA server/CDN, release via environment gate
4. Deploy: Device downloads package (HTTPS), verifies manifest/hash, streams to target slot
5. Commit: After self-tests → commit, otherwise rollback

# 1) Generate keys (offline)
cd signer && python -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt
python generate_keys.py  # keys/ed25519_priv.pem + ed25519_pub.pem

# 2) Build ESP32
idf.py -C esp32 set-target esp32 && idf.py -C esp32 build

# 3) Sign package
python signer.py \
  --pubkey keys/ed25519_pub.pem \
  --privkey keys/ed25519_priv.pem \
  --device esp32 --version 1.0.0 --slot ota_0 \
  --image ../builds/esp32-app.bin \
  --out ../dist/esp32-app-1.0.0.ota

# 4) Verify package
python verify_package.py --pubkey keys/ed25519_pub.pem --package ../dist/esp32-app-1.0.0.ota

• Device-side Ed25519 verification integrated/ported as a lightweight library (e.g., TweetNaCl port).
• Public key stored on the device (rotation via update possible).
• Optional: NVS encryption (ESP32) & Secure Boot/TrustZone/TPM integration.

• Unit: Signer/verifier (schema, canonicalization, round-trip), version logic
• Integration (ESP32): Happy path, signature error, hash mismatch, rollback path
• Integration (STM32): Slot matrix (A/B valid/invalid), brownout during update
• HIL: Network interruptions, watchdog triggers, recovery scenarios

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• v1.0 – Reference pipeline (tooling, ESP32/STM32, CI, docs)