P1: OTA Binary Verification — Verify firmware before flashing

[stritti]

Background

The OTA download path streams the firmware binary directly into Update.write() without any integrity check before committing. A MITM on the shared network can replace the .bin during download and the device will flash the attacker-controlled binary.

Required Changes

1. SHA-256 hash verification: Compute hash of downloaded stream and compare against expected hash from GitHub release metadata
2. Ed25519 signature verification (long-term): Verify binary signature using compiled-in public key
3. Graceful failure: On verification failure, abort update, keep UI state for retry

Affected Files

• src/OtaUpdater.cpp — downloadAndApply(), fetchLatestRelease()
• src/OtaUpdater.hpp — New constants and helpers
• scripts/sign-release.sh — New signing script (phase 2)
• .github/workflows/release.yml — Add signing step (phase 2)

OpenSpec

Full specification: openspec/specs/ota-binary-verification.spec.md
Change marker: openspec/changes/p1-ota-binary-verification/

Related

• Originally deferred from PR feat: OTA update from GitHub releases, semver release management, config safety #77 review
• Blocks: secure OTA update pipeline