docs(security): publish signing public key + verification recipe

CHANGELOG.md

@@ -10,6 +10,12 @@ Release notes for each tagged version are extracted from this file by the
 
 ## [Unreleased]
 
+### Added
+- `SECURITY.md` with release-verification recipe and key-rotation policy.
+- `docs/signing_public_key.pem` — public half of the release signing key,
+  committed so users can verify `.signed.bin` without coordination.
+- README "Verifying releases" section linking to the above.
+
 ## [0.1.1] - 2026-04-19
 
 First signed release. `SECRET_RSA_KEY` is now configured, so the release

README.md

@@ -73,6 +73,22 @@ fancontrol/status                           online|offline
 HA discovery is republished on every (re)connect under `homeassistant/{component}/{node_id}_*`.
 TLS is enabled automatically when the broker port is `8883` (insecure mode, matches SmartEVSE).
 
+## Verifying releases
+
+Every tagged release ships `firmware.bin` + SHA-256 + `firmware.signed.bin`.
+Verify signature with the public key committed at
+[`docs/signing_public_key.pem`](docs/signing_public_key.pem):
+
+```bash
+dd if=FanControl-vX.Y.Z.signed.bin of=firmware.sign bs=1 count=256 status=none
+dd if=FanControl-vX.Y.Z.signed.bin of=firmware.bin  bs=1 skip=256  status=none
+openssl dgst -verify docs/signing_public_key.pem -keyform PEM -sha256 \
+  -signature firmware.sign firmware.bin
+# → Verified OK
+```
+
+Full recipe + key rotation policy in [`SECURITY.md`](SECURITY.md).
+
 ## Documentation
 
 - [`PROJECT_PLAN.md`](PROJECT_PLAN.md) — v1.2, the single source of truth (features, test

SECURITY.md

@@ -0,0 +1,77 @@
+# Security Policy
+
+## Verifying release artifacts
+
+Every release attaches three files:
+
+| File | Purpose |
+|---|---|
+| `FanControl-vX.Y.Z.bin` | Unsigned firmware |
+| `FanControl-vX.Y.Z.bin.sha256` | SHA-256 checksum of the unsigned firmware |
+| `FanControl-vX.Y.Z.signed.bin` | The signed firmware: a **2048-bit RSA signature prepended** to the unsigned firmware |
+
+The public key used to sign releases lives at
+[`docs/signing_public_key.pem`](docs/signing_public_key.pem) in this repository.
+
+### Verify checksum
+
+```bash
+sha256sum -c FanControl-vX.Y.Z.bin.sha256
+# → FanControl-vX.Y.Z.bin: OK
+```
+
+### Verify RSA signature
+
+The signing scheme matches SmartEVSE-3.5 exactly: the first 256 bytes of
+`.signed.bin` are the RSA-SHA256 signature, and the remaining bytes are the
+original firmware.
+
+```bash
+# Split off the 256-byte signature
+dd if=FanControl-vX.Y.Z.signed.bin of=firmware.sign bs=1 count=256 status=none
+dd if=FanControl-vX.Y.Z.signed.bin of=firmware.bin  bs=1 skip=256  status=none
+
+# Verify against the public key
+openssl dgst -verify docs/signing_public_key.pem -keyform PEM -sha256 \
+  -signature firmware.sign firmware.bin
+# → Verified OK
+
+# Sanity: the extracted firmware should byte-match the unsigned release artifact
+cmp firmware.bin FanControl-vX.Y.Z.bin
+```
+
+If `openssl` prints anything other than `Verified OK`, **do not flash the
+firmware** — the artifact may have been tampered with. Open an issue under
+[Reporting a vulnerability](#reporting-a-vulnerability) below.
+
+### Key rotation
+
+If the current signing key is ever compromised or rotated, the replacement
+public key will be committed to `docs/signing_public_key.pem` in the same
+release (or the release preceding it), and the change will be called out in
+`CHANGELOG.md` under a `### Security` heading. Old releases remain signed
+with the old key; verify them against the key shipped in the corresponding
+tag rather than the current `HEAD`.
+
+## Scope
+
+FanControl runs on a LAN by design (home battery room controller, talking to a
+local MQTT broker and serving a local web UI). The following are **explicitly
+out of scope** for security hardening at this stage:
+
+- TLS verification for MQTT. Port `8883` enables `WiFiClientSecure::setInsecure()` —
+  no CA certificate check. This matches the SmartEVSE-3.5 approach and is adequate
+  for a trusted LAN broker. If you run this internet-exposed, pin a CA or
+  fingerprint first (tracked by `TODO(security)` in `src/mqtt.cpp`).
+- Authentication on the web UI settings pages. OTA is password-gated via
+  ElegantOTA; status/settings pages are open to anyone on the LAN.
+- Brute-force protection on the OTA form. ElegantOTA does not rate-limit;
+  protect the device network-side.
+
+## Reporting a vulnerability
+
+Please open a [private security advisory](https://github.com/basmeerman/FanControl/security/advisories/new)
+on GitHub. Do not file public issues for security bugs.
+
+For non-exploitable hardening suggestions (missing guards, weak defaults,
+documentation gaps), a regular issue with the `safety` label is fine.

docs/signing_public_key.pem

@@ -0,0 +1,9 @@
+-----BEGIN PUBLIC KEY-----
+MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA113xtiswiWB2GGg2f68a
+ll6WSPPgrXVZkp73871IGVLQ2PzDan3PFuLLIY8AuKXbG1yfVyRT4v69PyOfSgvV
+6x2dMedU3rYtGjm1WYZ9fTsCz0uGPgrwHjVC71VUUBUSi5nEAaZuZybBXhIkM0ad
+xfQcKz41QSC7u9q496iHaVcVKHTY06zHq6AaD9AQEPWwDufSCNXn2RuKdIVWDZdA
+3m+rH5y6/fMAB0IBw9yZYSJ+4IUZt8yacCZsBf+uLhPHjzcET4zyuKd1dnwW1JDc
+LzRUpWkBYQ9vbRWUa53zqER1so/grSrsUn2+SSW0hqXunx3PEXMpjzsNfJu0XQ2G
+0wIDAQAB
+-----END PUBLIC KEY-----