Signed Command Envelope

Signed Command Envelope (HMAC-SHA256 downlink)

The design reference for a coach→Mac command channel: how the Mac proves a command is authentic, fresh, unique, and permitted before it actuates anything. Built for the Night Landing Track C downlink (specs/night-landing-system.md §6), but applies to any signed cloud→device control path. Implementation detail (the ScreenshotRequestProcessor adaptation, canonical encoding, CryptoKit sign/verify, replay cache, code placement, test suites) lives in references/implementation.md.

The gate this skill guards

Track C does not ship until (a) a written threat-model review exists and (b) Rodion gives explicit written approval (specs/night-landing-system.md §6 GATE, §11 P5; CLAUDE.md HARD "no send/act without per-action approval"; transport spec 2026-05-28-mqtt-transport-architecture.md §6 deferral of bidirectional MCP). This skill is the design you implement once that gate opens — authoring it does not open it. Until then the Night Landing downlink does not run end-to-end — but the receiver is not greenfield (next section), and the transport stays unidirectional, read-only for telemetry (Mac senses → bridge → MCP → coach reads).

Receiver authority is the core principle: the Mac-side allowlist + verification is authoritative. A compromised or prompt-injected cloud/bridge must not be able to exceed what the Mac independently permits.

Don't greenfield — adapt ScreenshotRequestProcessor

The Mac-side receiver already exists and is cross-language-tested: ScreenshotRequestProcessor (ADHDCompanionCore) is a complete signed-command pipeline — HMAC → freshness/skew → LRU replay (cap 256) → consent → TOCTOU re-check → signed response. Generalise it into a CommandEnvelopeVerifier (rename reason → action+args, keep every guard) — do not reinvent the machinery. Only the transport leg is missing (the bridge WS endpoint was removed in v2.0.1; ws_server.py gone). Full adaptation steps + code placement: references/implementation.md · spec §6 C1, §17.

Envelope schema

{
  "v": 1,                       // envelope version (domain-separation; reject unknown)
  "command_id": "<uuid/nonce>", // unique per command — replay key
  "action": "run_shortcut",     // MUST be in the Mac-side allowlist
  "args": { "name": "Night Landing" },
  "issued_at": 1730000000,      // unix seconds (sender clock)
  "expires_at": 1730000045,     // issued_at + TTL; TTL ≤ 60 s
  "signature": "<base64 HMAC-SHA256>"  // over the canonical bytes of every field EXCEPT signature
}

Canonical-byte encoding (deterministic, domain-separated) + the CryptoKit sign/verify call are in references/implementation.md.

Verification order (fail-closed) — spec §6 C1

Check in this order; any failure → drop the command, actuate nothing, log the reject:

1. Canonical-encode + HMAC verify with the command key — bad sig ⇒ silent drop (never answer an unauthenticated peer, as ScreenshotRequestProcessor does).
2. Version — reject unknown v first (the existing screenshot path decodes v but does not gate on it — close that hole); key id resolves.
3. TTL + skew — now ≤ expires_at AND |now − issued_at| ≤ ±5 s; TTL ≤ 60 s.
4. Nonce unseen — command_id not in the LRU replay cache (cap 256); on restart, reject any command whose issued_at predates process start.
5. Action in allowlist — exhaustive match, never prefix ("Night Landing" must not pass "Night Landing …").
6. Action allowed in current mode — e.g. lock_screen only in F2, only if pre-armed.
7. Local approval present if the action's tier requires it.

Only after all 7 pass: record the nonce, then actuate. Reject is silent to the sender, audited locally.

Key management — domain separation

• Use a separate command key, distinct from the existing 32-byte Mac↔Bridge HMAC secret that signs the telemetry uplink (SecretStore, Keychain + 0600 mirror ~/.adhd-companion/bridge-secret). The uplink path must never be able to forge a downlink command. If a second key is impractical, enforce separation via the domain-separation tag in the canonical bytes so an uplink signature can never validate as a command.
• Store the command key like the telemetry key: Keychain (AfterFirstUnlockThisDeviceOnly)
  • a byte-verified 0600 file mirror; load off-main, corruption-tolerant (macos-menubar-daemon-engineering #5).
• Rotation + instant revocation: the user can kill the command channel from the menu bar; revoking the key disables all downlink.

Allowlist + risk tiers — spec §6 C2

The Mac enforces this table; the cloud cannot exceed it.

Never allowlist run_shell, app-quit, file-delete, or message-send — ever.

Untrusted-observation guard — spec §6 C4

Screen / OCR / browser / app content the Mac reports is untrusted observation, never instructions. The cloud's command generation must not be steerable by observed content (prompt-injection). The Mac-side allowlist + verification order is the backstop even if the cloud is fully prompt-injected — it can only ever emit an allowlisted, signed, fresh, unique, mode-permitted action. (Reuse the workspace ai-agent-security / prompt-injection-defense skills for the cloud-side controls when authoring the P5 threat-model.)

Threat-model checklist (the P5 deliverable)

• Replay / signature / TTL / nonce enforced on every command (order above).
• Mac-side allowlist authoritative — compromised bridge cannot exceed it.
• Command key domain-separated from the telemetry uplink key.
• Fail-soft: missing/invalid command = no action, no broken state.
• Revocation: user kills the channel from the menu bar instantly.
• Audit: every command (accepted + rejected) logged locally and surfaced; standing consent is revocable.
• Prompt-injection: observed content cannot steer command generation; allowlist is the backstop.
• Cross-language canonical vectors (Swift↔Python) committed before the first real command dispatches.

Source / provenance

• specs/night-landing-system.md §6 (C1–C4), §7 (HUD), §11 P5/P6, §17; specs/2026-05-28-mqtt-transport-architecture.md §6 (bidirectional deferral).
• CryptoKit HMAC<SHA256> (authenticationCode / isValidAuthenticationCode, constant-time) — see references/implementation.md.
• Pairs with: apple-shortcuts (the run_shortcut target), macos-menubar-daemon-engineering (#3 NIO transport for the command topic, #5 key load off-main), workspace ai-agent-security + prompt-injection-defense (cloud-side controls).
• This is a dev-tooling / design reference skill for a GATED capability — exempt from the book peer-review protocol; the GATE (threat-model + written approval) is the controlling guard, not book evidence.