keyrelay/AGENTS.md

# AGENTS.md — working on keyrelay

A WiFi-controlled USB HID keyboard: laptop browser → ESP32-S3 board → USB into the target.
The target runs no software and sees a Logitech G815. Everything the board does is gated on
one idea: **the difference between a remote keyboard and an unattended injection device is a
human deciding, live, with the arming light on.** Changes that quietly move that line are the
ones to catch here.

The README is the single source of truth for behaviour, protocol, safety and measurement.
This file is the operating manual: what must not break, how to build and verify, and where
the traps are. **Do not fork the protocol or the safety model into `docs/`** — a second copy
drifts, and two paths that can disagree about arming is the exact failure this project keeps
having to fix (see README, "Eleven bugs").

## Phased execution

This repository runs work through `.agents/phases/`:

1. Always read `.agents/PLAN.md` first to understand the project context and goals.
2. Follow the phased execution protocol in `.agents/phases/`.
3. Never modify `.agents/PLAN.md` or any files in `.agents/phases/complete/`.
4. If you need to update any file in `.agents/phases/todo/`, you must ask the user for permission first.
5. Strictly adhere to the **LOCKED DECISIONS** listed in `.agents/PLAN.md`.

## The invariants

1. **State, not events.** `S=<mods>:<usages>` restates the *entire* desired key state. Applying
   the same state twice is a no-op; any one message resynchronises the target completely. Never
   introduce key-up/key-down events, sequence numbers, or a resynchronisation timer. Every
   stuck-key horror story in HID injection tooling is an event-protocol bug.
2. **Disarmed is the only default.** Boot, reload, reconnect, focus loss, socket loss, target
   detach, bus suspend, queue overflow and 8 s of controller silence all disarm, and disarming
   releases every key **and every mouse button**. Arming needs the passphrase, the board
   button, or the serial console — nothing else may gain the ability. The mouse is captured,
   never grabbed: Pointer Lock, so `Esc` (the browser's own, never forwarded) always frees
   it. `mouse off` over serial removes the pointer interface at the descriptor level, which
   needs a reboot and is the pointer's rollback (the transport rollback is `-D KR_TLS=0`).
3. **Every fail-safe cancels work in flight; nothing "pauses"** — and **no fail-safe may be
   gated on the absence of the thing it detects.** `ControlServer::linked()` is false when a
   controller is gone, so gating the silence-disarm on it made the backstop for a vanished
   controller uncallable exactly when a controller vanished. Same shape: `closeLink()`
   releasing only `if (linked())` skipped the release for every path that cleared `_authed`
   first. If a rule fires when X stops existing, `X still exists` cannot be a precondition
   of it. See README bug 10; `test_relayctl.py` pins the three sentences.

   `releaseEverything()` in
   `main.cpp` is the single funnel: it clears the report queue, raises the keys, ends the chord
   and discards a pasted run. Any new state that can outlive a disconnect (a queue, a timer, a
   run) must be cancelled there too, or it will keep acting after the operator stopped asking.
4. **Nothing is stored.** Counters only. No keystrokes, characters, key names or text in NVS,
   logs, `Serial.printf`, error messages or status frames — `src/main.cpp` prints *lengths* for
   text runs, never content. No macro store, no schedule, no replay, no auto-arm.
5. **Reject, never guess.** A malformed frame leaves the previous state in force. A character
   the map cannot express is `E9` and *none* of that frame is typed — a paste that quietly loses
   a character is worse than one that refuses. Silent sanitising is never the default.
6. **`Ctrl+Alt+Del` is fixed on the device.** It is never composed from wire bytes; on a Linux
   VT it reboots the target. Same reasoning for the VT-switch chords.
7. **One frame is one command.** ASCII, no JSON inbound, ≤ `kWsMessageCap`. Long payloads are
   the *controller's* problem to split, with the board reporting how much it still holds.
8. **The keyboard is exclusive; the board is not.** A second `/ws` upgrade is refused (`E3`);
   read-only HTTP is served even while a controller holds the bridge. Keep both routes going
   through `serveReadonly()` so they cannot drift apart.

## Tables that must agree

| table | where | names | pinned by |
|---|---|---|---|
| `CODE_TO_USAGE` | `src/web_page.h` | physical position → usage | `tests/test_relayctl.py` |
| `CODE_TO_USAGE` | `tools/relayctl.py` | same, for the terminal | `tests/test_relayctl.py` |
| `strokeFor()` | `src/text.h` | character → modifier + usage | `tests/test_text.cpp` (whole printable range walked) + tool/board agreement on all 128 ASCII codes |
| mouse button bits | `src/mouse.h`, `src/web_page.h` (`MOUSE_BTN`), `tools/relayctl.py` (`MOUSE_BUTTONS`) | HID bit → DOM `MouseEvent.button` **and** name | `test_proto.cpp` (page↔config), `test_relayctl.py` (tool↔page↔config) |
| evdev axis codes | `tools/watch_target.py` (`REL_*`) | kernel code → label, for the *verifier* | `test_relayctl.py` — pinned to `<linux/input-event-codes.h>`, **not** to the report's byte order |
| certificate names | `src/tls_pem.h` (`sanEntries`, `kSanOidTlv`, `sanExtensionFullDer`) | the CN + SAN the certificate carries, **and the OID that labels the SAN**, and the same list rendered for the operator | `test_tls_pem.cpp` — the CN must be `kHostname` + `.local`, the 34 DER bytes were cross-checked against what OpenSSL writes for those three names, and so were the 43 bytes of the whole extension: the writer takes the OID as **DER content bytes** (`55 1D 11`), and handing it the dotted text produces a valid certificate with no SAN (README bug 11) |
| mbed TLS return codes + the refusal alert | `src/tls_session.h` (`kMbed*`, `mapIo`, `renderFatalAlert`) | "would block" vs "peer gone" vs "fatal" for every session call; the 7-byte RFC 5246 alert record a slot-cap refusal sends | `test_tls_session.cpp` walks the tables; `static_assert`s in `tls_session.cpp` weld the three copied error-code literals to the shipped library's macros — the code values differ between mbed TLS 2.x and 3.x, which is exactly how a confident transplant goes wrong |
| page build marker | `src/web_page.h` (`#if KR_TLS` between the raw-string chunks) | which build served the page — the TLS page refuses an `http:` origin outright; only the rollback page may speak plaintext | `test_page.js`, `test_relayctl.py` and `validate.sh` each assemble the served bytes from the chunks, and the two marker branches are asserted **equal length**, so one page byte count covers both builds |

The evdev row exists because it was guessed from the HID report's field order and was wrong
(`REL_WHEEL` is 8, not 2): a working wheel looked dead and the summary said no scroll
happened. Codes the verifier has no name for are now counted and printed rather than
dropped — the same fix as giving F13–F24 names. **When you add a table that decodes
something somebody else defined, test the table, not your confidence in it.**

Chord buttons carry their **wire bytes** in `data-chord`, not a name — which is why
`tests/test_proto.cpp` asserts every labelled chord against the combination its label promises.
A mislabelled button parses cleanly, is correctly gated, arrives intact and is the wrong key;
Alt+Tab was `05:04` (Ctrl+Alt+A) until that test existed. `TEXT_FRAME_MAX` / `TEXT_JOB_MAX` in
the page and `TEXT_FRAME_MAX` / `MAX_PASTE_CHARS` in `relayctl.py` are hand-copied from
`config.h` and are asserted equal in tests — change one, the suite fails, that is the point.

## Build, test, flash

```bash
tests/run_tests.sh                    # 4826 checks, no hardware
SANITIZE=1 tests/run_tests.sh         # + ASan/UBSan — use it for anything touching buffers
# PlatformIO. The venv below runs Python 3.15, which esptool REFUSES ("Python version must
# be 3.10..3.14") -- builds fine, dies at upload. Use uv's 3.14 for anything with -t upload:
uv run --python 3.14 --with platformio python -m platformio run -e bridge -e console -e rollback
#   `rollback` is [env:bridge] with -DKR_TLS=0: the plaintext way back. The gate builds it on
#   every run, because a way back that stopped compiling is not a way back.
python3 tools/to_bootloader.py        # 1200-baud touch → ROM download mode
uv run --python 3.14 --with platformio python -m platformio run -e bridge -t upload
```

Environment facts that will otherwise cost you a cycle:

- **`pio` on PATH is broken** here (`ModuleNotFoundError: platformio`), and
  `/tmp/piovenv` is Python 3.15 so it can *build* but not *upload*. Use the uv invocation
  above for both. If it is unavailable, find another interpreter with the package:
  `find / -maxdepth 6 -name platformio -type d -path '*site-packages*'`.
- **`pio run -t upload` reports SUCCESS on a stale binary if the build before it failed.**
  This happened: a diagnostic build failed to compile, the upload line right after it said
  SUCCESS, and the board was running the previous firmware. Never chain build and upload
  with `;` — check the build's own exit status first, or the verification you run next is
  verifying something that is not in the tree.
- **No `g++` on the host.** `run_tests.sh` falls back to a local podman image listed in its
  `images[]`. The page suite deliberately runs on the *host* so it works without node in there.
- **Build `console` too.** `ARDUINO_USB_MODE=1` compiles `KR_HAVE_HID == 0`, which is the only
  thing that checks the no-HID `#else` branches.
- **The page is served from flash.** A change to `src/web_page.h` is invisible on the network
  until the board is reflashed — and a tab that is already open keeps the old page in memory
  until it is reloaded. Confirm with `curl -sk https://keyrelay.local/ | wc -c` against the
  `page bytes:` line the gate prints (the raw-string chunks plus the 19-byte build marker;
  equal-length branches make it one number for both builds), and grep the served bytes for
  the element you added. Plain `http://` returns a 301 and no page bytes at all (task 04), so a `curl -s
  http://…` that prints a redirect body means the transport is working, not that the page is
  stale — fetch over `https://`, and `curl -s -L` is not a substitute: it follows to the same
  bytes whether or not the flash changed.
- **This directory is a git repository** with a short history that predates most of the
  current tree. Use it: `git diff` before and after any sweeping edit, and remember that
  `git checkout <file>` reverts far more than you asked for — restore a *region*, not a
  file.
- **Never do a multi-line `.*?` regex rewrite across a source file.** One did
  `#if KR_DEBUG_LINK.*?#endif` with `DOTALL` to remove a diagnostic block and deleted ~500
  lines of `main.cpp` — `onMessage()`, `setup()`, the console, the USB handlers — because
  the earliest `#if` in the file was elsewhere. The compiler was the only thing that
  noticed. Remove instrumentation with exact-string edits, or per-function slices, and
  compile immediately after. When it happens anyway: the last good `firmware.elf` still
  has the `.rodata` strings (console text, `printf` formats) and the symbol table, which is
  enough to rebuild faithfully against the surviving headers.

## Verification standard

A feature that lives on the device is not done when the source builds — it is done when the
device is running it and something observed it. Concretely:

- Protocol/page claims → the host suites, plus `curl` against the served page.
- Keystroke delivery → `tools/watch_target.py` on the target (evdev, focus-independent); it is
  the only thing that says what the target actually received, including *which* keys a chord
  really was.
- Text runs → check the `typed:` summary and press count, not the scrolling stream.

When you write a check, name the mechanism it exercises. "Silence disarms" sat in the README's
safety list while nothing compared `lastInboundMs()` to `kNoClientTimeoutMs`; the documented
verification passed because killing the controller process closes the socket, and the
socket-close path disarms. The check proved a different property from the one it was named after
and nobody noticed, until a run that keeps typing on its own made the difference matter. It is
wired now — but ask, of every green check, *what would still pass if this were broken?*

## Editing rules

- **Pure modules** (`keystate.h`, `mouse.h`, `output.h`, `proto.h`, `status.h`, `text.h`,
  `ws_codec.h`, `sha1.h`, `tls_pem.h`) include no Arduino or ESP-IDF headers. That is what lets the
  identical code run on a host under sanitisers. Keep it — and keep the *crypto library*
  out too: `tls_pem.h` handles the identity's strings, `tls.h`/`tls.cpp` handle the
  library, and the host suite includes the first and never the second. `tls_session.h`
  follows the same split inside one file: its policy half (deadline, admission,
  library-return table, alert record, queues) is pure and host-tested, and its
  `kr::tls::Session` class hides behind `KR_TLS && ESP_PLATFORM` — with the mbed TLS
  includes **outside** the namespace, because inside it they declare `::FILE` as
  `kr::tls::FILE` and every later stdio user in the translation unit dies. `conn.h` splits the
  same way and for the same reason: `plaintextRoute()`, the HTTP response head, the redirect
  builder and the page send cursor are pure and walked by `test_conn.cpp`; only `kr::Conn`
  itself needs `WiFiClient`.
- **`web_page.h`** is C++ raw string (plural since task 05: chunks joined by one
  `#if KR_TLS` build marker of equal-length branches): no compiler looks at the HTML or JS.
  Run `tests/test_page.js` after touching it (it executes the assembled page against a DOM
  shim and checks that every element id the script reaches for exists in the markup).
- **A protocol change is four edits, minimum**: `proto.h` (parser + `format*` + its header
  comment), `web_page.h`, `relayctl.py`, and the README Wire protocol table + error-code list.
  Tests that pin the wire format are `test_proto.cpp` and `test_relayctl.py`.
- **Docs move with the code.** Update in the same change: README intro bullets and check count,
  Safety model, Security model trust-boundary table, Controller page, Wire protocol,
  Architecture file map, Tests table, Backlog, and the docstring at the top of whichever module
  you changed. This README documents *why* decisions were made — keep the reasoning, don't just
  restate the behaviour. `.agents/validate.sh` checks the mechanical half so nobody has to
  remember it: the check count and the page byte count the docs quote must equal what the run
  measured, the Backlog must be numbered 1..N with nothing closed left inside it, and every line
  that mentions a plaintext `ws://` session must name the rollback build on the same line (wrap
  the sentence so it does — the point is that no sentence can read as if plaintext were default).
- Secrets: never in a file, a URL, a log or a shell history. The serial `token` command does not
  echo it; keep it that way. `/status` takes the token as a query parameter because `curl` is
  the point of that endpoint — do not add the token to any other URL. The TLS private key is
  the same rule with more teeth: it is generated on the board and lives only in NVS, so
  nothing may print it, hash-and-print it in a way that reads as the key, or be added to the
  tree as a `.pem`. Fingerprint yes, key never.
- `TODO.md` holds **open items only**; clear it when they are done. The durable record is the
  README: a fixed bug belongs in "Eleven bugs this verification caught" with its root cause and
  how it was proven, a new capability in the section that describes it, and a rule worth
  repeating here. A TODO that has accumulated resolved items tells the next reader nothing about
  what is left to do.

## Current defaults worth knowing

Default passphrase `changeme` (the page and `status` shout about it via `dflt:1`); AP
`keyrelay-ctl` / `keyrelay-ctl-pass`; **`https://keyrelay.local`** or `https://192.168.4.1`
(port 443 — `:80` answers a 301 to it, a 426 to a plaintext `/ws` upgrade or to any request
carrying `?token=`, and nothing else; plaintext only in the `-D KR_TLS=0` rollback, which is
`pio run -e rollback`); the TLS identity is an ECDSA secp256r1 keypair and self-signed
certificate generated on the board's first boot and stored in NVS (`tlscrt`, `tlskey`,
`tlscrtfp`) so it survives reflashing — rotated only by `nvs erase`, printed once at that first
boot and again on demand by the serial `cert` command, which prints the SHA-256 fingerprint, the
validity window and the names, and has no argument form that yields the key; at most 2 live TLS
sessions (`kTlsMaxSessions`, RAM-sized, a third is refused with an alert) and a 10 s handshake
deadline (`kTlsHandshakeMs`); paste pace
12 ms/character, 120 characters per frame, 2048 per run; silence disarm 8 s; NVS holds the
station credentials and passphrase and survives reflashing.