Skey: a FIDO2 security key on the Raspberry Pi Pico

Firmware for the Pico / Pico 2 that acts as a USB FIDO2 authenticator:
CTAP2 and U2F, passkeys, BOOTSEL as the touch button, and clientPIN with
PIN/UV auth protocols 1 and 2. A host simulator runs the same core so the
python-fido2 test suite exercises it without hardware.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

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+# Skey
+
+A FIDO2 / WebAuthn security key built on a Raspberry Pi Pico or Pico 2. It
+plugs in over USB and works with browsers for passkeys and 2FA, and with
+`ssh-keygen -t ecdsa-sk`. No drivers are needed.
+
+## What works
+
+- CTAP2: makeCredential, getAssertion, getNextAssertion, getInfo, reset, selection
+- U2F (CTAP1) register/authenticate, so older sites work too
+- ES256 (P-256) credentials with packed attestation
+- Up to 64 discoverable credentials (passkeys)
+- Touch to confirm, using the Pico's BOOTSEL button
+- PIN (clientPIN, PIN/UV auth protocols 1 and 2), for sites that require user
+  verification
+- Unlimited non-discoverable credentials. The credential ID carries everything
+  needed to re-derive the key, so nothing is stored for them.
+
+Not yet supported: hmac-secret, credential management.
+
+## Layout
+
+```
+src/        authenticator core: CTAPHID, CTAP1, CTAP2, CBOR, crypto, storage
+firmware/   Pico specifics: USB descriptors, flash, RNG, LED, BOOTSEL button
+host/       desktop simulator speaking CTAPHID over stdin/stdout
+tests/      python-fido2 tests run against the simulator
+tools/      attestation certificate generator
+```
+
+The core only talks to hardware through `src/platform.h`, so the simulator
+runs exactly the same protocol and storage code as the firmware. The
+simulator's flash file even follows NOR rules: erase whole sectors, and
+program whole pages that can only clear bits.
+
+## Setup
+
+```bash
+sudo apt install build-essential cmake ninja-build git gcc-arm-none-eabi \
+    libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib libmbedtls-dev \
+    python3-fido2 python3-pytest libusb-1.0-0-dev pkg-config
+
+git clone --depth 1 https://github.com/raspberrypi/pico-sdk deps/pico-sdk
+git -C deps/pico-sdk submodule update --init --depth 1 lib/tinyusb lib/mbedtls
+```
+
+Set `PICO_SDK_PATH` instead if you already have an SDK checkout.
+
+## Simulator and tests
+
+```bash
+cmake -B build-host -G Ninja -DSKEY_HOST=ON
+cmake --build build-host
+python3 -m pytest tests
+```
+
+The simulator approves every request unless `SKEY_BUTTON_FILE` is set. When it
+is set, creating that file counts as a button press. The tests in
+`tests/test_presence.py` use this.
+
+## Firmware
+
+```bash
+cmake -B build-pico -G Ninja -DPICO_BOARD=pico     # or pico2
+cmake --build build-pico
+```
+
+Hold BOOTSEL while plugging in the Pico, then copy `build-pico/skey.uf2` to the
+`RPI-RP2` (or `RP2350`) drive. The board restarts as "Skey FIDO2 Key".
+
+## Using the button
+
+The BOOTSEL button is the touch button. Press it briefly; holding it while
+plugging in still enters the bootloader as usual.
+
+| LED | Meaning |
+|---|---|
+| off | idle |
+| on | handling a request |
+| fast blink | waiting for a touch: press BOOTSEL |
+| slow blink | an older U2F site asked for a touch: press BOOTSEL |
+
+- A press only counts if it happens after the request arrives, and each press
+  approves one request.
+- An unanswered request times out after 30 seconds.
+- Closing the browser prompt cancels the wait.
+
+To try it out:
+
+- https://webauthn.io: register, then authenticate
+- `fido2-token -L` and `fido2-token -I <dev>` (package `fido2-tools`)
+- `ssh-keygen -t ecdsa-sk` for an SSH key backed by the device
+
+On Linux, systemd recognizes FIDO devices by their HID usage page, so no udev
+rules are needed. The exception is snap-packaged browsers, such as Ubuntu's
+Firefox. Snaps can only open keys on a fixed list of USB IDs, so without the
+rule below the browser waits forever and never sees the key:
+
+```bash
+sudo cp udev/69-skey-snap.rules /etc/udev/rules.d/
+sudo udevadm control --reload
+```
+
+Then unplug the key and plug it back in.
+
+## PIN
+
+The key works without a PIN. Once you set one:
+
+- every new registration asks for it;
+- logins ask for it when the site requires user verification;
+- the key returns account names, so the browser can show which passkey you are
+  picking.
+
+Set or change it in Chrome under Settings > Privacy and security > Security >
+Manage security keys. From a terminal, use `fido2-token` (package
+`fido2-tools`):
+
+```bash
+fido2-token -L                   # find the device, e.g. /dev/hidraw11
+fido2-token -S /dev/hidraw11     # set a PIN
+fido2-token -C /dev/hidraw11     # change it
+```
+
+A PIN must be at least 4 characters. 8 wrong guesses in a row lock the PIN
+for good, and only a reset (which also deletes every credential) clears it.
+After 3 wrong guesses, the key also needs unplugging before it accepts more.
+
+## Security notes
+
+- **Secrets live in flash.** The master secret sits in the last 64 KB of flash
+  in plain form. Anyone holding the key can read it via BOOTSEL. Pico 2 secure
+  boot plus OTP storage would fix that.
+- **Attestation key.** The first build creates one in `attestation/`
+  (gitignored). Every key you flash from that checkout shares it, just like a
+  vendor's batch attestation.
+- **Reset window.** `authenticatorReset` only works within 10 seconds of
+  plugging in.
+- **Randomness.** The Pico 2 has a hardware TRNG. The RP2040 relies on ROSC
+  jitter via `pico_rand`, whitened with SHA-256.
+
+## Roadmap
+
+1. Credential management (list and delete passkeys), then advertise FIDO_2_1
+2. hmac-secret / prf extension
+3. Pico 2: keep the master secret in OTP and enable secure boot
+4. Optional external button on a GPIO pin