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Hidden n_signer in KB2040 Media Controller

Overview

The goal is to embed the n_signer functionality into the existing keyboard_modules KB2040 media controller project. The device will normally function as a media controller (volume knob, play/pause, next, prev), but a specific button combination will unlock the n_signer mode, allowing the user to enter a mnemonic and sign requests.

Feasibility: Single USB Connection

Yes, this is entirely possible over a single USB connection.

The RP2040 microcontroller supports USB Composite Devices. By using a USB stack like TinyUSB (which is well-supported on the RP2040 via the Pico SDK or Adafruit TinyUSB for Arduino), the device can expose multiple USB interfaces simultaneously:

  1. HID Interface: Acts as the standard media controller (Keyboard/Consumer Control).
  2. Vendor-Specific / WebUSB Interface (or CDC): Acts as the communication channel for n_signer requests.

The host OS will see a single physical USB device with multiple virtual devices attached to it. The media keys will work normally, and the n_signer client software can connect to the custom interface concurrently.

Implementation Plan

1. Firmware Environment & USB Stack

  • Current State: The keyboard_modules project is an Arduino sketch using the standard <Keyboard.h>.
  • Action: Migrate the USB handling to use Adafruit TinyUSB (if staying in Arduino) or port the project to the Raspberry Pi Pico SDK (C/C++). TinyUSB is required to easily set up a composite device with both HID and WebUSB/Custom endpoints.

2. Integrate n_signer Core

  • Action: Bring the core n_signer C files (crypto, mnemonic handling, transport framing) into the KB2040 project.
  • Consideration: Ensure the cryptographic libraries used by n_signer (e.g., secp256k1, hashing) compile and run efficiently on the RP2040 (ARM Cortex-M0+).

3. Secret Mode Activation (UI/UX)

  • Action: Implement a state machine in the firmware.
    • State 1: Media Controller (Default). Buttons and knob send HID events. Display shows volume.
    • State 2: n_signer Mode. Triggered by a specific chord (e.g., holding Play/Pause + Next for 3 seconds).
  • Action: In n_signer mode, repurpose the hardware:
    • Knob: Scroll through letters/words for mnemonic entry, or scroll through request details.
    • Buttons: Select, Cancel, Approve, Reject.
    • Display: Show the n_signer UI (mnemonic entry, approval prompts) instead of the volume bar.

4. Display Management

  • Action: Refactor display.cpp to support multiple "views" or "screens". It currently hardcodes the volume UI. It needs to be able to switch to rendering text menus for the signer.

5. Host-Side Client

  • Action: The n_signer client on the host will need to be configured to look for the specific USB VID/PID and interface number of the composite KB2040 device.

Next Steps

  1. Verify the preferred development environment for the KB2040 (Arduino with Adafruit TinyUSB vs. Pico SDK).
  2. Create a proof-of-concept composite USB device on the KB2040 that exposes both HID and a dummy WebUSB endpoint.