n_signer BLE Wearable Signer
Status: Concept — brainstorming. No plan yet.
A small, battery-powered wearable hardware signer that communicates with a host over Bluetooth Low Energy (BLE). The host sends JSON-RPC requests over a BLE GATT characteristic; the signer shows an approval prompt on a tiny display; the user taps a button to approve; the signed response goes back over BLE.
Concept
flowchart LR
Host[Host: phone/laptop<br/>n_signer client] -->|BLE GATT| Signer[Wearable signer<br/>nRF52840 + OLED]
Signer -->|approve/deny button| User[User]
Signer -->|BLE GATT response| Host
The signer speaks the same algorithm-based API as the host and the CYD/Teensy
firmware (README.md §4). The auth envelope (kind 27235)
protects the BLE wire — even if BLE is sniffed, an attacker can't forge
requests without the caller's secp256k1 private key.
Why BLE
- Wearable form factor — always with you (wristband, pendant, card)
- No physical connection — no USB cable, no host-side driver, no dongle
- Universal host support — phones, laptops, tablets all have BT
- Low power — nRF52840 draws ~5 mA active, ~1 µA sleep
Hardware (preliminary)
| Component | Candidate | Notes |
|---|---|---|
| MCU | nRF52840 (Nordic) | Cortex-M4 @ 64 MHz, 1 MB flash, 256 KB RAM, BT 5.0, hardware AES/ECC, USB device, NFC-A. ~$5-8. |
| Display | 0.96" or 1.3" SSD1306 OLED (I2C) or 1.02" e-paper | Small is fine — only shows "approve kind 1 from ?" |
| Input | 2-3 tactile buttons (approve/deny/back) | No touch at this size |
| Power | 200 mAh coin cell or small LiPo | Weeks of battery life |
| Mnemonic entry | Buttons (scroll words), NFC from phone, or generate-on-device | The hard UX problem |
Security considerations
- BT stack attack surface: BLE has a large stack (pairing, GATT, L2CAP, SMP). A stack bug could allow code execution. Mitigations: use Nordic's audited SoftDevice, disable unnecessary services, require LE Secure Connections pairing.
- Radio range (~10 m): an attacker in the same room could potentially interact with the signer. The auth envelope + approval prompt protect against this, but the radio is omnidirectional.
- Pairing UX: BT pairing can be frustrating. LE Secure Connections (Numeric Comparison) is the most secure and user-friendly pairing method.
Open questions
- Mnemonic entry on a tiny screen: scroll through 2048 BIP-39 words with up/down buttons (like Coldcard)? Load via NFC from a phone? Generate on-device and display for the user to write down?
- PQ crypto on nRF52840: 256 KB RAM is enough for ML-DSA-65 (~6 KB heap) but SLH-DSA-128s is heavy. May need to limit the PQ algorithm set or stream the keygen.
- Display choice: OLED (fast refresh, high power) vs e-paper (slow refresh, zero power when static, persistent display).
- Form factor: wristband? pendant? card? What's the target use case — daily signing, emergency key access, or a backup signer?
Comparison to the IR air-gap signer
| BLE wearable | IR air-gap | |
|---|---|---|
| Air-gap | Medium (radio, ~10 m, omnidirectional) | High (light, line-of-sight, ~1 m) |
| Attack surface | Large (BT stack) | Small (no BT, dumb dongle) |
| Host compatibility | Universal (phones, laptops) | Requires USB dongle |
| Form factor | Wearable | Handheld (point at dongle) |
| Throughput | ~250 KB/s (BLE 5) | ~11 KB/s (raw IR) or ~400 KB/s (IrDA) |
| Novelty | Conventional | Novel (no hardware wallet uses IR) |
Next steps
- Decide on the MCU (nRF52840 vs RP2040+BT-module)
- Decide on mnemonic entry method
- Decide on display (OLED vs e-paper)
- Write a port plan (similar to
plans/teensy41_signer_port.md)