v0.1.0 - CYD firmware v0.0.2: algorithm-based API upgrade (all verbs, all algorithms), vendored Keccak/SHAKE + PSA ed25519/x25519 for IDF v5.4, Web Serial test page, CYD docs, Teensy 4.1 port plan (1TB SDXC OTP pad), brainstorming READMEs for BLE/IR/NFC/FPGA signer concepts

firmware/ble_wearable_signer/README.md

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+# 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
+
+```mermaid
+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`](../../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 <caller>?" |
+| 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`](../../plans/teensy41_signer_port.md))