Heartwood ESP32

A hardware signing device for Nostr on supported ESP32 boards: Heltec WiFi LoRa 32 V3/V4, LilyGO/TENSTAR T-Display, and Waveshare ESP32-C6-LCD-1.47. Pick the target with the board-aware build script. Heartwood holds multi-master nsec material and signs on request; master private keys never leave the chip. Signing is policy-gated: requests outside automatic authority need the physical button and are shown on the display, while an authenticated operator can install an exact per-client method/event-kind policy for unattended signing.

For the full architecture walkthrough with sequence diagrams and trust-boundary analysis, see docs/architecture.md.

The shared common crate (derivation, NIP-44/46, policy — no_std, host-tested) also powers heartwood-ledger, the same signer as a Ledger embedded app: same seed phrase, same npub, same personas, with curve operations on the secure element via the ledger-backend feature.

flowchart LR
    Bark["Bark<br/>(NIP-07 browser ext)"]
    Relays[("Nostr relays")]
    Bridge["heartwood-bridge<br/>on Pi<br/>(pure transport,<br/>holds no signing keys)"]
    HSM["Heartwood HSM<br/>(master nsecs,<br/>button / exact-policy signing)"]

    Bark <-->|"NIP-44<br/>ciphertext"| Relays
    Relays <-->|"wss"| Bridge
    Bridge <-->|"USB<br/>serial"| HSM

    style HSM fill:#1a1a2e,stroke:#16a34a,stroke-width:3px,color:#e8f4f8
    style Bridge fill:#0f1419,stroke:#3b82f6,stroke-width:2px,color:#e8f4f8

The device is always the component that produces the signature. Authority comes from either a physical button approval or a previously installed slot policy. New remotely managed v2 slots are exact and fail closed: methods and event kinds outside their ceiling are denied, while matching requests can run unattended only when auto_approve=true. The Pi-side bridge holds no master key and cannot expand that authority. The separate Sapwood operator key is more powerful: it can create/revoke clients and install a signing policy, so compromise of that key must be treated as compromise of the configured remote-management authority.

Deployment modes, selected at runtime from the NVS network config:

Home HSM — USB-attached to a Raspberry Pi (shipped)

Holds the master secrets (up to 8 masters across bunker / tree-mnemonic / tree-nsec modes). All radios are disabled in this mode. The Pi running heartwood-bridge handles networking (Nostr relays, NIP-46 transport). The ESP32 handles all cryptography — decryption, signing, envelope construction. A compromised Pi cannot extract a master key or broaden a slot policy; an already-authorised client can still receive whatever signatures its current policy permits. Management UI via Sapwood is served by the bridge.

WiFi-standalone — on-chip relay client, no Pi (shipped, opt-in)

The ESP32 joins WiFi and connects to Nostr relays directly, running the full NIP-46 signing loop on-chip (firmware/src/relay.rs) — no Raspberry Pi. Keys still never leave the chip and NIP-44 is still decrypted on-device. Exact v2 client policies can permit unattended signing for a bounded method/event-kind set; other requests are denied or require the OLED/button according to their legacy policy. An unbound relay peer cannot enter the 30-second button loop: remote physical approval is available only after the client has been provisioned and slot-bound, so strangers cannot keep a shelf signer busy with prompts. Enabled only when the device is provisioned with an SSID + relay list (mode="wifi" in the NVS net config); the USB cable stays fully usable in parallel, so a bad SSID or relay is recoverable over the cable. Relay-side device management (kind 24134) is authenticated to a provisioned operator pubkey, uses a durable one-time mutation challenge, and can manage clients and staged WiFi changes remotely. USB can read password-redacted network/operator state, patch network fields with keep/set/clear password semantics, and replace the operator only through a separate stale-revision-checked physical confirmation. Seed replacement, PIN changes, factory reset, and OTA remain local/USB operations.

This is the convenience tier — it accepts a larger attack surface (a live TCP/IP stack on a key-holding device) in exchange for dropping the Pi. The USB-attached mode above remains the high-assurance default; leave the radios off where that matters.

Portable signer — battery-powered, BLE to phone (roadmap)

Holds a child key derived by the home HSM (purpose="device/mobile"). Only BLE enabled — short range, requires physical proximity. If lost or compromised, burn that branch on the HSM and derive a new one at the next index. The master secret and all other branches are untouched.

Key hierarchy

Master secret (home HSM)
├── persona/social       — public Nostr identity
├── persona/forgesworn   — project identity
├── client/bray          — NIP-46 client key
├── device/mobile-0      — portable signer #0 ← child key lives here
├── device/mobile-1      — replacement if #0 is compromised
└── ...

The nsec-tree hierarchy means each device gets its own branch. Compromise of a child never threatens the root or siblings.

Hardware

Component	Detail
Boards	Heltec WiFi LoRa 32 V3/V4; LilyGO/TENSTAR T-Display; Waveshare ESP32-C6-LCD-1.47
Chip	ESP32-S3 (Heltec), classic ESP32 (T-Display), or ESP32-C6 (Waveshare)
Display	128x64 SSD1306 OLED (Heltec), ST7789 TFT (T-Display), or JD9853 TFT (Waveshare C6)
GNSS	L76K on V4 only (available for portable mode)
LoRa	SX1262 (never initialised -- no use case for signing)
WiFi	Built in (off in USB-bridged mode; enabled in WiFi-standalone mode)
BLE	Built in (reserved for future portable mode; not built)
USB-C (V4)	Wired direct to native USB-Serial-JTAG on GPIO19/20
USB-C (V3)	Wired through CP2102 bridge to UART0 on GPIO43/44
Battery	JST PH 2.0 connector + charging circuit (portable mode)
Buttons	Board-specific local confirmation controls; T-Display provides two buttons

Setup

Install the ESP Rust toolchain:

cargo install espup ldproxy espflash
espup install
source ~/export-esp.sh

Build

Three independent crates -- build each from its own directory:

cd common && cargo test                    # shared crypto tests
cd provision && cargo build                # host CLI tool
cd firmware && cargo build                 # ESP32 firmware (V4 default)

The firmware crate selects its board at compile time. The default cargo feature is heltec-v4, but production builds should use the wrapper so the cargo feature, target triple, MCU, and sdkconfig fragment cannot drift apart:

./scripts/build-firmware.sh v3 --release
./scripts/build-firmware.sh v4 --release
./scripts/build-firmware.sh tdisplay --release
./scripts/build-firmware.sh c6 --release

Or the manual form:

cd firmware
# V4 (default)
cargo build --release
# V3
ESP_IDF_SDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.defaults.heltec-v3" \
    cargo build --release --no-default-features --features heltec-v3

The wrapper script also writes a board-tagged ELF copy to target/heartwood-<board>.elf so you cannot accidentally flash the wrong binary onto the wrong hardware.

Flash & provision

Direct flash (USB cable to build machine)

# Heltec V4 (default, native USB CDC, enumerates as /dev/ttyACM*)
./scripts/build-firmware.sh v4 --release
espflash flash firmware/target/xtensa-esp32s3-espidf/release/heartwood-esp32

# Heltec V3 (CP2102 bridge, typically enumerates as /dev/ttyUSB* or /dev/cu.usbserial-*)
./scripts/build-firmware.sh v3 --release
espflash flash firmware/target/xtensa-esp32s3-espidf/release/heartwood-esp32

Always use --release -- debug builds (~2.2MB) exceed the 1.5MB OTA partition.

Remote flash via esptool (ESP32 attached to a Pi)

Build the app binary locally, transfer it, and flash with esptool on the Pi.

Serial port naming depends on the board. V4 enumerates as USB CDC (/dev/ttyACM0 on Linux) because it uses the ESP32-S3's native USB. V3 goes through a CP2102 UART bridge and enumerates as /dev/ttyUSB0. Substitute the right path in the commands below.

# Build and convert to app binary (0xE9 format, not raw ELF)
./scripts/build-firmware.sh v4 --release      # or: v3
espflash save-image --chip esp32s3 firmware/target/xtensa-esp32s3-espidf/release/heartwood-esp32 /tmp/heartwood-esp32.bin

# Transfer to the Pi
scp /tmp/heartwood-esp32.bin pi-host:/tmp/

# On the Pi: stop any process holding the serial port, then flash.
# PORT=/dev/ttyACM0 for V4, PORT=/dev/ttyUSB0 for V3.
PORT=/dev/ttyACM0
sudo systemctl stop <heartwood-services>
sudo fuser -k "$PORT"
python3 -m esptool --chip esp32s3 --port "$PORT" --before default-reset \
  write-flash 0x10000 /tmp/heartwood-esp32.bin

# Erase otadata to force boot from ota_0
python3 -m esptool --chip esp32s3 --port "$PORT" --before default-reset \
  erase-region 0xd000 0x2000

Important: if any daemon holds the serial port, the flash will fail mid-write. Use systemctl mask (not just stop) if services auto-restart, and stop ModemManager which probes new serial devices on USB re-enumeration.

OTA update (over serial, device already running)

# Stop the daemon holding the serial port
sudo systemctl stop <heartwood-service>

# Flash via OTA (requires button approval on the device -- hold 2s).
# PORT=/dev/ttyACM0 for V4, PORT=/dev/ttyUSB0 for V3.
heartwood-ota --port /dev/ttyACM0 --firmware /tmp/heartwood-esp32.bin

# Restart the daemon
sudo systemctl start <heartwood-service>

The OTA tool is built from the ota/ crate. The firmware binary must be an app binary (use espflash save-image), not the raw ELF from cargo build.

Provision

After first flash, wait for OLED to show "Awaiting secret...", then:

cd provision && cargo run -- --port /dev/cu.usbserial-*

Enter mnemonic and passphrase when prompted. After ACK, the device reboots with the stored identity.

To restore an existing key rather than a phrase, pick a mode: --mode bunker (the device signs as that exact key, same npub) or --mode tree-nsec (a fresh tree root is derived from it, new npub). Both prompt for the key and accept either an nsec1... or the 24-word key backup Sapwood writes out at import -- the key's own bytes as BIP-39 entropy, so the identical npub comes back. A 12-word phrase is a seed, not a key, and belongs in the default tree-mnemonic mode. Format and instructions: sapwood/docs/key-backup.md.

Subsequent boots display the master npub immediately (no provisioning needed).

Test vector

Parameter	Value
Root secret	`[0x01, 0x02, ..., 0x20]` (32 bytes, sequential)
Root path	Raw secret → SigningKey (no HMAC intermediate)
Purpose	`persona/test`
Index	`0`
Expected child npub	`npub1rx8u4wk9ytu8aak4f9wcaqdgk0lj4rjhdu4j9n7dj2mg68l9cdqs2fjf2t`

This must match heartwood-core's output for the same inputs. The nsec-tree derivation is:

context = b"nsec-tree\0" || b"persona/test" || 0x00 || 0x00000000
child_secret = HMAC-SHA256(key=root_secret, msg=context)
child_pubkey = SigningKey(child_secret).verifying_key()
npub = bech32_encode("npub", child_pubkey)

GPIO safety

Verified against Heltec factory test code, Meshtastic firmware, and ESPHome configs:

GPIO 17, 18, 21 -- OLED I2C, shared wiring on both V3 and V4.
GPIO 19, 20 -- native USB D-/D+ on V4. Not wired to USB-C on V3, so UsbSerialDriver will never enumerate on V3; the V3 code path uses UART0 instead.
GPIO 43, 44 -- UART0 TX/RX on both boards, but only V3 has them wired through to the USB-C port via the CP2102 bridge.
PSRAM pins on ESP32-S3R2 (V4 only) are GPIO 26-32 (quad-SPI). Nowhere near our I2C pins. The V3 has no PSRAM.
GPIO 33-37 are free on both boards (only reserved on octal PSRAM variants like S3R8).

Structure

common/                  Shared crypto, frame protocol, NIP-46/44/04 types
  src/
    lib.rs, derive.rs, encoding.rs, types.rs, hex.rs, frame.rs
    nip46.rs            NIP-46 event types, canonical serialisation, event id
    nip44.rs            NIP-44 v2 (XChaCha20 + HMAC-SHA256, conversation key)
    nip04.rs            NIP-04 legacy (AES-256-CBC)
firmware/                ESP32 firmware (NIP-46 signing bunker)
  src/
    main.rs             Boot flow → PIN unlock → frame dispatch loop
    sign.rs             BIP-340 Schnorr signing (secp256k1 C FFI)
    nvs.rs              NVS read/write for masters, bridge secret, policies, PIN
    provision.rs        Multi-master provisioning (add/remove/list)
    transport.rs        0x10 encrypted requests + 0x34 SIGN_ENVELOPE handler
    session.rs          Bridge session auth (0x21/0x22) + SET_BRIDGE_SECRET (0x23)
    policy.rs           Client approval policies, two-tier TOFU
    ota.rs              Serial OTA (0x30-0x33) with SHA-256 + auto-rollback
    pin.rs              Boot PIN, NVS-persisted failed-attempt counter
    nip46_handler.rs    NIP-46 dispatch (sign_event, connect, NIP-44/04, heartwood_*)
    identity_cache.rs   Derived persona cache for tree-mode masters
    approval.rs         Button approval loop with OLED countdown
    oled.rs             SSD1306 display helpers, boot animation
    masters.rs          LoadedMaster lookup, npub encoding
  build.rs, sdkconfig.defaults, rust-toolchain.toml, .cargo/config.toml
provision/               Host CLI tool
  src/
    main.rs             Mnemonic / nsec / hex → serial push to device
sign-test/               Signing test harness
  src/
    main.rs             Send NIP-46 requests over serial, display responses
heartwoodd/              Pi-side daemon (formerly bridge/)
  src/
    main.rs             Nostr relay ↔ NIP-44 transport ↔ serial ↔ ESP32
                        Master-routed, calls SIGN_ENVELOPE for outer events
    api.rs              Management HTTP API on :3100, bearer-token auth
    backend/            Soft mode (sealed keyfile) and hard mode (serial) backends
ota/                     Pi-side serial OTA tool
  src/
    main.rs             Chunked firmware upload with SHA-256 verification
scripts/
  setup-hsm.py          Interactive provisioning and bridge-start helper
  git-hooks/            Pre-commit secret scanner (64-hex + nsec1 detection)
  install-hooks.sh      Installer for the git hooks
docs/
  architecture.md       System architecture with mermaid diagrams
  plans/                Design notes, including the zero-trust bridge refactor plan
  specs/                Protocol specs

Roadmap

Phase 1 — Prove the crypto (current spike)

nsec-tree HMAC-SHA256 derivation on ESP32-S3
bech32 npub encoding
Runtime assertion against heartwood-core test vectors
Display npub on OLED
Sign a dummy 32-byte hash and display the signature

Phase 2 — Provisioning

CLI tool to derive 32-byte root secret from mnemonic + passphrase (offline PC)
NVS storage for root secret (plaintext — encryption deferred, see excluded)
First-boot provisioning mode: accept root secret over USB serial
Subsequent boots read from NVS, skip provisioning
Show master npub on OLED after boot

Phase 3 — USB signing oracle

NIP-46 JSON-RPC over serial (ESP32 is the bunker, Pi is a transport bridge)
Unified frame protocol: [magic][type][length][payload][crc32]
OLED shows what you're signing (identity, event kind, content preview, countdown)
Physical button: long hold (>=2s) to approve, short press to deny, 30s timeout
Per-request child key derivation with Heartwood extension field
Test harness CLI (sign-test/) for end-to-end validation
Flash and verify end-to-end signing flow on hardware (2026-04-03)
Pi-side relay bridge (bridge/) — NIP-46 over Nostr relays ←→ NIP-44 ←→ serial ←→ ESP32

Phase 4 — Full NIP-46 HSM (shipped)

Phase 5 — Hardening (shipped)

Phase 6 — Zero-trust bridge (shipped 2026-04-05)

SIGN_ENVELOPE frame (0x34) — HSM builds and signs NIP-46 kind:24133 envelope events on-device
Bridge queries device for master list at startup via PROVISION_LIST
Bridge routes NIP-46 traffic to the real master pubkey (no Pi-side bunker identity masquerade)
Pi-side bunker_keys demoted to ephemeral relay-layer transport identity with no signing authority
EventBuilder::sign_with_keys replaced with device round-trip in the response path
1.5 MB OTA partition slots (up from 896 KB) — accommodates current firmware size with 50% headroom
Dedicated on-device transport key distinct from user masters
NIP-46 transport architecture spec contribution

Phase 7 — Portable signer

Cargo feature flags: hsm (default, USB-only) vs portable (BLE, battery)
HSM provisions a child key onto the portable device (device/mobile-N)
BLE GATT service: NIP-46 request/response profile
Phone pairs to device over BLE
OLED shows signing request details, button to approve/deny
Battery management: deep sleep between requests, wake on BLE connect
Child key revocation: HSM increments index, re-provisions replacement device

Phase 8 — Portable extras (stretch)

GPS location stamp on signed events (opt-in, portable mode only)
QR code display of npub on OLED
Multi-identity: carry several child keys, select on OLED before signing

Deliberately excluded

WiFi signing as the default — the high-assurance default keeps all radios off and networks via the USB-attached Pi; a live TCP/IP stack is a real liability on a key-holding device. WiFi is off unless you explicitly opt into WiFi-standalone mode (see above), which trades that surface for dropping the Pi.
Master secret on portable device — only child keys leave the home HSM. If the portable device is lost, the damage is one branch.
LoRa signing — signing is a response to a request, and the requester needs internet anyway. LoRa solves a problem that doesn't exist for this use case. The SX1262 is never initialised (safe without antenna).
Flash encryption / eFuse burning — permanently locks the chip to one firmware, prevents reuse (e.g. Meshtastic), and risks bricking if anything goes wrong. Physical security is the protection model instead. May revisit on a dedicated production unit.

Part of the ForgeSworn Toolkit

ForgeSworn builds open-source cryptographic identity, payments, and coordination tools for Nostr.

Library	What it does
nsec-tree	Deterministic sub-identity derivation
ring-sig	SAG/LSAG ring signatures on secp256k1
range-proof	Pedersen commitment range proofs
canary-kit	Coercion-resistant spoken verification
spoken-token	Human-speakable verification tokens
toll-booth	L402 payment middleware
geohash-kit	Geohash toolkit with polygon coverage
nostr-attestations	NIP-VA verifiable attestations
dominion	Epoch-based encrypted access control
nostr-veil	Privacy-preserving Web of Trust

Name		Name	Last commit message	Last commit date
Latest commit History 438 Commits
.github		.github
common		common
docs		docs
esp8266-firmware		esp8266-firmware
esp8266-spike		esp8266-spike
firmware		firmware
heartwoodd		heartwoodd
ota-sign		ota-sign
ota		ota
provision		provision
scripts		scripts
sign-test		sign-test
site		site
sprite-gen		sprite-gen
ui-preview		ui-preview
.gitignore		.gitignore
CLAUDE.md		CLAUDE.md
CONTRIBUTING.md		CONTRIBUTING.md
Cross.toml		Cross.toml
LICENCE		LICENCE
README.md		README.md
SECURITY.md		SECURITY.md
deny.toml		deny.toml
llms.txt		llms.txt
this-is-my-goal.md		this-is-my-goal.md

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Heartwood ESP32

Home HSM — USB-attached to a Raspberry Pi (shipped)

WiFi-standalone — on-chip relay client, no Pi (shipped, opt-in)

Portable signer — battery-powered, BLE to phone (roadmap)

Key hierarchy

Hardware

Setup

Build

Flash & provision

Direct flash (USB cable to build machine)

Remote flash via esptool (ESP32 attached to a Pi)

OTA update (over serial, device already running)

Provision

Test vector

GPIO safety

Structure

Roadmap

Phase 1 — Prove the crypto (current spike)

Phase 2 — Provisioning

Phase 3 — USB signing oracle

Phase 4 — Full NIP-46 HSM (shipped)

Phase 5 — Hardening (shipped)

Phase 6 — Zero-trust bridge (shipped 2026-04-05)

Phase 7 — Portable signer

Phase 8 — Portable extras (stretch)

Deliberately excluded

Part of the ForgeSworn Toolkit

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