HERA

HERA turns a Raspberry Pi into a Bluetooth hands-free unit for a phone and relays the phone's calls, audio and call control, to a client application over an IP link. A typical client is an application running on the same phone, for example a voice assistant that answers calls.

 phone ──HFP/SCO──► device (Raspberry Pi) ══ TCP control + UDP audio ══► client
                          └──── BLE: discovery and provisioning ────────►

• Audio is PCM16 or G.711; no codec runs on the device.
• All IP traffic is encrypted and authenticated (X25519, HKDF-SHA256, ChaCha20-Poly1305).
• The device runs its own WiFi access point by default; the phone keeps cellular data as its default route.
• The audio block duration is negotiated among 20, 10 and 5 ms; with 5 ms blocks the transport latency is about 50 ms.

Repository layout

path	contents
PROTOCOL.md	protocol specification, version 4 (normative)
device/	device implementation in C for Raspberry Pi OS (BlueZ, hostapd, dnsmasq, libsodium)
client/hera_client/	Flutter client library
client/hera_audio/	pure Dart audio primitives used by the client (PCM16, G.711, resampling)

Getting started

Device, on Raspberry Pi OS:

cd device
bash setup.sh            # dependencies, build, BlueZ configuration
sudo -E ./hera-device

See device/README.md for configuration, running as a service and bring-up with tools/hera_probe.

Client, in a Flutter application's pubspec.yaml:

dependencies:
  hera_client:
    git:
      url: https://github.com/<owner>/<repository>.git
      path: client/hera_client

See client/hera_client/README.md for usage.

Tests

(cd device && make test)                 # any host with libsodium
(cd device && make test-link)            # Linux only
(cd client/hera_audio && dart test)
(cd client/hera_client && flutter test)

Both implementations are tested against the byte vectors of PROTOCOL.md §14 and §14.1.