BitChat
Open-source decentralized chat app (permissionlesstech/bitchat) built by Jack Dorsey's team. Dual-transport: Bluetooth LE mesh for offline/local messaging (multi-hop, up to 7 hops, adaptive power), falling back to Nostr relays (unresolved) for internet reach. No accounts, phone numbers, or central servers. Public domain (Unlicense).
Crypto: Noise Protocol for mesh sessions (forward secrecy on live sessions); a custom XChaCha20-Poly1305 "private envelope" scheme for Nostr messages — explicitly not compatible with standard Nostr encryption NIPs (17/44/59), i.e. bespoke crypto rather than an adopted, community-reviewed standard.
Caveats:
- No formal third-party security audit referenced in current docs.
- Mesh peers use a persistent per-device identifier derived from the identity key, observable to nearby Bluetooth devices — a metadata-leakage tradeoff inherent to the "no accounts" model.
- Repo has drawn takedown demands; README advises verifying builds against per-release hash manifests.
Verdict: technically ambitious (real BLE mesh routing, dual-transport fallback, geohash location channels), actively developed (~31.6k stars), but crypto is unaudited and non-standard — fine to experiment with, not yet something to trust for confidentiality-sensitive use.