bitchat/README.md

bitchat

A decentralized peer-to-peer messaging app with dual transport architecture: local Bluetooth mesh networks for offline communication and internet-based Nostr protocol for global reach. No accounts, no phone numbers, no central servers. It's the side-groupchat.

bitchat.free

📲 App Store

License

This project is released into the public domain. See the LICENSE file for details.

Features

• Dual Transport Architecture: Bluetooth mesh for offline + Nostr protocol for internet-based messaging
• Location-Based Channels: Geographic chat rooms using geohash coordinates over global Nostr relays
• Intelligent Message Routing: Automatically chooses best transport (Bluetooth → Nostr fallback)
• Decentralized Mesh Network: Automatic peer discovery and multi-hop message relay over Bluetooth LE
• Privacy First: No accounts, no phone numbers, no persistent identifiers
• Private Message End-to-End Encryption: Noise Protocol for mesh, BitChat private envelopes for Nostr fallback
• IRC-Style Commands: Familiar /slap, /msg, /who style interface
• Universal App: Native support for iOS and macOS
• Emergency Wipe: Triple-tap to instantly clear all data
• Performance Optimizations: LZ4 message compression, adaptive battery modes, and optimized networking

Technical Architecture

BitChat uses a hybrid messaging architecture with two complementary transport layers:

Bluetooth Mesh Network (Offline)

• Local Communication: Direct peer-to-peer within Bluetooth range
• Multi-hop Relay: Messages route through nearby devices (max 7 hops)
• No Internet Required: Works completely offline in disaster scenarios
• Noise Protocol Encryption: End-to-end encryption with forward secrecy
• Binary Protocol: Compact packet format optimized for Bluetooth LE constraints
• Automatic Discovery: Peer discovery and connection management
• Adaptive Power: Battery-optimized duty cycling

Nostr Protocol (Internet)

• Global Reach: Connect with users worldwide via internet relays
• Location Channels: Geographic chat rooms using geohash coordinates
• 290+ Relay Network: Distributed across the globe for reliability
• BitChat Private Envelopes: App-specific encrypted private messages over Nostr relays
• Ephemeral Keys: Fresh cryptographic identity per geohash area

BitChat's private-envelope format is proprietary and is not NIP-17, NIP-44, or NIP-59 compatible. It uses Nostr as a relay transport but only interoperates with BitChat clients. New envelopes use provisional, BitChat-specific public kind 1402 (not a formally reserved Nostr kind), encrypted inner kinds 1403/1404, and the bitchat-pm-v1: content prefix. For mixed-version delivery, clients publish both the primary kind-1402 envelope and a compatibility kind-1059 copy. There is no date-based cutoff: kind 1059 must remain enabled until a coordinated iOS/Android release confirms that supported older clients have migrated. Receivers subscribe to both kinds and deduplicate the authenticated embedded BitChat payload.

Private-envelope migration compatibility:

Sender	Receiver	Delivery path
New iOS	New iOS	Kind 1402 is primary; the kind-1059 twin is deduplicated
New iOS	Released iOS	Compatibility kind 1059
New iOS	Current Android	Compatibility kind 1059
Released iOS	New iOS	Kind 1059 with the released empty inner-tag shape
Current Android	New iOS	Kind 1059 with exactly the authenticated recipient p tag

New kind-1402 envelopes require an empty inner tag list. The Android recipient tag exception is intentionally confined to legacy kind 1059 and accepts only the exact addressed recipient. Mailbox subscriptions cover the 24-hour delivery window plus Android's full 48-hour timestamp randomization and 15 minutes of clock skew. Recovery uses one independent 500-event relay filter per wire kind so either format cannot consume the other's result budget.

The two outbound migration copies are admitted to the relay queue as one protected batch. Queue pressure evicts ephemeral traffic first, never one copy of a private pair; if protected capacity is exhausted, the entire new pair is rejected as a whole. User-message rejection becomes a visible failed delivery; acknowledgements and favorite notifications retain the exact pair in a process-wide 256-entry bounded retry queue. A sustained outage beyond that bound evicts the oldest whole control pair with an explicit warning, never half a pair. If either socket write fails, the same queued pair remains pending and both copies are replayed on the replacement connection. A terminal relay target is pruned after bounded retries so one dead relay cannot wedge healthy delivery.

Channel Types

mesh #bluetooth

• Transport: Bluetooth Low Energy mesh network
• Scope: Local devices within multi-hop range
• Internet: Not required
• Use Case: Offline communication, protests, disasters, remote areas

Location Channels (block #dr5rsj7, neighborhood #dr5rs, country #dr)

• Transport: Nostr protocol over internet
• Scope: Geographic areas defined by geohash precision
  • block (7 chars): City block level
  • neighborhood (6 chars): District/neighborhood
  • city (5 chars): City level
  • province (4 chars): State/province
  • region (2 chars): Country/large region
• Internet: Required (connects to Nostr relays)
• Use Case: Location-based community chat, local events, regional discussions

Direct Message Routing

Private messages use intelligent transport selection:

1. Bluetooth First (preferred when available)

   • Direct connection with established Noise session
   • Fastest and most private option

2. Nostr Fallback (when Bluetooth unavailable)

   • Uses recipient's Nostr public key
   • BitChat's app-specific private-envelope encryption
   • Routes through global relay network

3. Smart Queuing (when neither available)

   • Messages queued until transport becomes available
   • Automatic delivery when connection established

For detailed protocol documentation, see the Technical Whitepaper.

Setup

Option 1: Using Xcode

cd bitchat
open bitchat.xcodeproj

To run on a device there're a few steps to prepare the code:

• Clone the local configs: cp Configs/Local.xcconfig.example Configs/Local.xcconfig
• Add your Developer Team ID into the newly created Configs/Local.xcconfig
  • Bundle ID would be set to chat.bitchat.<team_id> (unless you set to something else)
• Entitlements need to be updated manually (TODO: Automate):
  • Search and replace group.chat.bitchat with group.<your_bundle_id> (e.g. group.chat.bitchat.ABC123)

Option 2: Using just

brew install just

Want to try this on macos: just run will set it up and run from source. Run just clean afterwards to restore things to original state for mobile app building and development.

Localization

• Base app resources live under bitchat/Localization/Base.lproj/. Add new copy to Localizable.strings and plural rules to Localizable.stringsdict.
• Share extension strings are separate in bitchatShareExtension/Localization/Base.lproj/Localizable.strings.
• Prefer keys that describe intent (app_info.features.offline.title) and reuse existing ones where possible.
• Run xcodebuild -project bitchat.xcodeproj -scheme "bitchat (macOS)" -configuration Debug CODE_SIGNING_ALLOWED=NO build to compile-check any localization updates.