e89fcbd7c4
* Document why rotating the service UUID does not work PEER-ID-ROTATION.md §2 lists "hiding that bitchat is in use" as a non-goal and says the service UUID is "tracked separately." Nothing tracked it, so the idea keeps coming back without its constraints. This is that document. The conclusion is negative and worth writing down: hiding that bitchat is running is not achievable for an open-source app that supports discovery between strangers. Two independent reasons, either sufficient. iOS requires background scanners to name the exact UUIDs they want, so any value a stranger can pre-compute to find us, an adversary running the same source derives identically — stranger discovery (G4) and invisibility to a code-reading adversary are mutually exclusive in one advertisement. And mechanically, third-party iOS apps cannot control advertisement contents at all, cannot change advertised services while backgrounded, and have their UUID collapsed into a 128-bit overflow bitmask in the background anyway — so the real exposure is foreground, where iOS permits rotation least. Also recorded: every comparable deployment (Exposure Notification, Find My, Fast Pair, Eddystone-EID) and every comparable mesh app (Bridgefy, Berty, qaul, Meshtastic) fixes the identifier and rotates the payload, if anything; Briar is the only structurally different answer and it gets there by not broadcasting. What survives any UUID change, ranked by how cheaply it can be used — GATT connect-and-probe is decisive at ~3.7s/device on a Raspberry Pi, and is invariant under rotation because app detection is a lookup, not a discrimination problem. Three ways rotation would make things actively worse (shrunken anonymity set, the restoration stale-identifier trap, anti-tracking standardisation surfacing us as an "unknown tracker"). And what is actually available instead, with peer-ID rotation ranked first because it targets the far larger exposure: identifying a specific person, not merely a bitchat user. Two load-bearing claims are undocumented by Apple and flagged for device verification rather than asserted (§6). RF-fingerprinting is deliberately described as a targeted tool rather than a dragnet; an earlier draft overstated it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> * Correct the foreground-rotation reading; drop the DULT claim Section 2.2 closed by saying rotation's value would sit where iOS permits it least, which reverses the section's own argument. The only mechanical constraint it cites on changing the UUID applies while backgrounded; the foreground is precisely where CoreBluetooth allows stopping advertising, swapping the service and starting again, a cycle BLEService+LinkLayerPeripheralRole already runs. Retitled from "forecloses" to "constrains", rewrote the closing passage, and softened the "two independent reasons" framing, since independence was the point of the objection. The conclusion is unchanged: foreground rotation defeats a stale-list scanner but not an adversary running the public derivation, and connect-and-probe identifies us regardless. Section 5's third downside claimed anti-tracking standardisation might flag us as an unknown tracker. DULT scopes to small location-tracking accessories identified by a specific service-data UUID, and platform alerts cover conformant tags rather than arbitrary advertisers; the spec also mandates address rotation for those accessories, which makes rotation a requirement of it rather than a trigger for it. Dropped, and the section is now two ways. Nothing else referenced it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
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.
Getting a copy you can trust
Install from the App Store, or build from source you have verified. A compiled build from anywhere else cannot be verified — see Verifying bitchat for how to check source against the per-release hash manifest, and for what to do if that is the only build you can get.
This matters more than it usually would: this repository has been the target of takedown demands, and when a repository or releases page disappears, mirrors appear that nobody can check.
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 servers. Note that the mesh does use a persistent per-device identifier derived from your identity key — see the whitepaper on identity and metadata for what a nearby radio can observe
- Private Message End-to-End Encryption: Noise Protocol for mesh, BitChat private envelopes for Nostr fallback
- IRC-Style Commands: Familiar
/slap,/msg,/whostyle 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 for live sessions (store-and-forward mail is sealed without it — see the whitepaper)
- 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
- 440+ 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: private payloads travel inside kind-1059
events whose v2:-prefixed content is a BitChat-specific XChaCha20-Poly1305
construction, not NIP-44 encryption.
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 levelneighborhood(6 chars): District/neighborhoodcity(5 chars): City levelprovince(4 chars): State/provinceregion(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:
-
Bluetooth First (preferred when available)
- Direct connection with established Noise session
- Fastest and most private option
-
Nostr Fallback (when Bluetooth unavailable)
- Uses recipient's Nostr public key
- BitChat's app-specific private-envelope encryption
- Routes through global relay network
-
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
open bitchat.xcodeproj
For a signed device build, create your ignored local configuration and replace the example team ID with your Apple Developer Team ID:
cp Configs/Local.xcconfig.example Configs/Local.xcconfig
Local.xcconfig.example derives unique app and App Group identifiers from that
team ID. The entitlement files already reference $(APP_GROUP_ID), so tracked
project or entitlement files do not need to be edited.
Useful command-line checks from the repository root:
# macOS Debug build without signing
xcodebuild -project bitchat.xcodeproj -scheme "bitchat (macOS)" \
-configuration Debug CODE_SIGNING_ALLOWED=NO build
# Full SwiftPM test suite
swift test
# iOS simulator tests
xcodebuild -project bitchat.xcodeproj -scheme "bitchat (iOS)" \
-sdk iphonesimulator \
-destination 'platform=iOS Simulator,name=iPhone 17' test
If iPhone 17 is unavailable, choose an installed simulator from:
xcodebuild -showdestinations -project bitchat.xcodeproj -scheme "bitchat (iOS)"
Option 2: Using just
brew install just
just check
just run
just build and just run use the current bitchat (macOS) scheme and keep
Xcode output in the ignored .DerivedData/ directory. They never patch source,
project, configuration, or entitlement files.
just clean removes only .DerivedData/ and .build/. It does not invoke Git
or restore tracked files, so uncommitted work is preserved. just test runs the
SwiftPM suite and just test-ios runs the iPhone 17 simulator suite.
Localization
- App localizations live in
bitchat/Localizable.xcstrings. - Share extension strings are separate in
bitchatShareExtension/Localization/Localizable.xcstrings. - 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 buildto compile-check any localization updates.