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* Harden REQUEST_SYNC and stop gossip-sync re-send loops Two fixes from an end-to-end review of the sync path: Efficiency: the GCS filter (400B, p=7) covers ~355 packet IDs, but stores hold up to 1000 messages + 600 fragments + 200 files. Once a mesh accumulates more than the filter can cover, responders re-sent the entire older tail to every requester every round — ~120KB per pair per 30s during file transfers, dropped by dedup after the airtime was already burned. Requesters now stamp the dormant sinceTimestamp TLV with the oldest timestamp their filter covers, and responders skip older packets (announces exempt: they carry the signing keys needed to verify everything else). Periodic sync also sends one request per type schedule instead of a union filter, so fragment floods can't crowd messages out of the filter budget. Security: a ~40-byte unsigned REQUEST_SYNC with an empty filter could elicit a full store replay (~900KB) — an unauthenticated >10,000x amplification vector, repeatable in a tight loop and relayable with crafted TTL to fan the drain out of every reachable node. Requests now require ttl == 0, a valid signature from the claimed sender's announced signing key, and a matching link binding; REQUEST_SYNC is never relayed regardless of TTL; and responses are rate-limited per peer (8 per 30s sliding window, ~3x the legitimate cadence). Cross-platform: verified against bitchat-android — it signs REQUEST_SYNC and sends SYNC_TTL_HOPS = 0, so both gates hold; it neither sends nor honors sinceTimestamp yet, so mixed pairs keep today's behavior with no regression. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> * Address Codex review: enforce no-relay on route path, exact since-cursor Two P2 findings from Codex on the REQUEST_SYNC hardening: - Route-forwarding bypass: handleRequestSync's early return for a rejected (nonzero-TTL / unsigned) request still fell through to forwardAlongRouteIfNeeded, which relays any routed packet with ttl > 1 regardless of type. The no-relay invariant was only enforced on the flood path. BLERouteForwardingPolicy now suppresses REQUEST_SYNC outright, so a crafted request with a route and TTL headroom can't be forwarded to the next hop either. - Inexact since-cursor: GCSFilter.buildFilter trimmed by hash order when the encoding overflowed the byte budget, so the cursor (computed from the untrimmed prefix) could claim coverage of timestamps whose packets were dropped from the filter — re-sending exactly those every round. buildFilter now trims from the input tail (oldest, since candidates are newest-first) and reports includedCount; the cursor is derived from that, so the covered set is always a contiguous newest-prefix and the cursor is exact. Adds GCSFilter includedCount coverage (full vs trimmed), a route-forwarding test for REQUEST_SYNC, and makes the truncated-cursor test robust to trim variance. Full suite: 1029 tests pass. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> --------- Co-authored-by: jack <jackjackbits@users.noreply.github.com> Co-authored-by: Claude Opus 4.8 <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.
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, NIP-17 for Nostr
- 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
- 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
- NIP-17 Encryption: Gift-wrapped private messages for internet privacy
- Ephemeral Keys: Fresh cryptographic identity per geohash area
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
- NIP-17 gift-wrapping for privacy
- 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
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)
- Bundle ID would be set to
- Entitlements need to be updated manually (TODO: Automate):
- Search and replace
group.chat.bitchatwithgroup.<your_bundle_id>(e.g.group.chat.bitchat.ABC123)
- Search and replace
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 toLocalizable.stringsand plural rules toLocalizable.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 buildto compile-check any localization updates.