ShortP2P

ShortP2P is a prototype mesh messenger: an app for exchanging messages between users without a central chat server, with delivery attempts through multiple nodes (limited network search depth), direct UDP, optional Bluetooth (RFCOMM on Windows), and relaying along a known or discovered route.

This is research/demo code: protocols and security parameters are not production-grade. The goal is to explore routing, multi-transport, and end-to-end encryption in one solution.

What the prototype already includes

• Local storage for chats and messages (SQLite via sqlite-net-pcl), local registration/login (password hash, user keys).
• Peer discovery on the LAN over UDP (ShortP2P.Discovery).
• Per-user shared gateway (SharedUserUdpGateway): one UDP socket for data, a separate presence/ping channel (port 17501, broadcast), periodic announcements and peer “visibility” expiry (~10 s interval, ~25 s timeout).
• Graph-style routing: configurable search depth of 1–3 hops (P2pRoutingSettings.MaxSearchHops), waiting for search replies, retries on send failure. A route can be direct UDP or a relay chain (ChatRelayRoute: first hop + strip relay).
• Adaptive chat transport (AdaptiveChatTransportLayer): if the peer’s last address from ping is known — send there when the transport is available; otherwise — send via the gateway along the current route (including relay).
• Two UIs: WinForms (Windows) and .NET MAUI (repo targets Windows; other platforms — see comments in the .csproj). Shared logic lives in ShortP2P.Client.
• The ShortP2P.UI project is a stub for a separate UI layer; the main shells are WinForms and MAUI.

Tech stack

Area	Implementation
Platform	.NET 10 (client), Server — .NET 8; Crypto — net8.0 + net10.0
Transport	ITransport abstraction → UDP (ShortP2P.Transport), Bluetooth WinRT RFCOMM (ShortP2P.Transport.Bluetooth.Windows), optional serial/IR (InfraredSerialTransport)
Cryptography	RSA (handshake), session on ECDH-like keys; AES-128-CBC + HMAC-SHA256 (truncated tag), upper bound of 128 bytes for an encrypted packet; large payloads are split in MessengerService
Messenger	ShortP2P.Messenger: encryption/fragmentation on top of ITransport + P2PSession
Client	QR (peer exchange), ZXing/ImageSharp, Polly for resilience, LAN scanning, on-disk routing settings
Tests	xUnit: ShortP2P.Crypto.Tests

Repository layout (logical layers)

ShortP2P.Crypto/              # Keys, handshake, P2PSession
src/
  ShortP2P.Transport.Abstractions/
  ShortP2P.Transport/         # UDP, BT stub, serial
  ShortP2P.Transport.Bluetooth.Windows/
  ShortP2P.Discovery/         # Overlay: UDP discovery (relay is a separate assembly of the same layer, not Client)
  ShortP2P.Messenger/         # Encrypted binary exchange over transport
  ShortP2P.Client/            # User scenarios, DB, QR; wires Messenger and the overlay layer
  ShortP2P.UI/                # UI stub
  ShortP2P.WinForms/          # Windows desktop
  ShortP2P.MauiApp/           # MAUI client
  ShortP2P.Crypto.Tests/

Each project has its own README.md with a short folder and file layout.

Build and run (brief)

• Solution: ShortP2P.sln.
• Tests: dotnet test from the repository root (after restoring packages).
• MAUI on non-Windows targets may need dotnet workload restore and extending TargetFrameworks (see src/ShortP2P.MauiApp/ShortP2P.MauiApp.csproj).
• For unpackaged MAUI on Windows, if you hit runtime environment errors, see comments in ShortP2P.MauiApp.csproj (Windows App SDK).

Prototype limitations

• Mesh logic is simplified: there is no full product-grade fault-tolerant mesh; depth and timeouts are constants/settings.
• Cryptography and key sizes (e.g. RSA-1024 in the generation code) are suitable for experiments, not as a reference for an internet threat model.
• Bluetooth is implemented for Windows; other OSes need their own ITransport implementations.

Development tools

This project was developed using Cursor — a Visual Studio Code–based IDE with built-in AI assistance.