RoseChat is a serverless, local area network (LAN) messaging application built with Flutter. It utilizes UDP Multicast for peer-to-peer discovery and communication, allowing users on the same network to chat without a central server or internet connection.

It is designed to be lightweight, private, and resilient, featuring distinct channels for public broadcasting and private direct messaging, all secured by AES-256 encryption.

• UDP Multicast: Automatically discovers other users on the LAN by broadcasting presence packets to a specific IP group (default 224.0.0.1) on port 5000.
• Zero Configuration: No servers to set up. Just launch the app, and it finds peers immediately.
• Offline Capable: Works entirely on local Wi-Fi or Ethernet; no internet access required.

• Public Broadcast Channel: A shared lobby visible to everyone on the same multicast group.
• Direct Messaging (DM): Private, one-on-one chats. Messages are filtered at the application layer so only the intended recipient displays them.
• Unread Indicators: A visual "red dot" notification appears next to users in the sidebar who have sent you new messages while you were away.

• AES-256 Encryption: All traffic—including presence announcements and chat messages—is encrypted using a shared secret key.
• Packet Envelope: Traffic is wrapped in an encrypted envelope. Sniffers (like Wireshark) only see randomized garbage data.
• Network Isolation: Users can change their Multicast Group IP and Secret Key in settings. Peers must share both to communicate, effectively creating private "rooms" on the same LAN.

• Modern Dark UI: Built with a "Slate" color palette tailored for desktop and mobile.
• Dynamic Sidebar: Lists discovered peers with real-time status updates and unread message tracking.
• Customizable Profiles: Users can set their display name and randomize their avatar (generated via DiceBear seeds).

RoseChat follows a clean, component-based architecture:

• main.dart: The application core. It manages the UI state, handles message routing (filtering public vs. private), and updates the peer list.
• multicast_service.dart: The networking engine. It binds to the system's RawDatagramSocket, handles the joinMulticast logic, and performs the AES encryption/decryption pipeline.
• models/: Defines the immutable data structures for Peer and ChatMessage.
• sidebar/ & chatwindow/: Modular UI components.

RoseChat uses a custom JSON-over-UDP protocol. To ensure security, every packet is wrapped in an encrypted envelope.

The Envelope (On the Wire)

{
  "iv": "base64_initialization_vector",
  "d": "aes_encrypted_payload_string"
}

The Payload (Decrypted) Once the app receives a packet and successfully decrypts it with the matching key, the payload looks like this:

• Message Packet:

  {
    "type": "msg",
    "group": "224.0.0.1",       // Ensures we don't process packets from other groups
    "senderId": "uuid-1234",    // Unique Instance ID
    "sender": "Alice",
    "seed": "avatar_seed",
    "content": "Hello World!",
    "recipientId": "uuid-5678"  // null for Public Broadcast, UUID for DM
  }

1. Binding: On startup, RoseChat binds to 0.0.0.0 (any IPv4) on port 5000 and joins the multicast group (e.g., 224.0.0.1).
2. Announcing: It broadcasts a "Presence" packet to the group.
3. Listening: It listens for incoming UDP datagrams.
4. Filtering Pipeline:
   • Decrypt: Attempt to decrypt d using the current Secret Key. If it fails, drop the packet.
   • Group Check: Does json['group'] match our current group? If no, drop it.
   • Peer Check: Is senderId new? If yes, add to the Sidebar.
   • Message Routing: Is recipientId null (Public)? Or does it match my ID (Private)? If neither, ignore it.

1. Clone the repository.
2. Install dependencies:

   flutter pub get

3. Run the application:

   flutter run

   (Note: To test locally, you can run flutter run -d windows (or macos/linux) in two separate terminals to spawn two distinct windows).

• Send Message: Type in the box and press Enter. Use Shift + Enter for a new line.
• Switch Channels: Click "Public Broadcast" or a user's name in the sidebar to switch views.
• Profile Settings: Click the Gear icon next to your name (bottom left) to change your Display Name or Avatar.
• Network Settings: Click the Ethernet/Gear icon next to the Multicast IP (sidebar) to change the Group IP or Encryption Key.
  • Reset: Inside Network Settings, click "Reset Defaults" to return to 224.0.0.1 and the default key.