ChainChat ⛓️💬

ChainChat is a decentralized, database-less, and end-to-end encrypted (E2EE) messaging application for Android. It operates on a custom peer-to-peer blockchain network where every device acts as a node, maintaining its own local ledger and synchronizing with others over a local network.

🌟 Key Features

• Decentralized Architecture: No central servers. Messaging happens directly between peers (P2P) using TCP sockets.
• End-to-End Encryption (E2EE):
  • Key Agreement: ECDH (Elliptic Curve Diffie-Hellman) using secp256r1.
  • Signatures: ECDSA with SHA-256 for integrity and authentication.
  • Encryption: AES-GCM (256-bit) for message payload privacy.
• Blockchain Ledger: Messages are stored as transactions within blocks. Each block is linked to the previous one via SHA-256 hashes.
• Proof-of-Work (PoW): Implements a mining mechanism to ensure ledger integrity and prevent spam.
• Modern UI: Built with Jetpack Compose featuring a dark-themed, glassmorphic design.
• Local Multi-Profile Switcher: Facilitates testing by allowing users to toggle between identities (e.g., Alice and Bob) on a single device.

🛠️ Tech Stack

• Language: Kotlin
• UI Framework: Jetpack Compose (Material 3)
• Asynchronous Programming: Kotlin Coroutines
• Networking: Java TCP Sockets (ServerSocket & Socket)
• Cryptography: Java Cryptography Architecture (JCA)

🏗️ Architecture

Blockchain Structure

• Transaction: Contains sender/recipient addresses (public key hashes), ephemeral public keys for ECDH, ciphertext, IV, and an ECDSA signature.
• Block: Contains an index, timestamp, list of transactions, previous block hash, nonce, and current block hash.
• Consensus: Follows the "longest chain" rule. Nodes validate incoming blocks based on hash integrity and PoW requirements.

P2P Networking

Nodes listen on a default port (9922). Communication involves:

• Handshake: Exchanging public keys and identity info.
• Sync: Fetching missing blocks from peers.
• Broadcast: Sending newly mined blocks to all connected peers.

🚀 Getting Started

Prerequisites

• Android Studio (Ladybug or newer recommended)
• Android SDK 34+
• Two or more devices (or emulators) on the same Wi-Fi network.

Installation

1. Clone the repository.
2. Open the project in Android Studio.
3. Sync Gradle and build the project.
4. Deploy to your Android devices.

How to Use

1. Initialize Profile: Enter a username and generate your cryptographic keys.
2. Find Your Address: Your public address is displayed at the top of the screen.
3. Connect to Peer:
   • Enter the recipient's Public Address.
   • (Optional) Enter the recipient's IP Address to establish a direct connection.
4. Send Message: Type your message and hit send. The app will:
   • Derive a shared secret via ECDH.
   • Encrypt the message with AES-GCM.
   • Sign the transaction with ECDSA.
   • Mine a new block (PoW).
   • Broadcast the block to the network.

📂 Project Structure

app/src/main/java/com/example/myapplication/
├── blockchain/         # Block and Transaction data models, Mining logic
├── crypto/             # ECDH, ECDSA, and AES implementation
├── network/            # P2P Server and Client socket management
├── ui/                 # ViewModels and UI state management
└── MainActivity.kt     # Jetpack Compose UI entries

🔒 Security Protocol

1. Identity: Every user has a persistent EC KeyPair. The Public Key acts as their address.
2. Encryption: For every message, an ephemeral EC KeyPair is generated. A shared secret is derived using the sender's ephemeral private key and the recipient's static public key.
3. Authentication: The recipient verifies the sender's signature using the sender's static public key, ensuring the message hasn't been tampered with and truly came from the claimed sender.

📄 License

This project is licensed under the MIT License.