README.md (Markdown content)

🔐 SecureChat — Java (Sockets) with Diffie–Hellman + AES

Java sockets chat using AES encryption and Diffie–Hellman key exchange.

Console chat over TCP sockets with a secure channel:

• Diffie–Hellman key exchange (per connection)
• AES message encryption (GCM-ready; current impl uses hashed shared secret)

✨ Features

• Server accepts multiple clients (thread per client)
• DH key exchange; shared secret derived & hashed for AES key bytes
• AES encrypt/decrypt for chat messages
• Simple CaesarCipher kept for comparison/learning

⚙️ Requirements

• JDK 17+ (tested with JDK 23)
• (Optional) lib/ with extra JARs if your IDE setup needs them

🧭 How it works (crypto)

1. Server generates a DH key pair and sends server public key to client.
2. Client extracts DH params from server public key, generates its key pair with the same params, sends client public key back.
3. Both sides compute the shared secret via KeyAgreement.
4. Shared secret is hashed → 16 bytes used as AES key (for demo).
5. Messages are AES-encrypted before sending; decrypted on receive.

🚀 Run (from IDE)

1. Run Server.java
2. Run Client.java (one or more times)
   Type messages in the client console; server broadcasts to all clients.

If you see “Illegal base64 character”, it means a non-Base64 line reached the decrypt step. (We’ll harden the protocol soon: message framing + type tags).

🧱 Project Structure

src/
  module-info.java
  server_client_chat_app/
    Server.java
    Client.java
    ClientHandler.java
    AESUtil.java
    CaesarCipher.java
    DiffieHellmanUtil.java
lib/           # optional jars
docs/          # screenshots/diagrams (optional)

🗺️ Roadmap

• Switch to AES-GCM with IV + auth tag
• Message framing (length + type)
• Basic UI (JavaFX) and user names
• Unit tests for DH/AES utilities
• CI build (once we add Maven/Gradle)

🏷️ Topics

java sockets diffie-hellman aes cryptography client-server

📝 License

MIT — see LICENSE.