CryptChat

A secure TCP messaging application with end-to-end encryption, supporting:

• RSA-2048 Public Key Exchange
• AES-256 Shared Key Exchange
• End-to-end Encrypted Messaging

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Dependencies

• C++ 11 or higher
• CMake 3.20 or higher (sudo dnf install cmake)
• Libuuid-devel (sudo dnf install libuuid libuuid-devel)
• OpenSSL (sudo dnf install openssl-devel)

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Formatting and Styling

Format using clang-format:

clang-format -i src/*.cpp include/*.h

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Build Instructions

1. Create a build directory:

   mkdir -p build
   cd build

2. Run CMake to configure the project:

   cmake ..

3. Build the project:

   make

4. Run the executable with a port number (e.g., 8090):

   export PORT=8090
   ../bin/cryptchat $PORT

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Command Reference

Command	Action
:help	Display the help menu.
:connect <host:port>	Connect to a remote peer.
:disconnect	Terminate the current connection and clear all shared keys.
:exchange pub [new]	Exchange RSA public keys. Use “new” to regenerate keys.
:exchange aes	Generate and share a new AES-256 session key (requires RSA keys).
:quit	Exit the application after performing cleanup.
<message>	Send a plaintext/encrypted message (if session key exists).

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Protocol Message Formats

Connection Establishment

CONN

TYPE:CONN
ID:<A's UUID>
NAME:<A's Username>
NONCE:<A's Nonce>
END

CONN-ACK

TYPE:CONN_ACK
ID:<B's UUID>
NAME:<B's Username>
REPLY_NONCE:<A's Nonce + 1>
END

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Plaintext Messaging

PLAIN

TYPE:PLAIN
ID:<Sender's UUID>
NONCE:<Sender's Nonce>
MSG:<Message Body>
END

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Public Key Exchange

PUBKEY-OFFER

TYPE:PUBKEY_OFFER
ID:<A's UUID>
NONCE:<A's Nonce>
PUBKEY:<A's RSA Public Key>
SIG:<Signature by A>
END

PUBKEY-RESP

TYPE:PUBKEY_RESP
ID:<B's UUID>
NONCE:<B's Nonce>
REPLY_NONCE:<A's Nonce>
PUBKEY:<B's RSA Public Key>
SIG:<Signature by B>
END

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Session Key Exchange

SESSKEY-OFFER

TYPE:SESSKEY_OFFER
ID:<A's UUID>
NONCE:<A's Nonce>
AESKEY:<AES-256 key encrypted with B's public key, base64>
SIG:<Signature by A>
END

SESSKEY-RESP

TYPE:SESSKEY_RESP
ID:<B's UUID>
NONCE:<B's Nonce>
REPLY_NONCE:<A's Nonce>
SIG:<Signature by B>
END

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Encrypted Messaging

CIPHER

TYPE:CIPHER
ID:<Sender's UUID>
NONCE:<Sender's Nonce>
CIPHER:<base64(IV || TAG || CIPHERTEXT)>
END

CIPHERTEXT Structure:

ID:<Sender's UUID>
NONCE:<Sender's Nonce>
MSG:<Plaintext Message>

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Control Messages

BYE / REJECT / ERROR

TYPE:<BYE/REJECT/ERROR>
NONCE:<Sender's Nonce>
REASON:<Reason (optional)>
END

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Usage Notes

• Public key exchange (:exchange pub) must precede AES key exchange.
• Messages are encrypted only after AES key exchange.
• Use :disconnect before switching peers.

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License

This work is licensed under the MIT License. See the LICENSE file for details.