Secure Message Exchange Protocol

This protocol simulates a basic SSL/TLS-style secure communication using netcat for transport and Python for encryption.

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Sequence Diagram

Key Setup

Each party generates an RSA key pair

python3 keygen_rsa.py --out sender
python3 keygen_rsa.py --out receiver

Exchange public keys (sender_pub.pem, receiver_pub.pem) out-of-band or hardcode them for testing.

Sending a Message (Sender Side)

1. Generate a random AES key and IV using your custom Python logic.
2. Encrypt the plaintext message using your custom AES-256 (CBC mode) implementation.
3. Sign the original plaintext using your custom SHA-256 hash function + RSA private key (custom implementation).
4. Encrypt the AES key and IV together using the recipient's RSA public key (custom implementation).
5. Base64-encode the outputs.
6. Send the three lines (in order) to the receiver via netcat:
   • encrypted_AES_key_and_IV
   • encrypted_message
   • digital_signature

python3 secure_send_custom.py receiver_pub.txt sender_priv.txt | nc 9999

Receiving a Message (Receiver Side)

1. Receive three base64-encoded lines over netcat and pipe them into your custom script:

   nc -l -p 9999 | python3 secure_receive_custom.py receiver_priv.txt sender_pub.txt

2. Read each line from standard input:

   Line 1: encrypted_AES_key_and_IV

   Line 2: encrypted_message

   Line 3: digital_signature

3. Base64-decode each part.

4. Decrypt the AES key and IV using the receiver's RSA private key (custom).

5. Decrypt the message using AES-256 in CBC mode with the decrypted key and IV.

6. Verify the digital signature using the sender’s public RSA key and your custom SHA-256.

Message Format (Sent over netcat)

The message consists of three base64-encoded lines, sent in the following order:

• encrypted_AES_key_and_IV
• encrypted_message
• digital_signature

Each line should be processed in sequence by the receiver.

Entry files

secure_receive_custom.py: to decrept an verify the received messages and either accept them or reject them.

secure_send_custom.py: to encrypt the messages before sending them over the socket.