## The problem

Send data from a client to a server through an attacker who can read, delete, and replay every packet, and still guarantee confidentiality, integrity, and that each side is talking to the other.

## Key decisions

Long-term keys only vouch for new ones

The long-term RSA keys are used only to sign fresh session keys, which in turn sign an elliptic-curve Diffie-Hellman exchange. The attacker in the middle sees every key but can't forge one.

A new key for every message

Every message carries a new Diffie-Hellman public key, and the shared secret goes through a key-derivation function to become that message's AES key. Like the ratchet in Off-the-Record messaging, leaking one key exposes only one message.

Separate keys for separate jobs

Messages are encrypted with AES and signed with a different key, so breaking one doesn't give away the other.

Counters against replay

Every message carries a signed counter, so replayed, dropped, or reordered packets are detected and reported.

## Results

• ✓Detects tampered, replayed, reordered, and dropped messages
• ✓Encryption and signing use separate keys

## Built with

Python, PyCryptodome, AES, RSA, Elliptic-curve DH