intercli

Context

intercli is a command-line chat written in C++. A server runs, clients connect to it, and they talk to each other: to one person by name, or to everyone at once. Everything that passes between a client and the server is encrypted.

It started much smaller. The first goal was simply to get one client to connect to the server and send one message. Once that worked, I kept going until several clients could connect at the same time and message each other through the server.

My role

This was a solo project. I built the server, the client, the encryption layer and the tests for them, working with an AI coding assistant throughout.

Key decisions

The server relays every message. Clients never talk to each other directly. Each one registers a name with the server, and the server routes every message, either to a single client by name or to everyone. It also keeps a list of who is online and announces when someone joins or leaves. That keeps each client simple: it only ever needs to know how to reach the server.

Encrypted, with keys that keep changing. Messages are encrypted with AES-256-GCM, which also detects if anything has been tampered with on the way. The client and server agree on their keys with a Diffie-Hellman exchange, so the keys themselves are never sent over the network. Each direction of a conversation has its own key. Each side replaces its sending key after every 10 messages and announces the change in the same stream, so both sides switch at exactly the same point.

The hard part

The hardest part at first was getting a client to connect to the server at all. When I finally got it to connect, the connection would drop shortly after, even though the server was still running. For a while I did not know what was causing it.

I worked through it by testing cases one at a time, and the cause turned out to be in how the session was set up. The client was working out its keys from its own session ID instead of the server's, and the server was changing keys every 10 messages without telling the client. Either way, the two sides soon stopped being able to read each other's messages, and the connection fell over. Once both sides used the same session values and changed keys together, the connection held. Each fix now has a test, so it stays fixed.

Outcome, and what I would do differently

intercli works: several clients can connect to one server and chat, directly or to everyone, with every message encrypted. A test runs three clients against one server to check it.

If I built it again, I would give the server files that clients can browse and download, and add uploads, so it could share more than messages.