This plugin, I guess, should do the trick.
Since there are two keys – private (on server) and public (in C2), it's harder (I think it's impossible, correct me if I'm wrong) for a hacker to decrypt it. Even if they know the public key, they can only encrypt data and send it to the server, but cannot decrypt it.
Also, it's a good idea to encrypt the public key a bit (instead of plain-pasting it to C2, maybe split it into lots of pieces, then use these pieces together to compose it, change numbers to equations etc.).