By no means an expert, bit I'll try: One technique would b asymmetric encryption. Every participant has two keys, a public and a private one. When I want to send you an encrypted message, I encrypt the message with your public key. This key you can make available in any way, it can't be used in a harmful way. The message I encrypted with you public, you can decrypt using your private key, and only with that. Like this, you only need to exchange public keys used only for encryption. So no useful information for an attacker. And private keys never need to leave your hands.