Other Public-Key Systems

Now we know two ways that Alice can send Bob a secret message securely without a secure meeting first. They can use a key-agreement system to choose a secret key for a symmetric-key cipher, or they can use an asymmetric-key system, where Alice knows Bob’s public encryption key but only Bob knows the private decryption key. There’s a third way that uses symmetric-key cryptography to allow Alice to send Bob a message without them exchanging or agreeing on any keys at all, public or private. It’s called the three-pass protocol; it’s too inefficient for general use, but it’s interesting and