9.7.4. Public Key Ciphers
The AES cipher that you have seen in the preceding section is a symmetric cipher. The same key is used for both encryption and decryption. The Achilles heel of symmetric ciphers is key distribution. If Alice sends Bob an encrypted method, Bob needs the same key that Alice used. If Alice changes the key, she needs to send Bob both the message and, through a secure channel, the new key.
But perhaps she has no secure channel to Bob—which is why she encrypts her messages to him in the first place.
Public key cryptography solves that problem. In a public key cipher, Bob has a key pair consisting of a public key and a matching private key. Bob can publish the public key anywhere, but he must closely guard the private key. ...