Key exchange, plus the fact that the key is the same size of the message, and can only be used once. This means that securely exchanging keys is as hard as secu

Key exchange, plus the fact that the key is the same size of the message, and can only be used once. This means that securely exchanging keys is as hard as securely exchanging the message itself -- if you had a way to exchange the keys, you might as well exchange the message itself and be done with it.