Chapter 5 Question 9

9. How can dual-key encryption be used to authenticate a message?

See the Figure 5.12. The keys must always be used in pairs. Since the private key is never available to anyone but the author, when the message sender encrypts using his key, only his matching public key will decrypt the message. No one can fake the message because they will not have access to the secret private key, so using the public key would not decrypt the message properly.