A possible way to break public key encryption: a criminal with "man in the middle" access could wait for the victim to request a public key, and then the criminal could reply with a their own public key (for which the criminal has the private key), thus tricking the victim into encrypting with the wrong key, and allowing the criminal to read the encrypted message.

Why can't this attack work on a web site whose public key is part of a digital certificate issued by a certificate authority?