Initial exchanges of public keys are vulnerable to man-in-the-middle attacks. Describe as many defences against it as you can. PGP is often used to secure email communication. Describe the steps that...

Initial exchanges of public keys are vulnerable to man-in-the-middle attacks. Describe as many defences against it as you can. PGP is often used to secure email communication. Describe the steps that a pair of users using PGP must take before they can exchange email messages with privacy and authenticity guarantees. What scope is there to make the preliminary key negotiation invisible to users? (The PGP negotiation is an instance of the hybrid scheme.)