$(10$ points$)$ Alice is sending an email to Bob. The email message $($could include

attachments of files up to $10M$ Bytes$)$ should be encrypted. Normally the email

delivery do not employ mutual authentication. Instead it just uses one way

communication.

Based on the above requirements, please

Design a one message security protocol between the email sending server

$($Alice$\text{'}$s side$)$ and the email receiving server $($Bob$\text{'}$s side$).$ Note that we

assume that the two email servers have the public key of each other before

hand.

Explain how did the receiver authenticate the sender and obtain the plaintext

email message.