Security at the application layer is also important. One of the functions of application layer security protocols is to ensure email security. The main protocols working at application layer for email security are PGP and S/MIME. Considering these protocols answer the following: 

a. Assume Alice needs to send an e-mail to Bob. Explain how the confidentiality of the e-mail is achieved using PGP. 

b. Assume Alice needs to send an e-mail to Bob. Explain how the authentication of the e-mail is achieved using S/MIME.