$($PGP$,6\%)$ In Figure $3,$ we show the operations of PGP for sending an email from Alice

to Bob. Assume that the public keys are available to each other. The email consists of

two parts: one is a session key encoded by Bob's public key, and the other part, encoded

by the session key, is the message and a digital signature signed by Alice.

a$)(2\%)($Confidentiality$)$ Explain briefly why the email is a secret between Bob and

Alice.

b$)(2\%)($Authentication and non$-$repudiation$)$ Once the email is decoded by Bob,

explain briefly why Bob knows that it is an email sent by Alice.

c$)2\%)($Data integrity$)$ Explain briefly why Bob can be sure that the email has not

been altered.