5) Bob is concerned about the integrity of any message from Alice. He is not concerned with confidentiality. He also does not want to get involved with encryption algorithms. What can Alice do to assure him of the integrity of the messages she sends him without using encryption?

a Alice sends Bob a signed message digest along with the messages.

b Alice sends Bob her digital signature along with the message.

c Alice first creates a message authentication code (MAC) by (1) concatenating the message m, with their authentication key, a (which she shares with Bob), (2) then calculates a hash H(m + a). She then appends the MAC to , and sends both to Bob.

d Alice first creates a message authentication code (MAC) by (1) concatenating the message m with her public key, Ka + and (2) then calculates a hash H(m + a). She then appends the MAC to m , and sends both to Bob.