Question $4$

Considering digital envelopes, message authentication and digital signatures, answer parts $($a$),$

$($b$)$ and $($c$).$ The general approach of a digital envelope is shown in Figure $4\mathrm{.}1.$ Suppose Bob

wishes to send a confidential message to Alice, but they do not share a symmetric secret key.

Bob does the following:

Prepare a message.

Generate a random symmetric key that will be used this one time only.

Encrypt that message using symmetric encryption with a one$-$time key.

Encrypt the one$-$time key using public$-$key encryption with Alice's public key.

Attach the encrypted one$-$time key to the encrypted message and send it to Alice.

$($a$)$ Discuss the benefits of this approach.

$($b$)$ Discuss what algorithms could provide the most computationally secure encryption for the

digital envelope.

$($c$)$ Construct a figure similar to Figure $4\mathrm{.}1$ that includes a digital signature to authenticate the

message in the digital envelope.