Joe must send Ann a message and provide Ann with assurance that he was the actual sender. Which of
the following will Joe need to use to BEST accomplish the objective?

A.
A pre-shared private key

B.
His private key

C.
Ann’s public key

D.
His public key

Explanation:
To achieve both authentication and confidentiality, Joe should include Ann’s name in the message, sign it
using his private key, and then encrypt both the message and the signature using Ann’s public key.
Incorrect Answers:
A: Because pre-shared keys are held by both parties to the communication, it can be compromised at one
end, without the knowledge of anyone at the other.
C: Anybody encrypt the message using Anne’s public key, so this would not prove that Joe is the actual
sender.
D: Joe would sign the message with his private key, not his public key.

https://en.wikipedia.org/wiki/Public-key_cryptography