Which of the following offers confidentiality to an e-mail message?

A.
The sender encrypting it with its private key.

B.
The sender encrypting it with its public key.

C.
The sender encrypting it with the receiver’s public key.

D.
The sender encrypting it with the receiver’s private key.

Explanation:
A message encrypted using a public key can only be decrypted using the corresponding private key. The
receiver should be the only person in possession of the recipient’s private key. The recipient’s public key can be
freely distributed.
Therefore, if the sender encrypts a message with the recipient’s pubic key, the sender will know that the
recipient is the ONLY person who can decrypt the message. This ensures the confidentiality of the message.
Incorrect Answers:
A: A public key can be freely distributed. If the sender encrypts a message with his private key, ANYONE in
possession of the sender’s public key could decrypt the message. This offers no confidentiality.
B: A message encrypted using a public key can only be decrypted using the corresponding private key. If the
sender encrypts a message with his public key, only the sender would be able to decrypt it as he is the only
person in possession of the private key that corresponds to his public key.
D: The receiver should be the only person in possession of the recipient’s private key. The sender should never
be in possession of the receiver’s private key.