Which of the following, using public key cryptography, ensures authentication, confidentiality and
nonrepudiation of a message?

A.
Encrypting first by receiver’s private key and second by sender’s public key

B.
Encrypting first by sender’s private key and second by receiver’s public key

C.
Encrypting first by sender’s private key and second decrypting by sender’s public key

D.
Encrypting first by sender’s public key and second by receiver’s private key

Explanation:

Encrypting by the sender’s private key ensures authentication. By being able to decrypt with the
sender’s public key, the receiver would know that the message is sent by the sender only and the
sender cannot deny/repudiate the message. By encrypting with the sender’s public key secondly,
only the sender will be able to decrypt the message and confidentiality is assured. The receiver’s
private key is private to the receiver and the sender cannot have it for encryption. Similarly, the
receiver will not have the private key of the sender to decrypt the second-level encryption. In the

case of encrypting first by the sender’s private key and. second, decrypting by the sender’s public
key, confidentiality is not ensured since the message can be decrypted by anyone using the
sender’s public key. The receiver’s private key would not be available to the sender for secondlevel encryption. Similarly, the sender’s private key would not be available to the receiver for
decrypting the message.