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

A. Encrypting first by receivers private key and second by senders public key

B. Encrypting first by senders private key and second by receivers public key

C. Encrypting first by senders private key and second decrypting by senders public key

D. Encrypting first by senders public key and second by receivers private key

Explanation:

Encrypting by the senders private key ensures authentication. By being able to decrypt with the senders 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 senders public key secondly, only the sender will be able to decrypt the message and confidentiality is assured. The receivers 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 senders private key and. second, decrypting by the senders public key, confidentiality is not ensured since the message can be decrypted by anyone using the senders public key. The receivers private key would not be available to the sender for second-level encryption. Similarly, the senders private key would not be available to the receiver for decrypting the message.