Joe is exchanging encrypted email with another party. Joe encrypts the initial email with a key. When Joe receives a response, he is unable to decrypt the response with the same key he used initially. Which of the following would explain the situation? 

Answer options:

A. An ephemeral key was used for one of the messages
B. A stream cipher was used for the initial email; a block cipher was used for the reply
C. Out-of-band key exchange has taken place
D. Asymmetric encryption is being used

D

Asymmetric algorithms use two keys to encrypt and decrypt data. These asymmetric keys are referred to as the public key and the private key. The sender uses the public key to encrypt a message, and the receiver uses the private key to decrypt the message; what one key does, the other one undoes.