CompTIA Security+ (SY0-501) — Question 282

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

Correct answer: D

Explanation

The correct answer is D because asymmetric encryption uses a pair of keys: a public key for encryption and a private key for decryption. The other options do not directly address the situation where the same key cannot decrypt a message, as ephemeral keys, cipher types, and out-of-band exchanges do not inherently cause this issue.