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?
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
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I think the most feasible answer should be A.
Explanation:
Perfect forward secrecy for securing email with ephemeral key (short-term keys)
https://www.semanticscholar.org/paper/Secure-E-mail-protocols-providing-perfect-forward-Sun-Hsieh/14853cff384d9dd18158b347169106c6f4b026cd