In what type of attack does an attacker try, from several encrypted messages, to figure out the key used in the
encryption process?

A.
Known-plaintext attack

B.
Ciphertext-only attack

C.
Chosen-Ciphertext attack

D.
Plaintext-only attack

Explanation:
In this question, the attacker is trying to obtain the key from several “encrypted messages”. When the attacker
has only encrypted messages to work from, this is known as a Ciphertext-only attack.
Cryptanalysis is the act of obtaining the plaintext or key from the ciphertext. Cryptanalysis is used to obtain
valuable information and to pass on altered or fake messages in order to deceive the original intended
recipient. This attempt at “cracking” the cipher is also known as an attack.
The following are example of some common attacks:
Chosen Ciphertext. Portions of the ciphertext are selected for trial decryption while having access to the
corresponding decrypted plaintext
Known Plaintext. The attacker has a copy of the plaintext corresponding to the ciphertext
Chosen Plaintext. Chosen plaintext is encrypted and the output ciphertext is obtained
Ciphertext Only. Only the ciphertext is available
Incorrect Answers:
A: With a Known Plaintext attack, the attacker has a copy of the plaintext corresponding to the ciphertext. This
is not what is described in the question.
C: With a Chosen-Ciphertext attack, the attacker has a copy of the plaintext corresponding to the ciphertext.
This is not what is described in the question.
D: With a Plaintext-only attack, the attacker does not have the encrypted messages as stated in the question.

Krutz, Ronald L. and Russel Dean Vines, The CISSP Prep Guide: Mastering the Ten Domains of Computer
Security, John Wiley & Sons, New York, 2001, p. 154