Which of the following cryptographic attacks describes when the attacker has a copy of the plaintext and the
corresponding ciphertext?

A.
known plaintext

B.
brute force

C.
ciphertext only

D.
chosen plaintext

Explanation:
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:
Brute Force. Trying every possible combination of key patterns — the longer the key length, the more
difficult it is to find the key with this method
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:
B: A Brute Force attack involves trying every possible combination of key patterns. This is not what is described
in the question.
C: With a Ciphertext Only attack, only the ciphertext is available. The plaintext is not available.
D: In a Chosen Plaintext attack, chosen plaintext is encrypted and the output ciphertext is obtained. This is not
what is described 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