Which of the following can best be defined as a cryptanalysis technique in which the analyst tries to determine
the key from knowledge of some plaintext-ciphertext pairs?

A.
A known-plaintext attack

B.
A known-algorithm attack

C.
A chosen-ciphertext attack

D.
A chosen-plaintext attack

Explanation:
In this question, the attacker is trying to obtain the key from several “some plaintext-ciphertext pairs”. When the
attacker has a copy of the plaintext corresponding to the ciphertext, this is known as a known-plaintext 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:
Known Plaintext. The attacker has a copy of the plaintext corresponding to the ciphertext
Chosen Ciphertext. Portions of the ciphertext are selected for trial decryption while having access to the
corresponding decrypted plaintext
Chosen Plaintext. Chosen plaintext is encrypted and the output ciphertext is obtained
Ciphertext Only. Only the ciphertext is available
Incorrect Answers:
B: A known-algorithm attack is not a defined type of attack.
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 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