# CISSP PRACTICE QUESTIONS – 20211121

Which of the following is the block cipher mode of operation that confuses or diffuses plaintext as the cipher’s input? (Wentz QOTD)
A. Electronic codebook (ECB)
B. Cipher feedback (CFB)
C. Output feedback (OFB)
D. Counter (CTR)

Kindly be reminded that the suggested answer is for your reference only. It doesn’t matter whether you have the right or wrong answer. What really matters is your reasoning process and justifications.

My suggested answer is A. Electronic codebook (ECB).

Wentz’s book, The Effective CISSP: Security and Risk Management, helps CISSP and CISM aspirants build a solid conceptual security model. It is a tutorial for information security and a supplement to the official study guides for the CISSP and CISM exams and an informative reference for security professionals.

My justification, as follows, is an excerpt of Wikipedia:

Summary of modes
Mode Formulas Ciphertext
Electronic codebook (ECB) Yi = F(PlainTexti, Key) Yi
Cipher block chaining (CBC) Yi = PlainTexti XOR Ciphertexti−1 F(Y, Key); Ciphertext0 = IV
Propagating CBC (PCBC) Yi = PlainTexti XOR (Ciphertexti−1 XOR PlainTexti−1) F(Y, Key); Ciphertext0 = IV
Cipher feedback (CFB) Yi = Ciphertexti−1 Plaintext XOR F(Y, Key); Ciphertext0 = IV
Output feedback (OFB) Yi = F(Yi−1, Key); Y0 = F(IV, Key) Plaintext XOR Yi
Counter (CTR) Yi = F(IV + g(i), Key); IV = token() Plaintext XOR Yi

Note: g(i) is any deterministic function, often the identity function.

ECB and CBC transform plaintext as input into ciphertext:

The following modes transform the key into a keystream used in a stream cipher instead of transforming plaintext into ciphertext:

A. 電子碼本 (ECB)
B. 密碼反饋 (CFB)
C. 輸出反饋 (OFB)
D. 計數器 (CTR)