Which of the following block cipher modes of operation in which a block cipher doesn’t use plaintext as the direct input but operates on its output and the plaintext to produce the ciphertext?
A. Cipher Block Chaining (CBC)
B. Cipher feedback (CFB)
C. Electronic codebook (ECB)
D. Initialization Vector (IV)

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 B. Cipher feedback (CFB).

• Electronic codebook (ECB) accept plaintext as inputs.
• Cipher Block Chaining (CBC) accept “Plaintext XOR IV” as inputs.
• Cipher FeedBack (CFB) and Output FeedBack (OFB) accept IV as inputs.
• Counter (CTR) concatenates nonce and counter as the input.
• Initialization Vector (IV) is not a block cipher mode of operation.

Block Cipher Mode of Operation

The “block cipher encryption” box in each diagram of block cipher mode can be expressed as the following function, which accepts variables “Input” and “Key” as inputs, confuses inputs, and produces the output.

Output = E(Input, Key)

Mode	Input	Ciphertext	Block Chaining
ECB	Plaintext	Output	N/A
CBC	Plaintext XOR IV	Output	Ciphertext (Chaining)
CFB	IV	Output XOR Plaintext	Ciphertext (Feedback)
OFB	IV	Output XOR Plaintext	Output (Feedback)
CTR	Nonce | Counter	Output XOR Plaintext	N/A

Electronic codebook (ECB)

Cipher block chaining (CBC)

Cipher feedback (CFB)

Output feedback (OFB)

Counter (CTR)

Reference

• Block cipher mode of operation

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下列哪種區塊型加密器(cipher block)的操作模式(mode of operation)的加密器不使用明文作為直接輸入(input)，而是對其輸出(output)和明文(plaintext)進行操作以產生密文(ciphertext)？
A. Cipher Block Chaining (CBC)
B. Cipher feedback (CFB)
C. Electronic codebook (ECB)
D. Initialization Vector (IV)