CISSP PRACTICE QUESTIONS – 20210206

Effective CISSP Questions

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)

ModeInputCiphertextBlock Chaining
ECBPlaintextOutputN/A
CBCPlaintext XOR IVOutputCiphertext (Chaining)
CFBIVOutput XOR PlaintextCiphertext (Feedback)
OFBIVOutput XOR PlaintextOutput (Feedback)
CTRNonce | CounterOutput XOR PlaintextN/A
IV refers to Initialization Vector

Electronic codebook (ECB)

Image credit: Wikipedia

Cipher block chaining (CBC)

Image credit: Wikipedia

Cipher feedback (CFB)

Image credit: Wikipedia

Output feedback (OFB)

Image credit: Wikipedia

Counter (CTR)

Image credit: Wikipedia

Reference

A BLUEPRINT FOR YOUR SUCCESS IN CISSP

My new book, The Effective CISSP: Security and Risk Management, helps CISSP aspirants build a solid conceptual security model. It is not only a tutorial for information security but also a study guide for the CISSP exam and an informative reference for security professionals.

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)

Leave a Reply