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MCQ | Modes of Operations in Block Cipher

Here, we have a set of multiple-choice questions and answers (quiz) on Modes of Operations in Block Cipher.
Submitted by Monika Sharma, on March 23, 2020

1) Which of the following is a mode of operation for the Block ciphers in cryptography?

  1. Electronic Code Book (ECB)
  2. Cipher Block Chaining (CBC)
  3. Counter (CTR) mode
  4. All of the above

Answer: d. All of the above

Explanation:

All the mentioned modes are followed by the block cipher techniques. Therefore, the correct answer is d. All of the above.


2) For which of the following should EBC (Electronic Code Book) process not be used for encryption?

  1. For large block sizes
  2. For fixed block sizes
  3. For small block sizes
  4. None of the above

Answer: c. For small block sizes

Explanation:

It is preferred that the block size in the EBC technique must be greater than 64 bits. If not, the text is padded to make it of the required length. This is due to some particular words and phrases that may be reused again often so that the same repetitive part of ciphertext can emerge as mixed.


3) Which of the following is the main disadvantage of the ECB (Electronic Code Book)?
or
Which of the following is the major drawback of ECB?

  1. It requires large block size
  2. Padding is done to make the plain text divisible into blocks of fixed size
  3. It is prone to cryptanalysis since there is a direct relationship between plain text and cipher text.
  4. None of the above

Answer: c. It is prone to cryptanalysis since there is a direct relationship between plain text and cipher text.

Explanation:

In ECB, there lies a direct relation between the plain text and the ciphertext. Therefore, it is easy for an outsider to break the encryption logic and steal the data.


4) Which of the following options is not correct according to the definition of the Cipher Block Chaining (CBC)?

  1. CBC is a mode of operation for stream ciphers.
  2. Initialization vector (IV) is used in CBC in the initial phase.
  3. It has better resistive nature towards cryptanalysis than ECB
  4. None of the above

Answer: a. CBC is a mode of operation for stream ciphers

Explanation:

CBC which stands for Cipher Block chaining is a mode of operation for block ciphers and not for stream ciphers.


5) Which of the following modes of operations can be followed for both stream ciphers as well as block ciphers?

  1. CBC (Cipher Block Chaining)
  2. ECB (Electronic Code Book)
  3. CFB (Cipher text Feed Back)
  4. All of the above

Answer: c. CFB (Cipher text Feed Back)

Explanation:

CFB is primarily a mode to derive some characteristics of a stream cipher from a block cipher on the cryptography in cryptoanalysis.








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