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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.


6) Amongst which of the following is/ are showing the goodness of electronic code book,

  1. Parallel encryption of blocks of bits is possible
  2. It is a faster way of encryption.
  3. Simple way of the block cipher.
  4. All of the mentioned above

Answer: d. All of the mentioned above

Explanation:

The electronic code book is the most straightforward block cypher manner of operation. It is feasible to do parallel encryption of blocks of bits in this manner. This is a straightforward implementation of the block cypher.


7) Cipher block chaining or CBC is an advancement made to electronic code book,

  1. True
  2. False

Answer: a. True

Explanation:

Cipher block chaining, often known as CBC, is an improvement on the ECB algorithm since the ECB algorithm breaches some security standards. After XORing the previous cypher block with the original plaintext block, the previous cypher block is sent on as input to the next encryption algorithm in the CBC scheme.


8) In Cipher block chaining mode, the current plaintext block is added to the ____.

  1. Previous ciphertext block
  2. Next ciphertext block
  3. Middle ciphertext block
  4. None of the mentioned above

Answer: a. Previous ciphertext block

Explanation:

If you are using CBC mode, the current plaintext block is added to the previous ciphertext block, and the result is then encrypted with the secret key. To put it another way, encryption is the opposite process of decryption, which consists in decrypting the current ciphertext and then adding the preceding ciphertext block to the result.


9) With reference to Cipher Feedback (CFB) Mode, amongst which of the following is /are true,

  1. Each ciphertext block gets 'fed back' into the encryption process
  2. Encrypt the next plaintext block
  3. Both A and B
  4. None of the mentioned above

Answer: c. Both A and B

Explanation:

As part of the Cipher Feedback (CFB) Mode encryption process, each ciphertext block is "fed back" into the overall encryption process in order to encrypt the next plaintext block. CFB mode requires an initialization vector to be used as the first random n-bit input block in order to function properly. It is not necessary to keep the initialization vector a secret.


10) An initialization vector (IV) or starting variable (SV) is a block of bits that is used by several modes to _____.

  1. Randomize the decryption
  2. Randomize the encryption
  3. Minimize and maximize the randomization
  4. None of the mentioned above

Answer: b. Randomize the encryption

Explanation:

An initialization vector, also known as a starting variable (SV), is a block of bits that is used by several encryption modes to randomize the encryption process and, as a result, to produce distinct ciphertexts even if the same plaintext is encrypted multiple times without the need for a more time-consuming re-keying process.


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