MCQ | Block Cipher as IDEA, DES, AES, RSA in Cryptography

Here, we have a set of multiple-choice questions and answers (quiz) on Block Cipher as IDEA, DES, AES, RSA in Cryptography.
Submitted by Monika Sharma, on March 22, 2020

1) In the AES-128 algorithm there are mainly __________ similar rounds and _________ round is different from other round.

  1. 5 similar rounds having 2 pair ; every alternate
  2. 9 ; the last
  3. 8 ; the first and last
  4. 10 ; no

Answer: b. 9; the last


Basically, in the AES-128 there are 9 rounds and the last round is different from other rounds.

2) Which of the following modes of operation in DES is used for operating?

  1. Cipher Feedback Mode (CFB)
  2. Cipher Block chaining (CBC)
  3. Electronic code book (ECB)
  4. Output Feedback Modes (OFB)

Answer: c. Electronic code book (ECB)


Mainly, the Electronic codebook mode is used for operating on short data as the same key is used for each block in this algorithm. Thus convert in Plain Text lead to repetitions in Cipher Text in cryptography.

3) Using Rivest, Shamir, Adleman cryptosystem with p=7 and q=9. Encrypt M=24 to find ciphertext. The Ciphertext is:

  1. 42
  2. 93
  3. 114
  4. 103

Answer: c. 114


    Calculate n = p × q = 63
    Phi(n)=(p-1) x (q-1)=48
    Let e=5 , where gcd(e,phi(n))=1
    Plaintext M = 24
    Ciphertext = C ≡ M^e (mod n)
    = 24^5 mod 63 = 7962624 mod 161
    Ciphertext transmitted = 114.

4) International Data Encryption Algorithm (IDEA) was developed by

  1. Xuejia Lai and James Massey
  2. Xuejia Lai and Bruce Schneier
  3. Xuejia Lai and Carlisle Adams
  4. Xuejia Lai and Stafford Tavares

Answer: a. Xuejia Lai and James Massey


It is a symmetric key block cipher designed by James Massey of ETH Zurich and Xuejia Lai and in 1991 it firstly described it.

5) When do we compare the AES with DES, which of the following functions from DES does not have an equivalent AES function in cryptography?

  1. f function
  2. permutation p
  3. swapping of halves
  4. xor of subkey with function f

Answer: c. swapping of halves


Mainly, there is no equivalent to swapping of halves in the AES algorithm in cryptography.

6) Data encryption standard is a block cipher and encrypts data in blocks of size of _____ each.

  1. 16 bits
  2. 64 bits
  3. 32 bits
  4. All of the mentioned above

Answer: b. 64 bits


When using the Data Encryption Standard, data is encrypted in blocks of 64 bits apiece, which means that the data encryption standard requires 64 bits of plain text to be entered as input?

7) The process of decryption of an AES ciphertext is similar to the encryption process in the ______.

  1. Reverse order
  2. Next order
  3. Both A and B
  4. All of the mentioned above

Answer: a. Reverse order


The process of decrypting an AES ciphertext is quite similar to the process of encrypting it, except that it is done in the opposite order. Each round is comprised of the four processes, which are carried out in the opposite sequence of the previous round. Due to the fact that sub-processes in each round are carried out in reverse order, as opposed to a Feistel Cipher, the encryption and decryption algorithms must be developed separately, despite the fact that they are extremely closely connected.

8) AES stands for Advanced Encryption Standard,

  1. True
  2. False

Answer: a. True


Advanced Encryption Standard (AES) is an abbreviation for Advanced Encryption Standard. It was thought to be vulnerable to an exhaustive key search attack because of the increase in computational capacity. AES is an iterative cypher, as opposed to a Feistel cypher. Based on the ‘substitution–permutation network,' it is a powerful tool. It is made up of a sequence of connected operations, some of which require substituting certain inputs with specified outputs (substitutions), and others which entail shuffling bits around (permutations).

9) RSA algorithm is ____ cryptography algorithm.

  1. Systematic
  2. Symmetric
  3. Asymmetric
  4. None of the mentioned above

Answer: c. Asymmetric


The RSA algorithm is a symmetric cryptography algorithm that uses a random number generator. Asymmetric refers to the fact that it operates on two separate keys, namely the Public Key and the Private Key.

10) Amongst which of the following is / are true with reference to the rounds in AES –

  1. Byte Substitution
  2. Shift Row
  3. Mix Column and Key Addition
  4. All of the mentioned above

Answer: d. All of the mentioned above


The rounds in AES are as follows: Byte Substitution, Shift Row, Mix Column, and Key Addition (in order of appearance).




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