# 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.**

- 5 similar rounds having 2 pair ; every alternate
- 9 ; the last
- 8 ; the first and last
- 10 ; no

**Answer:** b. 9; the last

**Explanation:**

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?**

- Cipher Feedback Mode (CFB)
- Cipher Block chaining (CBC)
- Electronic code book (ECB)
- Output Feedback Modes (OFB)

**Answer:** c. Electronic code book (ECB)

**Explanation:**

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:**

- 42
- 93
- 114
- 103

**Answer:** c. 114

**Explanation:**

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**

- Xuejia Lai and James Massey
- Xuejia Lai and Bruce Schneier
- Xuejia Lai and Carlisle Adams
- Xuejia Lai and Stafford Tavares

**Answer:** a. Xuejia Lai and James Massey

**Explanation:**

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?**

- f function
- permutation p
- swapping of halves
- xor of subkey with function f

**Answer:** c. swapping of halves

**Explanation:**

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

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