This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “Rabin/ Elgamal Algorithm”.

1. “Rabin Cryptosystem is a variant of the Elgamal Cryptosystem”

a) True

b) False

View Answer

Explanation: Rabin Cryptosystem is a variant of the RSA Cryptosystem.

2. Using Rabin cryptosystem with p=23 and q=7

Encrypt P=24 to find ciphertext. The Cipher text is

a) 42

b) 93

c) 74

d) 12

View Answer

Explanation: Calculate n = p × q = 161

Plaintext P = 24

Ciphertext = C ≡ P2 (mod n)

= 242 mod 161 = 93 mod 161

Ciphertext transmitted = 93.

3. Which Cryptographic system uses C1 = (e1^{r}) mod p and C1 = (e2^{r} x P) mod p at the encryption side?

a) Elgamal

b) RSA

c) Rabin

d) Whirlpool

View Answer

Explanation: The Elgamal cryptographic system uses the above formulae to compute the CT.

4. Sender chooses p = 107, e1 = 2, d = 67, and the random integer is r=45. Find the plaintext to be transmitted if the ciphertext is (28,9).

a) 45

b) 76

c) 66

d) 13

View Answer

Explanation: P = [C2 (C1d)-1] mod p = 66.

5. In Elgamal cryptosystem, given the prime p=31.

Choose e1= first primitive root of p and d=10, calculate e2.

a) 24

b) 36

c) 25

d) 62

View Answer

Explanation: We choose e1=3 (a primitive root of p = 31) and d=10. Then we have e2 = 3

^{10}mod 31 = 25.

6. In Elgamal cryptosystem, given the prime p=31.

Encrypt the message “HELLO”; use 00 to 25 for encoding. The value of C2 for character ‘L’ is

a) 12

b) 07

c) 20

d) 27

View Answer

Explanation:The Common factor for the calculation of C2 is e

^{7}mod 31 = 25

^{7}mod 31 = 25.

P = “L” = 11; C1 = 3

^{7}mod 31 = 17; C2 = 11 x 25 mod 31 = 27; C = (17,27).

7. In Elgamal cryptosystem, given the prime p=31.

Encrypt the message “HELLO”; use 00 to 25 for encoding. The value of C2 for character ‘O’ is

a) 09

b) 07

c) 23

d) 27

View Answer

Explanation:The Common factor for the calculation of C2 is e

^{7}mod 31 = 25

^{7}mod 31 = 25.

P = “O” = 14; C1 = 3

^{7}mod 31 = 17; C2 = 14 x 25 mod 31 = 09; C = (17,09).

8. In Elgamal cryptosystem, given the prime p=31.

What is the respective plaintext character for C = (27, 20)?

a) H

b) L

c) O

d) M

View Answer

Explanation: The Common factor for the calculation of C2 is e

^{7}mod 31 = 25

^{7}mod 31 = 25.

C = 17, 20); P = 20 X (17

^{10})

^{-1}mod 31 = 07; “07” = “H”.

**Sanfoundry Global Education & Learning Series – Cryptography and Network Security.**

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