This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “Overview – 1”.

1. In the elliptic curve group defined by y2= x3- 17x + 16 over real numbers, what is P + Q if P = (0,-4) and Q = (1, 0)?

a) (15, -56)

b) (-23, -43)

c) (69, 26)

d) (12, -86)

View Answer

Explanation: P=(x1, y1)= (0,-4)

Q=(x2, y2)= (1,0)

From the Addition formulae:

λ= (0-(-4)) / (1-0) = 4

x3= = 16 – 0 – 1 = 15 and

y3= 4(0 – 15) –(-4) = -56

Thus R=P + Q = (15, -56).

2. Bob selects E67(2, 3) as the elliptic curve over GF(p). He selects e1 = (2, 22) and d = 4.

Then he calculates e2 = d × e1 and the publicly announces the tuple (E, e1, e2). Now, Alice wants to send the plaintext P = (24, 26) to Bob and she selects r = 2. What are C1 and C2?

a) C1=(35,1) ; C2 =(21,44)

b) C1=(44,21) ; C2 =(1,35)

c) C1=(44,21) ; C2 =(44,21)

d) C1=(21,44); C2 =(35,1)

View Answer

Explanation: Alice finds the points C1= r × e1 ; C1= (35, 1),

C2=P + r × e2 ; C2= (21, 44).

3. For the point P (7, 0) defined in the curve E13(1, 1). What is –P?

a) (7,1)

b) (8,12)

c) (8,1)

d) (7,0)

View Answer

Explanation: The inverse of P(11,2) is (11,11) or (11,-2).

4. Consider knapsack that weighs 23 that has been made from the weights of the superincreasing series {1, 2, 4, 9, 20, and 38}. Find the ‘n’.

a) 011111

b) 010011

c) 010111

d) 010010

View Answer

Explanation: v0=1, v1=2, v2=4, v3=9, v4=20, v5=38

K=6, V=23

Starting from largest number:

v5 > V then ϵ_5=0

v4 < V then V = V – v4 = 23 – 20 = 3 ϵ_4=1

v3 > V then ϵ_3=0

v2>V then ϵ_2=0

v1 < V then V = V – v1= 3 – 2 = 1 ϵ_1=1

v0 =1 then V = V – v0= 1 – 1 = 0 ϵ_0=1

n= ϵ_5 ϵ_4 ϵ_3 ϵ_2 ϵ_1 ϵ_0 = 010011.

5. The plaintext message consist of single letters with 5-bit numerical equivalents from (00000)2 to (11001)2. The secret deciphering key is the superincreasing 5-tuple (2, 3, 7, 15, 31), m = 61 and a = 17. Find the ciphertext for the message “WHY”.

a) C= (148, 143, 50)

b) C= (148, 143, 56)

c) C= (143, 148, 92)

d) C= (148, 132,92)

View Answer

Explanation: {wi }= {a vi mod m}

{wi} = { 17×2 mod 61, 17×3 mod 61, 17×7 mod 61, 17×15 mod 61, 17×31 mod 61}

{wi} = {34, 51, 58, 11, and 39}

PlainText In binary Ci

W- 22 10110 148

H – 7 00111 143

Y – 24 11000 50

So that the ciphertext sent will be C= (148, 143, 50).

6. Suppose that plaintext message units are single letters in the usual 26-letter alphabet with A-Z corresponding to 0-25. You receive the sequence of ciphertext message units 14, 25, 89. The public key is the sequence {57, 14, 3, 24, 8} and the secret key is b = 23, m = 61.

Decipher the message. The Plain text is

a) TIN

b) INT

c) KIN

d) INK

View Answer

Explanation: Solve using Knapsack Cryptosystem.

Wi = {57, 14, 3, 24, 8}

b = 23 and m = 61

a = b-1 mod m

61 = 2 x23 + 15

23 = 1x 15 + 8 Therefore 1= 8 x 23 – 3 x 61

15 = 1x 8 + 7 b-1 = 23-1= 8

8 = 1x 7 + 1 a = 8

v_i=a

^{(-1)}w_i mod m

=bw_i mod m

v_i={ 30, 17, 8, 3, 1}

Cipher text V = bC mod m Plaintext

14 23 x 14 mod 61 = 17 01000 = 8 = I

25 23 x 25 mod 61 = 26 01101 = 13 = N

89 23 x 89 mod 61 = 34 10011 = 19 = T.

7. How many primitive roots does Z<19> have?

a) 5

b) 8

c) 7

d) 6

View Answer

Explanation: Z<19> has the primitive roots as 2,3,10,13,14 and 15.

8. Find the order of group G= <Z7*, x>

a) 6

b) 4

c) 3

d) 5

View Answer

Explanation: |G| = f(7) = (71-70) = 6

G =

9. Which among the following values: 17, 20, 38, and 50, does not have primitive roots in the group G = <Zn∗, ×>?

a) 17

b) 20

c) 38

d) 50

View Answer

Explanation: The group G = <Zn*, ×> has primitive roots only if n is 2, 4, pt, or 2pt

‘p’ is an odd prime and‘t’ is an integer.

G = <Z17∗, ×> has primitive roots, 17 is a prime.

G = <Z20∗, ×> has no primitive roots.

G = <Z38∗, ×> has primitive roots, 38 = 2 × 19 prime.

G = <Z50∗, ×> has primitive roots, 50 = 2 × 52 and 5 is a prime.

10.Find the solution of x^{2}≡3 mod 23

a) x≡±16 mod 23

b) x≡±13 mod 23

c) x≡±22 mod 23

d) x≡±7 mod 23

View Answer

Explanation: a=3 3

^{((23+1)/4)}≡3

^{6}≡1 (QR and there is solution)

x ≡ ±3(23 + 1)/4 (mod 23) ≡±16 i.e. x = 7 and 16

11. 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).

12. 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”.

13. For 1000 nodes in IP level, how many keys would be required?

a) 499000

b) 499500

c) 500500

d) 500000

View Answer

Explanation: Use N(N-1)/2 where N=1000.

14. “Meet in the middle attack” is an attack

a) where the timing required for the attack via brute force is drastically reduced

b) where the adversary uses 2 or more machines to decrypt thus trying to reduce the time

c) where messages are intercepted and then either relayed or substituted with another message

d) where cryptanalysis takes lesser time than the brute force decryption

View Answer

Explanation: “Meet in the middle attack” is an attack where messages are intercepted and then either relayed or substituted with another message.

15. Which systems use a timestamp?

i) Public-Key Certificates

ii) Public announcements

iii) Publicly available directories

iv) Public-Key authority

a) i) and ii)

b) iii) and iv)

c) i) and iv)

d) iv) only

View Answer

Explanation: Public announcements and Public Certificates involve the use of timestamps.

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