A directory of Objective Type Questions covering all the Computer Science subjects. Here you can access and discuss Multiple choice questions and answers for various compitative exams and interviews.

1. Bob selects E67(2, 3) as the elliptic curve over GF(p). He selects e1 = (2, 22) and d = 4.
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)
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Answer: (a).C1=(35,1) ; C2 =(21,44)

2. 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
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Answer: (b).20

3. 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
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Answer: (a).x≡±16 mod 23

4. 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
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Answer: (d).27

5. 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
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Answer: (a).H

6. The multiplicative Inverse of 550 mod 1769 is –
a. 434
b. 224
c. 550
d. Does not exist
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Answer: (a).434

7. It is desirable to revoke a certificate before it expires because –
a. the user is no longer certified by this CA
b. the CA’s certificate is assumed to be compromised
c. the user’s private key is assumed to be compromised
d. all of the mentioned
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Answer: (d).all of the mentioned

8. Suppose that A has obtained a certificate from certification authority X1 and B has obtained certificate authority from CA X2. A can use a chain of certificates to obtain B’s public key. In notation of X.509, the correct order of this chain is represented as –
a. X2 X1 X1 B
b. X1 X1 X2 A
c. X1 X2 X2 B
d. X1 X2 X2 A
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Answer: (c).X1 X2 X2 B

9. A total of seven messages are required in the Public-Key distribution scenario. However, the initial five messages need to be used only infrequently because both A and B can save the other’s public key for future – a technique known as ____________
a. time stamping
b. polling
c. caching
d. squeezing
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Answer: (c).caching

10. Which of the following statements are true?

i) In the CBC mode, the plaintext block is XORed with previous ciphertext block before encryption.
ii) The CTR mode does not require an Initialization Vector
iii) The last block in the CBC mode uses an Initialization Vector
iv) In CBC mode repetitions in plaintext do not show up in ciphertext
a. iii)
b. ii) and iv)
c. All the Statements are true
d. i) ii) and iv)
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Answer: (d).i) ii) and iv)

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