# Symmetric Ciphers Questions and Answers – Pseudorandom Number Generators and Stream Ciphers – I

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This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “Pseudorandom Number Generators and Stream Ciphers”.

1. Using the Linear Congruential Method (LCM), for a=7, c=0 and m=32. The period is
a) 13
b) 4
c) 11
d) 7

Explanation: The sequence generated is {7,17,23,1,7..}. Thus the period is 4.

2. Using the Linear Congruential Method, for a=5, c=0 and m=32. The period is
a) 8
b) 4
c) 9
d) 11

Explanation: The sequence generated is {2,25,29,17,21,9,13,1,5,2…}. Thus the period is 8.

3. The appropriate value for m (in LCM) is
a) 2(31) – 1
b) 2(31)
c) 2(32)
d) 2(32) – 1

Explanation: Typically 2^(31) – 1 is chosen as the value for m (a large prime number).
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4. A CSPRBG is defined as one that passes the __ test.
a) Runs test
b) Maurer’s Universal statistical test
c) Frequency Test
d) On-bit test

Explanation: A CSPRBG is defined as one that passes the one-bit test.

5. Find the first 8 bits for Blum Blum Shub Bit Generator when seed = 101355 and n = 192649.
a) 10101010
b) 11100010
c) 11001011
d) 11001110

Explanation: The blum blum shub algorithm is as follows-
Xo = s2 mod n
for i=1 to 8
Xi = X(i-1)2 mod n
Bi = Xi mod 2
Using this we compute the bits as – 11001110.

6. Which of these modes is an appropriate mode for PRNG?
a) ECB
b) CBC
c) CFB
d) CTR

Explanation: CTR and OFB are suitable modes for PRNG.

7. Which mode is recommend for the X9.82 Random number generator?
a) OFB
b) CBC
c) CFB
d) CTR

Explanation: OFB is the recommended mode for the X9.82 Random number generator.

8. AES PRNG produces bits outputs of size
a) 512 bits
b) 128 bits
c) 256 bits
d) 64 bits

Explanation: AES PRNG produces bits outputs of size 128 bits.

9. Which mode is recommend for the ANSI standard X9.82 Random number generator?
a) OFB
b) CTR
c) CFB
d) CFB

Explanation: CTR is the recommended mode for the ANSI standard X9.82 Random number generator.

10. The CTR algorithm for PRNG is known as
a) CTR_PRNG
b) X-SESS
c) CTR-SESS
d) CTR_DRBG

Explanation: The CTR algorithm for PRNG is known as CTR_DRBG.

11. Which mode is recommend for the RFC 4086 Random number generator?
a) CFB
b) CBC
c) OFB
d) CTR

Explanation: OFB is the recommended mode for the RFC 4086 Random number generation.

12. while (len(temp) < requested_number_of_bits) do
V = E(Key, V)
temp = temp || V

The above algorithm is for the CTR mode.
a) True
b) False. It is for the ECB mode
c) False. It is for the OFB mode
d) False. It is for the CFB mode

Explanation: The algorithm above is the OFB mode representation.

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