# Symmetric Ciphers Questions and Answers – Principles of PRNG – II

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This set of Cryptography Interview Questions and Answers for experienced focuses on “Principles of PRNG”

1. Which among the following cannot be used in the case of stream ciphers?
a) TRNGs
b) PRNGs
c) PRFs
d) All of the mentioned

Explanation: TRNGs are not practical for stream ciphers.

2. A stream cipher key is usually of size ___ or 128 bits.
a) 56
b) 64
c) 54
d) 48

Explanation: A stream cipher key is usually of size 54 or 128 bits.

3. Which among the following has the lowest bitrate ?
a) PRF
b) TRNG
c) PRNG
d) All have the same bitrate

Explanation: TRNGs are the slowest. The mechanism used to generated True Random numbers may not be able to generate bits at a rate sufficient to keep up with the application requiring the random bits.

4. RC4 was built on the sole purpose of generating random bits i.e. to act as a PRNG.
a) True
b) False

Explanation: The statement is true. RC4 is a purpose built algorithm.

5. Maurer’s Universal Statistical Test is –
a) a test to determine whether the number of runs of ones and zeros of various lengths is as expected
b) a test to detect if a sequence can be significantly compressed without loss of information
c) a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p
d) a test for the check of evenness in the 1s and 0s

Explanation: Maurer’s Universal Statistical Test is a test to detect if a sequence can be significantly compressed without loss of information.

6. Runs Test is
a) a test to determine whether the number of runs of ones and zeros of various lengths is as expected
b) a test to detect if a sequence can be significantly compressed without loss of information
c) a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p
d) a test for the check of evenness in the 1s and 0s

Explanation: Runs Test is a test to determine whether the number of runs of ones and zeros of various lengths is as expected.

7. Frequency Test is
a) a test to determine whether the number of runs of ones and zeros of various lengths is as expected
b) a test to detect if a sequence can be significantly compressed without loss of information
c) a test for the check of evenness in the 1s and 0s
d) a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p

Explanation: Frequency Test is a test to determine whether the number of ones and zeros in a sequence is approx the same as in a truely random o/p.

8. We should not be able to determine the seed from any previous knowledge of the bits sequence. This is the property of backward unpredictability.
a) True
b) False

Explanation: The statement is true.

9. Forward Unpredictability is a property when the seed is unknown, and the next output bit is the sequence should be unpredictable.
a) True
b) False

Explanation: The statement is true.