# Symmetric Ciphers Questions and Answers – RC4 and RC5 – I

This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “RC4 and RC5”.

1. Which of the following is true for the RC5 algorithm?
i) Has variable number of rounds
ii) Has fixed Key length
iii) High memory Requirements
iv) Uses only primitive computational operations commonly found on microprocessors

a) i) and iv)
b) i) ii) and iv)
c) iv)
d) i) ii) and iii)

Explanation: RC5 algorithm can have a variable key length and requires low amount of memory.

2. What are the allowable values of word size in bit for RC5 algorithm?
a) 16, 32
b) 16, 32, 64
c) 8, 16, 32
d) 16, 32, 48

Explanation: Word sizes in bits for RC5 algorithm can be 16, 32 or 64.

3. The number of rounds in RC5 can range from 0 to _____________
a) 127
b) 63
c) 255
d) 31

Explanation: Number of Rounds range from 0 to 255.

4. The standard/nominal version of the RC5-w/r/b has parameters w/r/b as
a) 32/18/16
b) 16/18/16
c) 32/12/16
d) 32/16/18

Explanation: Nominal/Standard version is RC5-32/12/16.
-i.e. 32-bit words, so encrypts 64-bit data blocks
-using 12 rounds
-with 16 bytes (128-bit) secret key.
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5. The total number of subkeys t used in the RC5 algorithm is given by the formula (r corresponds to number of rounds)
a) t=2r+4
b) t=2r
c) t=2r+2
d) t=2r-2

Explanation: The total number of keys used in the RC5 algorithm is given by the formula t=2r=2.

6. RC5 uses 2 magic constants to define their subkeys. These are
a) Base of natural Logarithm and Golden ratio
b) Base of natural Logarithm and Pi
c) Golden Ratio and Pi
d) Pi and Golden Ration

Explanation: The initialize operations makes use of magic constants defined as follows:
P_w=Odd[(e-2) 2w].
Q_w=Odd[(φ-1) 2w].

7. The value of the base of natural logarithms is
a) e= 2.7073
b) e= 2.7183
c) e= 3.7183
d) e= 1.7273

Explanation: The base of natural log i.e. ‘e’ has a value 2.7183.

8. The value of the golden ratio is
a) ᶲ =2.618
b) ᶲ =1.622
c) ᶲ =1.743
d) ᶲ =1.618

Explanation: The value of the golden ratio is 1.618.

9. In RC5, the initialization operations makes use of magic constants defined as follows:
P_w=Odd[(e-2) 2w].
Q_w=Odd[(φ-1) 2w].
What is the hexadecimal value of Pw for word size of 16 bits?
a) B7DF
b) B7D4
c) B7E1
d) B6E2

Explanation: Pw is B7E1 for ‘w’ of the size 16 bits.

10. In RC5, the initialization operations makes use of magic constants defined as follows:
P_w=Odd[(e-2) 2w].
Q_w=Odd[(φ-1) 2w].
What is the hexadecimal value of Qw for word size of 32 bits?
a) 9D3779B4
b) 9D7779F9
c) 9E36D9B2
d) 9E3779B9

Explanation: Pw is 9E3779B9 for ‘w’ of the size 32 bits.

11. Calculate the number of subkeys required in RC5 for 18 rounds of computation.
a) 40
b) 38
c) 36
d) 34

Explanation: Using the formula t=2r+2 we get number of subkeys as 38.

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