# Symmetric Ciphers Questions and Answers – The Simplified Data Encryption Standard (SDES)

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This set of Cryptography Multiple Choice Questions & Answers (MCQs) focuses on “The Simplified Data Encryption Standard (SDES)”.

1. What is the size of the key in the SDES algorithm?
a) 24 bits
b) 16 bits
c) 20 bits
d) 10 bits

Explanation: The size of the key in the SDES algorithm is 10 bits.

2. Assume input 10-bit key, K: 1010000010 for the SDES algorithm. What is K1?
a) 10100100
b) 01011011
c) 01101000
d) 10100111

Explanation: The permuted key P10 = 1000001100. Input to P8: 0000111000 and
K1 is 10100100.

3. Assume input 10-bit key, K: 1010000010 for the SDES algorithm. What is K2?
a) 10100111
b) 01000011
c) 00100100
d) 01011010

Explanation: Input to P8: 0010000011 and K2 is 01000011.

4. The Ciphertext for the Plaintext 01110010, given that the keys K1 is 10100100 and K2 is 01000011 is
a) 01110111
b) 10010110
c) 01010110
d) 01000101

Explanation: Perform the SDES algorithm and compute the cipher text.

5. The Ciphertext for the Plaintext 11010101, given that the key is 0111010001 is
a) 00010001
b) 10110010
c) 11010010
d) 01110011

Explanation: Perform the SDES Encryption algorithm and compute the cipher text.

6. The Plaintext for the Ciphertext 00100010, given that the key is 1111111111 is
a) 01100111
b) 00001010
c) 01001000
d) 01001100

Explanation: Perform the SDES Decryption algorithm and compute the cipher text.

7. In SDES, Encryption algorithm can be written as a composition of functions:
IP-1 o fK2 o fK1 o SW o IP
a) True
b) False

Explanation: The SDES algorithm follows the order – IP-1 o fK2 o SW o fK1 o IP.

8. Assume input 10-bit key, K: 0010010111 for the SDES algorithm. What is K1?
a) 00101111
b) 01011011
c) 01101000
d) 10100111

Explanation: The permuted key P10 = 1000010111. Input to P8: 0000101111 and K1 is 00101111.

9. The Plaintext for the Ciphertext 00001111, given that the key is 1111111111 is
a) 01100111
b) 00001010
c) 11111111
d) 01101101

Explanation: Perform the SDES Decryption algorithm and compute the cipher text.

10. The Plaintext for the Ciphertext 11110000, given that the key is 0000000000 is
a) 01100111
b) 00000000
c) 01001000
d) 01101100

Explanation: Perform the SDES Decryption algorithm and compute the cipher text.

11. Assume input 10-bit key, K: 0010010111 for the SDES algorithm. What is K2?
a) 11101010
b) 11011011
c) 01101000
d) 10101111

Explanation: The permuted key P10 = 0000101111. Input to P8: 0010011101 and K2 is 11101010.

12.The Plaintext for the Ciphertext 10100101, given that the key is 0010010111 is
a) 01100111
b) 00110110
c) 01001000
d) 01001100

Explanation: Perform the SDES Decryption algorithm and compute the cipher text.

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