# Symmetric Ciphers Questions and Answers – The Data Encryption Standard (DES) and It’s Strength – II

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This set of Cryptography Assessment Questions focuses on “The Data Encryption Standard (DES) and It’s Strength”.

1. During decryption, we use the Inverse Initial Permutation (IP-1) before the IP.
a) True
b) False

Explanation: IP-1 is the first step and the last step is IP during decryption.

2. A preferable cryptographic algorithm should have a good avalanche effect.
a) True
b) False

Explanation: Thus statement is true as a change in one bit of the plaintext or one bit of the key should produce a change in many bits of the ciphertext. This is referred to as the avalanche effect.

3. The number of tests required to break the DES algorithm are
a) 2.8×1014
b) 4.2×109
c) 1.84×1019
d) 7.2×1016

Explanation: There are 256 keys =7.2×1016.
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4. The number of tests required to break the Double DES algorithm are
a) 2112
b) 2111
c) 2128
d) 2119

Explanation: For Double DES key is 2112 bits, should require 2111 tests to break.

5. How many keys does the Triple DES algorithm use?
a) 2
b) 3
c) 2 or 3
d) 3 or 4

Explanation: For Triple DES we can either have 2 or 3 keys.
Using two keys: c = Ek1(Dk2(Ek1(m)))
Using three keys: c = Ek3(Ek2(Ek1(m))).

6. In triple DES, the key size is ___ and meet in the middle attack takes ___ tests to break the key.
a) 2192 ,2112
b) 2184,2111
c) 2168,2111
d) 2168,2112

Explanation: The key size is 2168 and meet in the middle attack takes 2112 tests to break.

7. Using Differential Crypt-analysis, the minimum computations required to decipher the DES algorithm is
a) 256
b) 243
c) 255
d) 247

Explanation: Differential Crypt-analysis requires only 247 computations to decipher the DES algorithm.

8.Using Linear Crypt-analysis, the minimum computations required to decipher the DES algorithm is
a) 248
b) 243
c) 256
d) 264

Explanation: Linear Crypt-analysis requires only 243 computations to decipher the DES algorithm.

Sanfoundry Global Education & Learning Series – Cryptography and Network Security. 