Feistel Networks, DES, 3DES, and AES ECES Practice Questions (Page 2)

Questions 6–10

1. 6expert · medium

   You are analyzing a DES implementation for a legacy system. You notice that the S-boxes are being applied to the full 32-bit right half before the expansion step. What is the likely effect of this implementation error?

   • AThe cipher will still be invertible, but the security will be significantly reduced because the expansion and S-box interaction is lost.
   • BThe cipher will be equivalent to DES with a different key schedule, but still secure.
   • CThe cipher will produce the same ciphertext as standard DES because S-boxes are linear.
   • DThe cipher will become non-invertible, making decryption impossible.
   Reveal Solution Discussion Select an answer first
2. 7application · medium

   A company is choosing between AES and 3DES for a new high-throughput encryption appliance. The appliance will handle large volumes of data and must meet modern security standards. Which choice is most appropriate and why?

   • A3DES, because it is a NIST-approved standard and has no known practical attacks.
   • BAES, because it offers larger key sizes and is designed for efficient implementation in both software and hardware.
   • CAES, because it uses a Feistel network that is faster than 3DES's substitution-permutation network.
   • D3DES, because its 168-bit key length is stronger than AES-256.
   Reveal Solution Discussion Select an answer first
4. 8application · medium

   A student is implementing the DES round function and is unsure about the S-box substitution step. They have a 48-bit input to the S-boxes. What is the output size after the S-box substitution?

   • A64 bits
   • B48 bits
   • C56 bits
   • D32 bits
   Reveal Solution Discussion Select an answer first
5. 9application · medium

   You are reviewing a legacy system that uses DES for encrypting files. The system administrator wants to understand why DES is structured the way it is. Which statement accurately describes the DES round structure?

   • AEach round uses a different S-box for each bit position, selected by the subkey.
   • BEach round applies the round function to the entire 64-bit block before splitting into halves.
   • CEach round applies the initial permutation to the right half before the round function.
   • DEach round applies an expansion, XOR with a subkey, S-box substitution, and a P-box permutation to the right half.
   Reveal Solution Discussion Select an answer first
6. 10foundation · easy

   What is the first step in a standard Feistel network round?

   • APerform an initial permutation on the block
   • BXOR the block with the round key
   • CSplit the data block into two equal halves
   • DApply the S-box substitution to the entire block
   Reveal Solution Discussion Select an answer first