Feistel Networks, DES, 3DES, and AES ECES Practice Questions (Page 7)

Questions 31–35

1. 31application · medium

   A security engineer is reviewing an AES-256 implementation and wants to verify the key expansion process. They know that the cipher key is 256 bits. How many round keys are generated for AES-256, and what is the size of each round key?

   • A16 round keys, each 128 bits.
   • B14 round keys, each 128 bits.
   • C14 round keys, each 256 bits.
   • D15 round keys, each 128 bits.
   Reveal Solution Discussion Select an answer first
2. 32application · medium

   A team is designing a Feistel cipher and wants to ensure that decryption uses the same round function as encryption. Which design choice guarantees this?

   • AUse a different round function for decryption that is the inverse of the encryption round function.
   • BUse a symmetric key schedule where the same subkey is used for every round.
   • CUse the same round function and subkeys in reverse order for decryption.
   • DUse an invertible round function so that decryption can invert each round.
   Reveal Solution Discussion Select an answer first
4. 33foundation · easy

   Why is AES efficient in both software and hardware implementations?

   • AIt uses a 64-bit block size, which reduces the number of rounds
   • BIts operations are simple byte-level operations that map well to modern processors and can be accelerated with dedicated instructions
   • CIt uses a Feistel network, which is inherently fast
   • DIt only uses XOR operations, which are very fast
   Reveal Solution Discussion Select an answer first
5. 34application · medium

   A developer is implementing DES and wants to verify that the initial permutation (IP) and final permutation (FP) are correctly applied. They know that the IP is applied to the 64-bit plaintext block before the 16 rounds. What is the purpose of the initial and final permutations in DES?

   • AThey increase the key length.
   • BThey have no cryptographic significance and are primarily for implementation convenience.
   • CThey are essential for the Feistel structure to be invertible.
   • DThey provide diffusion across the block.
   Reveal Solution Discussion Select an answer first
6. 35foundation · easy

   What is the block size and key size of the original DES algorithm?

   • A64-bit block, 56-bit key
   • B64-bit block, 64-bit key
   • C128-bit block, 56-bit key
   • D128-bit block, 128-bit key
   Reveal Solution Discussion Select an answer first