Feistel Networks, DES, 3DES, and AES ECES Practice Questions (Page 4)

Questions 16–20

1. 16application · medium

   A developer is designing a block cipher and wants to use a Feistel network. They plan to use a round function that is not invertible. They are concerned that this will make decryption impossible. What should they know about Feistel networks?

   • ADecryption is possible because the round function does not need to be invertible; the structure itself is invertible.
   • BDecryption is possible only if the round function is applied to both halves.
   • CDecryption is impossible unless the round function is invertible.
   • DDecryption is possible only if the round function is a permutation.
   Reveal Solution Discussion Select an answer first
2. 17application · medium

   A developer is implementing AES in a hardware accelerator and needs to understand the purpose of each transformation in the AES round. They are particularly interested in the transformation that provides diffusion across columns. Which AES transformation is responsible for mixing bytes within each column?

   • AMixColumns
   • BShiftRows
   • CAddRoundKey
   • DSubBytes
   Reveal Solution Discussion Select an answer first
4. 18foundation · easy

   What is the primary security weakness of DES that makes it vulnerable to brute-force attacks?

   • AThe 56-bit key length is too short
   • BThe initial permutation is not secret
   • CThe S-boxes are too nonlinear
   • DThe block size of 64 bits is too large
   Reveal Solution Discussion Select an answer first
5. 19foundation · easy

   In 3DES with three keys (K1, K2, K3), what is the encryption process?

   • AEncrypt with K1, decrypt with K2, encrypt with K3
   • BEncrypt with K1, encrypt with K2, decrypt with K3
   • CEncrypt with K1, encrypt with K2, encrypt with K3
   • DDecrypt with K1, encrypt with K2, decrypt with K3
   Reveal Solution Discussion Select an answer first
6. 20application · medium

   A junior developer on your team is designing a custom block cipher for an embedded system. They propose a Feistel network with a round function that is not invertible. The developer worries that decryption will be impossible. What should you tell them?

   • ADecryption is still possible because the Feistel structure reconstructs the halves in reverse order, regardless of the round function's invertibility.
   • BDecryption will fail unless the round function is made invertible, so you must redesign the round function.
   • CDecryption is possible only if the round function is a permutation, so you must add a P-box.
   • DDecryption is possible only if the number of rounds is even, so you must add one more round.
   Reveal Solution Discussion Select an answer first