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

Questions 11–15

1. 11foundation · easy

   How is the AES state represented internally?

   • AAs a 4x8 matrix of bits
   • BAs a 2x8 matrix of 64-bit words
   • CAs a 1x16 array of 16-bit words
   • DAs a 4x4 matrix of bytes
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2. 12application · medium

   A junior developer is implementing a custom block cipher for an internal tool and has chosen a Feistel network structure. They ask you to review the design. The round function F is intentionally kept secret and is not invertible. The developer is concerned that decryption will be impossible because F cannot be reversed. How should you respond?

   • ADecryption is possible only if the round function is a permutation.
   • BDecryption is impossible unless F is replaced with an invertible function.
   • CDecryption is possible only if the round function is applied to the left half instead of the right half.
   • DDecryption is still possible because the Feistel structure uses the same round function in reverse order, and the XOR operation is its own inverse.
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4. 13application · medium

   A developer is implementing DES for a legacy protocol and needs to understand the key schedule. Which statement about DES subkey generation is correct?

   • AThe 64-bit key is used directly in each round without any permutation or shifting.
   • BEach subkey is generated by applying the S-boxes to the previous subkey.
   • CThe key schedule uses a different S-box for each round to generate subkeys.
   • DThe 56-bit key is divided into two 28-bit halves, each shifted left by one or two bits per round, then combined and permuted to produce a 48-bit subkey.
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5. 14application · medium

   A legacy payment system still uses DES for encrypting transaction data. The compliance team has mandated that the system move to a cipher with a larger effective key size, but the hardware security module (HSM) in use only supports DES and 3DES operations. The team must choose a migration path that maximizes security while staying within the HSM's constraints. Which approach should they take?

   • AUse 3DES with two keys (2TDEA) in EDE mode, reusing the first key as the third.
   • BContinue using DES but increase the number of rounds from 16 to 32.
   • CReplace the HSM with a software-based AES implementation on the application server.
   • DConfigure the HSM to use 3DES with three independent keys (3TDEA) in EDE mode.
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6. 15foundation · easy

   What is the current status of 3DES in modern cryptographic standards?

   • AIt is only used for legacy systems and is still fully supported for new designs
   • BIt is still the recommended standard for all government use
   • CIt is deprecated and disallowed for most new applications, with AES being the preferred alternative
   • DIt is considered secure for all uses and is widely recommended
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