Cryptographic Principles (Diffusion, Confusion, Kerckhoff Principle) ECES Practice Questions (Page 6)

Questions 26–30

1. 26application · medium

   A security architect is evaluating a legacy block cipher for encrypting database backups. The cipher has a fixed 64-bit block size and a simple substitution-permutation network. The architect is concerned that patterns in the plaintext (e.g., repeated headers) might appear in the ciphertext. Which property of the cipher should the architect verify to ensure that a single plaintext bit change affects many ciphertext bits?

   • AThe cipher uses a long key schedule, so each round uses a different subkey.
   • BThe cipher exhibits strong diffusion, so changing one plaintext bit flips roughly half the ciphertext bits.
   • CThe cipher's block size is 64 bits, which limits the amount of plaintext that can be encrypted before a collision.
   • DThe cipher's S-boxes are non-linear, making the substitution step resistant to linear cryptanalysis.
   Reveal Solution Discussion Select an answer first
2. 27application · medium

   A database administrator is encrypting a table that contains many rows with identical values in a column. The administrator is concerned that an attacker might infer the frequency of these values from the ciphertext. Which property of the encryption algorithm would most directly mitigate this risk?

   • AUse a longer key to increase the key space.
   • BStrong diffusion, so that identical plaintext values produce very different ciphertext values.
   • CUse a block cipher mode with an initialization vector (IV) to randomize ciphertext for identical plaintexts.
   • DStrong confusion, so that the key-ciphertext relationship is complex.
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4. 28expert · hard

   A security architect is evaluating two block ciphers for a high-throughput encryption appliance. Cipher X has excellent diffusion but poor confusion. Cipher Y has excellent confusion but poor diffusion. The appliance will encrypt a large volume of structured data with many repeated fields. The architect must choose one cipher. Which consideration is most critical?

   • AChoose Cipher Y because poor diffusion will allow statistical patterns to leak, making the ciphertext vulnerable to frequency analysis.
   • BChoose Cipher X because diffusion is more important than confusion for all applications.
   • CChoose Cipher X because poor confusion will allow key recovery from known plaintext-ciphertext pairs.
   • DChoose either cipher because diffusion and confusion are interchangeable and both are needed.
   Reveal Solution Discussion Select an answer first
5. 29application · medium

   A cryptography student is implementing a simple Feistel cipher for a class project. The instructor emphasizes that the round function must include both a substitution step and a permutation step. What is the primary reason for including the permutation step?

   • ATo comply with Kerckhoffs's principle by making the algorithm public.
   • BTo achieve diffusion by spreading the influence of each plaintext bit across the block.
   • CTo achieve confusion by making the key relationship nonlinear.
   • DTo increase the key size without changing the algorithm.
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6. 30application · medium

   A government agency is considering a new encryption standard. They require that the algorithm be openly published and subject to public scrutiny, but they are concerned that this will weaken security. Which principle directly addresses this concern?

   • AConfusion, which ensures that the key is hard to recover.
   • BDiffusion, which ensures that plaintext patterns are hidden.
   • CThe avalanche effect, which ensures that small changes produce large differences.
   • DKerckhoffs's principle, which states that security should not depend on the secrecy of the algorithm.
   Reveal Solution Discussion Select an answer first