Cryptographic Principles (Diffusion, Confusion, Kerckhoff Principle) ECES Practice Questions (Page 2)

Questions 6–10

1. 6application · medium

   A software developer is implementing a cipher and wants to ensure that a single-bit change in the plaintext causes a significant change in the ciphertext. Which design feature should the developer focus on?

   • AA non-linear S-box that substitutes values.
   • BA permutation layer that spreads bits across the block.
   • CA larger block size.
   • DA longer key schedule.
   Reveal Solution Discussion Select an answer first
2. 7application · medium

   A cryptographer is designing a cipher and wants to make it resistant to a known-plaintext attack where the attacker has many plaintext-ciphertext pairs. Which design goal is most important?

   • AEnsure that the block size is large enough to prevent collisions.
   • BEnsure that the algorithm is kept secret.
   • CEnsure that the plaintext bits are spread across many ciphertext bits.
   • DEnsure that the relationship between the key and ciphertext is highly non-linear.
   Reveal Solution Discussion Select an answer first
4. 8expert · hard

   A security engineer is designing a new encryption scheme for a low-power IoT device. The device has limited computational resources, so the engineer wants to minimize the number of rounds in the block cipher. However, the engineer also needs to ensure that the cipher is resistant to differential cryptanalysis. Which design trade-off best balances these constraints?

   • AKeep the algorithm secret to prevent attackers from knowing the round structure.
   • BUse more rounds but with a simpler substitution box to reduce per-round cost.
   • CUse fewer rounds but increase the diffusion layer's branch number to achieve faster avalanche.
   • DUse a longer key to compensate for fewer rounds.
   Reveal Solution Discussion Select an answer first
5. 9application · medium

   A startup is developing a new encryption product and is deciding whether to open-source the algorithm or keep it proprietary. The CTO wants to ensure the product is trusted by security-conscious customers. Which approach best aligns with Kerckhoffs's principle?

   • AKeep the algorithm proprietary to prevent attackers from analyzing it.
   • BOpen-source the algorithm so that its security is based on the key, not on secrecy.
   • COpen-source only the key management, not the algorithm.
   • DKeep the algorithm proprietary but publish a whitepaper describing its design.
   Reveal Solution Discussion Select an answer first
6. 10application · medium

   A network engineer is analyzing a stream cipher used for VPN traffic. The engineer notices that if two different plaintext messages are encrypted with the same key, the resulting ciphertext streams show a similar pattern in the first few bytes. Which principle is being violated, allowing this statistical leakage?

   • AThe avalanche effect, because a single bit change in the key does not change the ciphertext enough.
   • BConfusion, because the key relationship is too linear.
   • CKerckhoffs's principle, because the algorithm is not public.
   • DDiffusion, because the plaintext influence is not spread enough across the ciphertext.
   Reveal Solution Discussion Select an answer first