Cryptographic Principles (Diffusion, Confusion, Kerckhoff Principle) ECES Practice Questions (Page 7)

Questions 31–35

1. 31application · medium

   A startup is building a secure messaging app. The lead engineer argues that the encryption algorithm should be kept secret to add an extra layer of protection. A senior security consultant disagrees, citing Kerckhoffs's principle. Which statement best reflects the consultant's position?

   • AThe algorithm should be changed frequently to prevent cryptanalysis.
   • BThe algorithm should be kept secret to prevent attackers from analyzing it.
   • CThe security of the system should rely only on the secrecy of the key, not on the secrecy of the algorithm.
   • DThe key should be derived from the algorithm to ensure compatibility.
   Reveal Solution Discussion Select an answer first
2. 32expert · hard

   A security team is evaluating two ciphers for a high-security application. Cipher A has a very complex substitution box but weak diffusion. Cipher B has strong diffusion but a simple linear substitution. The team wants to minimize the risk of both statistical attacks and key recovery attacks. Which cipher should they choose, and why?

   • AEither, because Kerckhoffs's principle ensures security regardless of the internal structure.
   • BNeither, because both lack a balance of confusion and diffusion.
   • CCipher B, because strong diffusion is more important than confusion.
   • DCipher A, because strong confusion is more important than diffusion.
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4. 33foundation · easy

   What is the main goal of confusion in a cipher?

   • ATo make the relationship between the ciphertext and the encryption key as complex and obscure as possible
   • BTo ensure that the ciphertext is the same length as the plaintext
   • CTo make the encryption and decryption processes identical
   • DTo spread the influence of each plaintext bit across many ciphertext bits
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5. 34application · medium

   A security team is reviewing a proprietary cipher that uses a simple linear transformation. An analyst points out that the cipher is vulnerable to a known-plaintext attack because the key can be recovered by solving linear equations. Which principle is the cipher violating?

   • ADiffusion, because the plaintext bits are not spread enough.
   • BKerckhoffs's principle, because the algorithm is proprietary.
   • CAvalanche effect, because a small change in plaintext does not cause a large change in ciphertext.
   • DConfusion, because the relationship between the key and ciphertext is too simple and linear.
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6. 35expert · hard

   A cryptanalyst is analyzing a cipher and finds that the ciphertext bits can be expressed as a linear function of the key bits and plaintext bits. This allows the analyst to recover the key with moderate computational effort. Which principle is the cipher violating, and what design change would most directly address the vulnerability?

   • AViolates confusion; introduce non-linear substitution boxes (S-boxes) into the round function.
   • BViolates the avalanche effect; increase the key length.
   • CViolates diffusion; increase the number of permutation rounds.
   • DViolates Kerckhoffs's principle; keep the algorithm secret.
   Reveal Solution Discussion Select an answer first