Cryptographic Principles (Diffusion, Confusion, Kerckhoff Principle) ECES Practice Questions (Page 1)

Questions 1–5

1. 1foundation · easy

   According to Kerckhoffs's principle, what is the only part of a cryptosystem that must be kept secret?

   • AThe ciphertext format
   • BThe plaintext format
   • CThe encryption algorithm
   • DThe key
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2. 2expert · hard

   A cryptographer is designing a new block cipher and must balance two goals: resistance to linear cryptanalysis (which exploits linear relationships between plaintext, ciphertext, and key) and resistance to differential cryptanalysis (which exploits how differences in plaintext propagate to differences in ciphertext). Which combination of design choices best addresses both?

   • AIncrease the block size to 256 bits without changing the round function.
   • BUse a single round with a strong S-box to minimize latency.
   • CUse only linear transformations to keep the cipher fast and simple.
   • DUse non-linear S-boxes to increase confusion and a permutation layer to increase diffusion.
   Reveal Solution Discussion Select an answer first
4. 3application · medium

   A software developer is implementing a block cipher and wants to test the avalanche effect. The developer encrypts a plaintext block, then flips a single bit in the plaintext and encrypts again. The two ciphertexts are compared. What result indicates good diffusion?

   • AOnly the corresponding bit in the ciphertext changes.
   • BApproximately half of the ciphertext bits change.
   • CThe key changes, but the ciphertext stays the same.
   • DThe ciphertext remains identical.
   Reveal Solution Discussion Select an answer first
5. 4application · medium

   A government agency is considering using a classified encryption algorithm for sensitive data. The algorithm's design is kept secret. A security auditor argues that this violates a fundamental cryptographic principle. What is the auditor's main concern?

   • AThe algorithm likely lacks confusion, so key recovery is easy.
   • BIf the algorithm is ever leaked, the security of the system is completely compromised because it relies on secrecy of the algorithm rather than the key.
   • CThe algorithm's key length is probably too short.
   • DThe algorithm likely lacks diffusion, so plaintext patterns will appear in ciphertext.
   Reveal Solution Discussion Select an answer first
6. 5application · medium

   A cryptanalyst is attempting a chosen-plaintext attack on a new cipher. The analyst notices that the ciphertext reveals a linear relationship with the key bits, allowing the key to be solved as a system of linear equations. Which principle is the cipher failing to implement?

   • ADiffusion, because the plaintext bits are not spread enough.
   • BThe avalanche effect, because a single bit change in the key does not change the ciphertext enough.
   • CKerckhoffs's principle, because the algorithm is not public.
   • DConfusion, because the key-ciphertext relationship is not sufficiently non-linear.
   Reveal Solution Discussion Select an answer first