Cryptanalysis and Attacks CEH Practice Questions (Page 1)

Questions 1–5

1. 1foundation · easy

   Which countermeasure is commonly used to prevent replay attacks?

   • AUsing a different cipher mode
   • BUsing a nonce or timestamp in each message
   • CUsing a longer encryption key
   • DUsing a stronger hash function
   Reveal Solution Discussion Select an answer first
2. 2application · medium

   A security analyst suspects that a legacy encryption system uses a simple XOR cipher with a repeating key. The analyst has captured a plaintext message and its corresponding ciphertext. Which attack would be most effective to recover the key?

   • AConduct a chosen-ciphertext attack by submitting modified ciphertexts to the system.
   • BUse a rainbow table to reverse the ciphertext back to the plaintext.
   • CXOR the known plaintext with the ciphertext to derive the keystream, then analyze the repeating pattern to determine the key length.
   • DPerform a brute-force attack by trying every possible key length from 1 to the message length.
   Reveal Solution Discussion Select an answer first
4. 3expert · hard

   A security researcher is analyzing a smart card that performs RSA decryption. The researcher has physical access to the card and can measure power consumption during decryption operations. The researcher also has the ability to submit chosen ciphertexts to the card. Which combination of attacks would be most effective to recover the private key?

   • AUse a timing attack alone, since power analysis is not effective against RSA.
   • BCombine a chosen-ciphertext attack with a power analysis side-channel attack.
   • CUse a fault injection attack to cause the card to output the private key directly.
   • DPerform a brute-force attack on the RSA key, which is feasible for 1024-bit keys.
   Reveal Solution Discussion Select an answer first
5. 4application · medium

   A cryptanalyst is analyzing a system that uses double encryption with two independent 56-bit keys (effective key length 112 bits). The analyst has access to a known plaintext-ciphertext pair. Which attack would reduce the effective key space to approximately 2^57 operations?

   • ABirthday attack to find a collision in the intermediate values.
   • BBrute-force attack trying all 2^112 possible key combinations.
   • CMeet-in-the-middle attack, encrypting the plaintext with all possible first keys and decrypting the ciphertext with all possible second keys, then matching intermediate values.
   • DRelated-key attack exploiting the relationship between the two keys.
   Reveal Solution Discussion Select an answer first
6. 5application · medium

   A cryptanalyst is testing a custom encryption protocol and has access to a decryption oracle that returns the plaintext for any ciphertext submitted, except for the specific challenge ciphertext. Which attack should the cryptanalyst use to recover the plaintext of the challenge?

   • APerform a brute-force attack over all possible keys.
   • BSubmit a modified version of the challenge ciphertext to the oracle and use the resulting plaintext to deduce the original.
   • CApply a birthday attack to find a collision in the ciphertext.
   • DUse a known-plaintext attack with previously captured pairs.
   Reveal Solution Discussion Select an answer first