Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 2)

Questions 6–10

1. 6expert · hard

   A development team is selecting a block cipher mode for a new file encryption feature. The requirements are: (1) support parallel encryption for performance, (2) allow random access to encrypted blocks, (3) minimize error propagation, and (4) avoid the need for padding. Which mode satisfies all requirements?

   • AOFB mode
   • BCTR mode
   • CCFB mode
   • DCBC mode
   Reveal Solution Discussion Select an answer first
2. 7application · medium

   A small company stores encrypted user profile images in a database. Each image is encrypted independently with AES-128 using the same key. An analyst notices that two different users have identical profile pictures, and the resulting ciphertexts are identical. Which mode is most likely being used, and what is the primary risk?

   • ACBC mode; the missing IV causes identical plaintexts to encrypt identically.
   • BCTR mode; the counter is not incremented, so each block encrypts the same keystream block.
   • COFB mode; the keystream is reused because the IV is not unique.
   • DECB mode; identical plaintext blocks produce identical ciphertext blocks, revealing data patterns.
   Reveal Solution Discussion Select an answer first
4. 8foundation · easy

   Which block cipher mode is parallelizable for both encryption and decryption?

   • ACBC
   • BCFB
   • CCTR
   • DOFB
   Reveal Solution Discussion Select an answer first
5. 9expert · hard

   A cloud storage service encrypts each object with AES-CTR. To support random access, the service stores the 64-bit nonce and the starting counter value for each object. A security review finds that the nonce is derived from the object ID, which is sequential. Which statement best describes the risk?

   • AAn attacker who can predict the nonce can cause keystream reuse if the same key is used for multiple objects.
   • BThe counter value must be random to prevent an attacker from predicting the keystream.
   • CThe nonce must be kept secret to prevent an attacker from decrypting the object.
   • DThe risk is low because the nonce is unique for each object ID.
   Reveal Solution Discussion Select an answer first
6. 10application · medium

   A network protocol encrypts each byte of a real-time data stream using CFB mode. During transmission, one ciphertext byte is corrupted by noise. What is the impact on decryption?

   • AOnly the corrupted byte decrypts incorrectly; all subsequent bytes are unaffected.
   • BThe corrupted byte causes the entire block to fail, but the next block decrypts correctly.
   • CAll subsequent bytes decrypt incorrectly until the end of the stream.
   • DThe corrupted byte and the next block-size worth of bytes decrypt incorrectly, then decryption recovers.
   Reveal Solution Discussion Select an answer first