Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 6)

Questions 26–30

1. 26expert · hard

   A security architect is selecting a block cipher mode for a low-power IoT device that transmits short, latency-sensitive messages over an unreliable radio link. The device cannot afford a MAC, but the application can tolerate occasional corrupted messages being dropped. The messages are variable length and often not multiples of the block size. Which mode is the best choice?

   • ACTR mode, because it is parallelizable and does not require padding.
   • BOFB mode, because it is a synchronous stream cipher.
   • CCBC mode, because it provides better error propagation.
   • DCFB mode, because it is self-synchronizing and does not require padding.
   Reveal Solution Discussion Select an answer first
2. 27expert · hard

   A satellite communication system uses CFB mode with a segment size of 8 bits for encrypting telemetry. The link occasionally drops bits, causing the receiver to lose synchronization. The system can tolerate a small amount of corrupted data but must recover quickly. Which property of CFB mode is most relevant to this requirement?

   • ACFB is a synchronous stream cipher, so any bit slip causes permanent loss of synchronization.
   • BCFB requires the receiver to request retransmission of the entire message after any bit slip.
   • CCFB uses a counter that can be reset to re-synchronize, but this requires a new IV.
   • DCFB is self-synchronizing: after a bit slip, the receiver recovers once the corrupted segment shifts out of the shift register.
   Reveal Solution Discussion Select an answer first
4. 28application · medium

   A forensic analyst is examining a database that was encrypted with a block cipher in ECB mode. The analyst notices that the ciphertext contains many repeated 16-byte blocks. What can the analyst infer?

   • AThe encryption key is weak and should be replaced.
   • BThe plaintext contains repeated 16-byte sequences, and the encryption does not hide data patterns.
   • CThe ciphertext is corrupted and needs to be decrypted again.
   • DThe IV was reused across multiple records.
   Reveal Solution Discussion Select an answer first
5. 29application · medium

   An auditor reviews a legacy system that encrypts database columns using ECB mode. The auditor recommends switching to CBC. What is the primary security improvement?

   • ACBC provides authentication, ensuring data integrity.
   • BCBC is faster than ECB because it can be parallelized.
   • CCBC uses an IV and chaining, so identical plaintext blocks produce different ciphertext blocks.
   • DCBC eliminates the need for padding.
   Reveal Solution Discussion Select an answer first
6. 30application · medium

   A storage system encrypts fixed-size 4 KB blocks using CTR mode. To maximize throughput on a multi-core server, the design uses a unique 64-bit nonce per block and a 64-bit counter. Which statement about this design is correct?

   • ACTR mode propagates a single-bit error in a ciphertext block to all subsequent blocks in the same 4 KB block.
   • BCTR mode requires the nonce to be kept secret to prevent an attacker from predicting the keystream.
   • CCTR mode allows each block to be encrypted independently, so the 4 KB blocks can be processed in parallel across cores.
   • DCTR mode requires padding to the block size because it is a block cipher mode.
   Reveal Solution Discussion Select an answer first