Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 8)

Questions 36–38

1. 36application · medium

   A cloud storage service encrypts each file with AES-CTR. The service uses a random 96-bit nonce and a 32-bit counter. What is the maximum number of blocks that can be encrypted with a single nonce/key pair?

   • A2^128 blocks
   • BUnlimited, because the counter can be extended.
   • C2^32 blocks
   • D2^96 blocks
   Reveal Solution Discussion Select an answer first
2. 37foundation · easy

   Why is CFB mode considered a self-synchronizing stream cipher?

   • ABecause it can recover from a lost or corrupted ciphertext block after a few blocks, as long as the receiver has the correct key
   • BBecause it uses a public key to synchronize the sender and receiver
   • CBecause it uses a fixed keystream that does not depend on the plaintext or ciphertext
   • DBecause it automatically pads the plaintext to a multiple of the block size
   Reveal Solution Discussion Select an answer first
3. 38application · medium

   A developer chooses OFB mode for encrypting a satellite downlink. The receiver occasionally loses a chunk of data. What is the main consequence of using OFB in this scenario?

   • AThe lost chunk causes a one-block error, but the next block decrypts correctly.
   • BThe keystream becomes unsynchronized, and all subsequent decryption fails until the keystream is re-synchronized.
   • CThe lost chunk only affects the corresponding plaintext, and decryption continues normally.
   • DThe receiver can recover after the lost chunk because OFB self-synchronizes from the received ciphertext.
   Reveal Solution Discussion Select an answer first