Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 7)

Questions 31–35

1. 31foundation · easy

   Why is CTR mode considered parallelizable?

   • ABecause it uses a public key algorithm that supports parallel processing.
   • BBecause each keystream block is generated independently from a counter value, so multiple blocks can be encrypted simultaneously.
   • CBecause it uses a fixed keystream that is precomputed for all messages.
   • DBecause it does not require an initialization vector.
   Reveal Solution Discussion Select an answer first
2. 32application · medium

   A developer is encrypting a 30-byte file with AES-256-CBC. The block size is 16 bytes. How many bytes of padding will be added, and why?

   • A16 bytes, because padding is always a full block.
   • B0 bytes, because CBC does not require padding.
   • C1 byte, because the file is not a multiple of 16.
   • D2 bytes, because 30 mod 16 = 14, so 2 bytes are needed to reach the next block boundary.
   Reveal Solution Discussion Select an answer first
4. 33foundation · easy

   In which mode does a single-bit error in a ciphertext block corrupt the corresponding plaintext bit and also the first bit of the next plaintext block?

   • AOFB
   • BCBC
   • CCTR
   • DECB
   Reveal Solution Discussion Select an answer first
5. 34application · medium

   A real-time sensor network uses CFB mode to encrypt variable-length messages. The protocol does not include any authentication mechanism. What is the most important security limitation of this design?

   • ACFB mode is vulnerable to keystream reuse if the IV is not secret.
   • BCFB mode leaks the plaintext length because it does not pad messages.
   • CAn attacker can flip bits in the ciphertext, which causes predictable bit flips in the plaintext.
   • DCFB mode is vulnerable to replay attacks because it does not use an IV.
   Reveal Solution Discussion Select an answer first
6. 35expert · hard

   A cryptographic library is being hardened. The library currently allows the caller to supply the IV for CBC and CTR modes. A security audit recommends that the library enforce certain IV requirements. Which requirement is correct for both CBC and CTR?

   • ACBC requires an unpredictable IV; CTR requires a unique nonce/counter pair.
   • BBoth CBC and CTR require the IV to be kept secret.
   • CCBC requires a unique IV; CTR requires an unpredictable IV.
   • DBoth CBC and CTR require a random IV of the same length as the block size.
   Reveal Solution Discussion Select an answer first