Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 4)

Questions 16–20

1. 16expert · hard

   A developer is implementing CBC mode and must decide how to handle the IV for multiple messages encrypted with the same key. Which practice is most secure?

   • AUse the previous message's last ciphertext block as the IV for the next message.
   • BGenerate a new random IV for each message and transmit it with the ciphertext.
   • CUse a fixed IV and rely on the key to provide security.
   • DUse a counter as the IV and increment it for each message.
   Reveal Solution Discussion Select an answer first
2. 17expert · hard

   A security architect is evaluating OFB mode for encrypting a high-speed data stream. The system can precompute keystream blocks during idle time to reduce latency. However, the architect is concerned about the risk of keystream reuse if the IV is ever repeated. Which design decision best mitigates this risk?

   • AUse a fixed IV and rely on the precomputed keystream being discarded after each use.
   • BUse a 128-bit random IV for each stream and ensure the key is changed periodically.
   • CUse a random IV but do not change the key, relying on the IV alone for uniqueness.
   • DUse a counter as the IV and increment it for each stream, but allow it to wrap around.
   Reveal Solution Discussion Select an answer first
4. 18expert · hard

   A team is implementing CBC mode with PKCS#7 padding. They are concerned about padding oracle attacks. Which mitigation is most effective?

   • AUse a random IV and always return a generic error message on decryption failure.
   • BUse an authenticated encryption mode such as AES-GCM instead of CBC with padding.
   • CUse CTR mode with PKCS#7 padding.
   • DUse a fixed IV and validate the padding before decryption.
   Reveal Solution Discussion Select an answer first
5. 19foundation · easy

   In Cipher Block Chaining (CBC) mode, what is XORed with the plaintext block before encryption?

   • AThe previous plaintext block
   • BThe previous ciphertext block (or the initialization vector for the first block)
   • CThe output of the block cipher applied to the previous plaintext block
   • DA counter value that increments for each block
   Reveal Solution Discussion Select an answer first
6. 20expert · hard

   A developer is implementing CBC mode and must decide how to handle the IV. The application encrypts many messages with the same key. Which approach is most secure?

   • AGenerate a random IV for each message and transmit it with the ciphertext.
   • BUse a counter as the IV, incrementing it for each message.
   • CUse a fixed IV derived from the key to save bandwidth.
   • DUse the previous message's ciphertext as the IV.
   Reveal Solution Discussion Select an answer first