Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 5)

Questions 21–25

1. 21foundation · easy

   In Counter (CTR) mode, what is encrypted to produce the keystream block that is XORed with the plaintext?

   • AThe previous ciphertext block
   • BThe plaintext block itself
   • CThe previous keystream block
   • DA counter value that is incremented for each block
   Reveal Solution Discussion Select an answer first
2. 22application · medium

   A file-encryption tool uses AES-CBC. During decryption, a single bit in ciphertext block N is corrupted. Which plaintext blocks are affected?

   • ABlock N is completely garbled, and block N+1 has a single bit flipped in the same position.
   • BAll subsequent blocks are garbled until the end of the file.
   • CBlock N has a single bit flipped, and block N+1 is completely garbled.
   • DOnly block N is garbled; all other blocks decrypt correctly.
   Reveal Solution Discussion Select an answer first
4. 23foundation · easy

   Which security weakness is most characteristic of ECB mode?

   • AIdentical plaintext blocks produce identical ciphertext blocks, revealing data patterns.
   • BThe mode requires a unique nonce for every message to avoid keystream reuse.
   • CA single-bit error in one ciphertext block corrupts all subsequent plaintext blocks.
   • DThe mode cannot encrypt data that is not a multiple of the block size without padding.
   Reveal Solution Discussion Select an answer first
5. 24foundation · easy

   What is the primary purpose of the initialization vector (IV) in CBC mode?

   • ATo allow the block cipher to encrypt data of any length without padding
   • BTo authenticate the ciphertext and detect tampering
   • CTo increase the effective key length of the block cipher
   • DTo ensure that encrypting the same plaintext with the same key produces different ciphertext when a different IV is used
   Reveal Solution Discussion Select an answer first
6. 25application · medium

   A high-throughput encryption service must encrypt multiple independent data blocks in parallel on a multi-core server. Which mode is best suited for this requirement?

   • ACFB mode, because it generates keystream from the previous ciphertext block.
   • BOFB mode, because it generates a keystream that can be precomputed.
   • CCBC mode, because each block depends on the previous ciphertext block.
   • DCTR mode, because each block's keystream depends only on the counter value and key.
   Reveal Solution Discussion Select an answer first