Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 1)

Questions 1–5

1. 1foundation · easy

   What is the primary purpose of an initialization vector (IV) in block cipher modes like CBC and CFB?

   • ATo authenticate the ciphertext and ensure integrity
   • BTo ensure that encrypting the same plaintext with the same key produces different ciphertexts when different IVs are used
   • CTo increase the effective key length of the cipher
   • DTo eliminate the need for padding in block cipher modes
   Reveal Solution Discussion Select an answer first
2. 2expert · hard

   A legacy system encrypts a database column using ECB mode. The column contains a small set of possible values (e.g., status codes). An attacker has access to the ciphertext and knows the possible plaintext values. Which attack is most feasible?

   • AExploit the IV reuse to recover the plaintext.
   • BBuild a codebook of known plaintext-ciphertext pairs and match ciphertext blocks to plaintext values.
   • CUse a birthday attack to find a collision in the ciphertext.
   • DPerform a padding oracle attack to recover the key.
   Reveal Solution Discussion Select an answer first
4. 3application · medium

   A software team is choosing a block cipher mode for encrypting a large file that will be stored on a distributed file system. The file is read in random-access chunks, and the team wants to minimize error propagation while allowing parallel encryption of chunks. Which mode is the best fit?

   • ACTR mode with a unique nonce per chunk and a per-chunk counter.
   • BOFB mode with a unique IV per chunk.
   • CCFB mode with a segment size equal to the block size.
   • DCBC mode with a unique IV per chunk.
   Reveal Solution Discussion Select an answer first
5. 4expert · hard

   A security consultant is evaluating a system that encrypts a database of employee records. The system currently uses ECB mode. The consultant must choose a replacement mode that provides confidentiality and allows random access to individual records. The system does not require authentication. Which mode is the best recommendation?

   • ACTR mode, because it allows random access and avoids the pattern leakage of ECB.
   • BCBC mode, because it provides better confidentiality than CTR.
   • COFB mode, because it is self-synchronizing.
   • DCFB mode, because it provides authentication.
   Reveal Solution Discussion Select an answer first
6. 5application · medium

   A security engineer is reviewing a legacy system that encrypts database records with a block cipher in ECB mode. The records are fixed-length and contain a small number of distinct plaintext values. The engineer notices that identical plaintext records produce identical ciphertext, allowing an attacker to infer patterns. Which change would most directly address this weakness while keeping the ability to encrypt records independently?

   • ASwitch to CBC mode with a unique random IV for each record.
   • BSwitch to OFB mode with a fixed IV reused for all records.
   • CKeep ECB but add a random padding to each record before encryption.
   • DUse CTR mode with a counter that resets to zero for each record.
   Reveal Solution Discussion Select an answer first