Block Cipher Modes (ECB, CBC, CFB, OFB, CTR) ECES Practice Questions (Page 3)

Questions 11–15

1. 11application · medium

   A legacy payment system encrypts credit card numbers using AES-ECB. Each card number is padded to 16 bytes and encrypted with a static key. Which attack is most feasible?

   • AAn attacker can build a codebook of known plaintext-ciphertext pairs and identify card numbers by matching ciphertext blocks.
   • BAn attacker can flip bits in the ciphertext to change the card number without detection.
   • CAn attacker can recover the key by analyzing the padding.
   • DAn attacker can decrypt the data without the key because ECB is reversible.
   Reveal Solution Discussion Select an answer first
2. 12expert · hard

   A network protocol uses CFB mode to encrypt a stream of packets. Each packet is encrypted independently with a unique IV. An attacker captures two packets that were encrypted with the same key but different IVs. What can the attacker learn?

   • AThe XOR of the two plaintexts, because CFB is a stream cipher.
   • BNothing, because CFB with unique IVs is secure against ciphertext-only attacks.
   • CThe plaintext of both packets, because CFB does not use an IV.
   • DThe encryption key, because CFB is weak.
   Reveal Solution Discussion Select an answer first
4. 13application · medium

   A security engineer is configuring a VPN that uses AES in CTR mode. The documentation warns that the IV/counter must never be reused with the same key. Why is this requirement critical?

   • AReusing the same IV/counter with the same key produces the same keystream, allowing an attacker to recover plaintext via XOR.
   • BReusing the IV/counter causes the block cipher to fail and produce incorrect ciphertext.
   • CReusing the IV/counter makes the encryption reversible, allowing decryption without the key.
   • DReusing the IV/counter is acceptable if the key is changed frequently.
   Reveal Solution Discussion Select an answer first
5. 14foundation · easy

   In Output Feedback (OFB) mode, what is the input to the block cipher for each keystream block?

   • AA counter value that increments for each block
   • BThe previous ciphertext block
   • CThe previous keystream block (or the IV for the first block)
   • DThe previous plaintext block
   Reveal Solution Discussion Select an answer first
6. 15application · medium

   A developer is comparing OFB and CTR modes for a new encryption library. Which statement correctly distinguishes the two modes?

   • AOFB provides authentication; CTR does not.
   • BOFB generates a keystream by repeatedly encrypting the previous keystream block; CTR generates a keystream by encrypting a counter value.
   • COFB requires a unique IV; CTR does not.
   • DOFB is parallelizable; CTR is not.
   Reveal Solution Discussion Select an answer first