Which of the following encryption modes can make protocols without integrity protection even more susceptible to replay attacks, since each block gets decrypted in exactly the same way?

Question

Options

• ACipher feedback mode
• BCipher block chaining mode
• COutput feedback mode
• DElectronic codebook mode

• A
  6% (2)
• B
  3% (1)
• C
  18% (6)
• D
  73% (24)

Explanation

Electronic Codebook (ECB) mode encrypts every block independently using only the key - identical plaintext blocks always produce identical ciphertext blocks, with no relationship between blocks. This deterministic, stateless behavior means an attacker can capture and replay any ciphertext block and it will decrypt to the same plaintext every time, with no context to reveal tampering.

Why the distractors are wrong:

• CBC (B) XORs each plaintext block with the previous ciphertext block before encrypting, so blocks are interdependent - replaying a single block corrupts all subsequent blocks.
• CFB (A) feeds previous ciphertext back into the cipher to generate the next keystream segment - same chaining dependency as CBC, so isolated block replay breaks decryption.
• OFB (C) generates a keystream from the IV/previous output, independent of plaintext - while it has weaknesses, the keystream is positionally tied to the IV and sequence, not a static per-block mapping like ECB.

Memory tip: Think "ECB = Every Cipher Block is isolated" - no chain, no feedback, no memory. Isolation = determinism = replay heaven for attackers.

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