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CISSP study material · question 27 of 500

A payments platform encrypts stored card tokens with AES-128. An architect proposes migrating to AES-256 so that each encrypted block becomes larger, cutting the number of blocks and the padding overhead on every record. How should the security engineer assess this proposal?

  1. AES-256 transforms 256-bit blocks only when a chaining mode of operation is selected.
  2. AES transforms 128-bit blocks at every key length, so block counts and padding overhead are unchanged.
  3. AES-256 transforms 256-bit blocks, so the same plaintext needs roughly half as many blocks.
  4. The mode of operation sets the block width, so a chaining mode with AES-256 doubles it.
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Answer: B. AES transforms 128-bit blocks at every key length, so block counts and padding overhead are unchanged.

AES fixes the block at 128 bits for all key lengths; the suffix names key size only. Longer keys add rounds, leaving padding and block counts unchanged.

Source: NIST FIPS 197 (NIST) — FIPS 197 > Abstract

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