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CISSP study material · question 30 of 500

A protocol team needs a derivation step that emits 512 bits of keying material now and may need 1024 bits for a later revision, produced by a single primitive invocation rather than by concatenating several digests. The design is restricted to functions specified in FIPS 202. Which function meets the requirement?

  1. SHA3-512, a fixed-width digest that can be repeated internally to whatever length is requested.
  2. SHA3-224, the cheapest fixed-width digest, invoked once for each output segment requested.
  3. SHA3-256, invoked twice with distinct domain separators to reach whatever length is requested.
  4. SHAKE256, an extendable-output function whose result can be produced at whatever length is requested.
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Answer: D. SHAKE256, an extendable-output function whose result can be produced at whatever length is requested.

SHAKE128 and SHAKE256 are the extendable-output functions in FIPS 202; the SHA3-224 through SHA3-512 members emit fixed widths and need concatenation tricks to stretch.

Source: NIST FIPS 202 (NIST) — FIPS 202 > Abstract

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