KDFs & Password Hashing Checkpoint

Worked scenarios: trace state, diagnose mistakes, and apply the module contract.

Get every question correct to pass this checkpoint. You can review feedback, revise your answers, and submit again.

1. A uniformly chosen 4-digit PIN is expanded with HKDF and a public salt to 64 bytes. An attacker can check guesses. Which claim is justified?

Select one answer.

The 64-byte output guarantees 512-bit guessing security The public salt makes every PIN guess untestable At most 10,000 PIN candidates still suffice; output length does not create secret entropy HKDF forces memory-hard work for each guess

2. How many SHA-256 HKDF expansion blocks are required for 65 output bytes?

3. HKDF receives L=8161 with SHA-256 and this exercise's API. What should it return?

Select one answer.

nil, because 256 blocks would exceed the one-byte counter range 8161 bytes after counter wraps to zero The first 8160 bytes silently An empty PRK with length 32

4. PBKDF2-SHA256 derives 48 bytes using five iterations per block. How many HMAC evaluations are required?

5. A protocol derives two directional keys with the same PRK, identical info, and identical lengths. What happens?

Select one answer.

HKDF silently adds a direction byte Keys differ because calls automatically randomize HKDF Only their last bytes coincide Both keys are identical; distinct contexts need distinct unambiguous labels

6. For identical PRK/info, is the 16-byte HKDF result the prefix of the 32-byte result? Answer yes or no.

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