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Interview question · Cryptography, PKI & TLS

How and why do you derive multiple keys from one secret with a KDF such as HKDF?

Using one key for several purposes quietly couples their security. The scored answer explains extract and expand, shows key separation by context, and says when HKDF is the wrong tool.

mediumNewPreparation tags: Google, Amazon, MicrosoftUpdated Nov 2026

Using one key for several purposes quietly couples their security. The scored answer explains extract and expand, shows key separation by context, and says when HKDF is the wrong tool.

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  1. 01 State the assumptions your answer depends on.
  2. 02 Explain the mechanism and choose a control.
  3. 03 Name a condition that would change your decision.

The worked answer includes an approach, scoring criteria and follow-up questions. Compare the reasoning with your own.

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What if · the follow-ups in this answer

The answer works through 6 follow-ups like an interviewer would

A service needs separate encryption and MAC keys from one shared secret, and per-tenant keys from a master secret.

  1. Why derive separate keys at all?
  2. How does HKDF work?
  3. What should the info string contain?
  4. Can HKDF turn a password into a key?
  5. What if the master secret leaks?
  6. How would you design key derivation?

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