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Interview question · Cryptography, PKI & TLS

Why is HMAC built the way it is, rather than simply hashing the key followed by the message?

A classic that tests whether you know how hash functions work inside. The scored answer explains length extension, the nested HMAC construction, and how to verify a MAC safely.

mediumNewPreparation tags: Google, Amazon, MicrosoftUpdated Nov 2026

A classic that tests whether you know how hash functions work inside. The scored answer explains length extension, the nested HMAC construction, and how to verify a MAC safely.

Make your first attempt

What would you say before reading the answer?

  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 developer builds a message authentication code for API requests as SHA-256(key + message) and asks why a reviewer flagged it.

  1. What is a length-extension attack?
  2. Why does that break this MAC?
  3. How does HMAC fix it?
  4. Are there other options?
  5. What else matters when verifying tags?
  6. What would you tell the developer?

Each rung has a nudge, a model answer, the principle that carries over and the weak answer to avoid. Premium opens every answer and ladder.

UP NEXT ON YOUR JOURNEY
Next in this trackCompress then encrypt, or encrypt then compress? Why does it matter for security?Next in this trackYou are reviewing code that implements its own encryption. What do you look for?Next in this trackWhere do randomness failures come from, and how have they broken real systems?
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