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Interview question · Cryptography, PKI & TLS

You are reviewing code that implements its own encryption. What do you look for?

Most cryptographic failures are misuse of good primitives, not broken algorithms. The scored answer has a checklist that catches the common ones and a strong default to replace them.

mediumNewPreparation tags: Google, Microsoft, AmazonUpdated Nov 2026

Most cryptographic failures are misuse of good primitives, not broken algorithms. The scored answer has a checklist that catches the common ones and a strong default to replace them.

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

You are reviewing code that encrypts customer data with a custom wrapper around a low-level crypto library.

  1. What is the first question to ask?
  2. Which mode mistakes do you look for?
  3. What about nonces, keys and randomness?
  4. Which home-made constructions are red flags?
  5. What about operations around the crypto?
  6. What would your review recommend?

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 trackWhere do randomness failures come from, and how have they broken real systems?Next in this trackWhen do you need a hardware security module rather than a cloud KMS, and what does each protect against?Next in this trackDesign encryption and key custody for high-value model weights.
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