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Interview question · Cryptography, PKI & TLS

Design an internal PKI for tens of thousands of services, devices and developers: hierarchy, issuance, revocation and failure.

Issuing a certificate is easy. Knowing every certificate you issued, renewing them before they expire, and surviving the day a CA key leaks is the design.

expertNewPreparation tags: Google, Microsoft, AmazonUpdated Nov 2026

Issuing a certificate is easy. Knowing every certificate you issued, renewing them before they expire, and surviving the day a CA key leaks is the design.

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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 must design an internal PKI for service-to-service TLS across thousands of workloads, laptops and a code-signing pipeline.

  1. How should the root CA be handled?
  2. How should issuing CAs be separated?
  3. How should certificates be issued?
  4. How long should certificates last?
  5. What operational pieces are needed?
  6. What PKI design would you propose?

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 trackWhat is crypto agility, how do downgrade attacks exploit it, and how would you plan a migration to post-quantum algorithms?Next in this trackWalk through a TLS 1.2 handshake message by message, then explain exactly what TLS 1.3 changed and why each change matters for security.Next in this trackTake a TLS certificate apart field by field. Which fields and extensions do validators actually check, and which mistakes do they catch?
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