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Interview question · Cryptography, PKI & TLS

Take a TLS certificate apart field by field. Which fields and extensions do validators actually check, and which mistakes do they catch?

A certificate is a signed statement with a short list of claims: this key, these names, these uses, this period, issued by this CA. Every validation failure you have seen is one of those claims not holding.

mediumEssentialNewPreparation tags: Google, Microsoft, AmazonUpdated Nov 2026

A certificate is a signed statement with a short list of claims: this key, these names, these uses, this period, issued by this CA. Every validation failure you have seen is one of those claims not holding.

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 partner gives you a certificate for api.partner.example and asks why your client rejects it.

  1. What does an X.509 certificate bind?
  2. Which name field matters for hostname checks?
  3. What else might cause rejection?
  4. What does basicConstraints do?
  5. How would you diagnose it quickly?
  6. What would you tell the partner?

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 trackWho decides which root CAs your devices trust? Explain root programs, trust stores per runtime, distrust events and root expiry.Next in this trackHow does a CA verify you control a domain before issuing a certificate (ACME, HTTP-01, DNS-01, TLS-ALPN-01), and how is that subverted?Next in this trackWhere should mutual TLS terminate (app, load balancer, sidecar), and how does the client's identity reach the application safely?
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