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Interview question · Cryptography, PKI & TLS

Design mutual TLS between internal services. How does its trust model differ from the web PKI?

mTLS authenticates both ends, but only as well as your private CA and identity scheme. The scored answer designs issuance, identity, rotation and authorisation, and contrasts it with the public web.

hardEssentialNewPreparation tags: Google, Amazon, MicrosoftUpdated Nov 2026

mTLS authenticates both ends, but only as well as your private CA and identity scheme. The scored answer designs issuance, identity, rotation and authorisation, and contrasts it with the public web.

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

Two internal services, orders and ledger, authenticate each other with mutual TLS using certificates from an internal CA, and ledger accepts any valid certificate.

  1. What does mutual TLS give you?
  2. What is wrong with accepting any valid certificate?
  3. How should identities be encoded and checked?
  4. How should certificates be issued?
  5. What if a service is compromised?
  6. What would you change?

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 problem does Certificate Transparency solve, and how would you use it defensively?Next in this trackWhy is certificate revocation hard, and what do CRLs, OCSP and short-lived certificates each trade off?Next in this trackWhy is HMAC built the way it is, rather than simply hashing the key followed by the message?
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