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Interview question · Cryptography, PKI & TLS

How 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?

Domain-validated certificates prove one thing: whoever asked could answer a challenge on that domain at that moment. Every attack on issuance is a way to answer that challenge without owning the domain.

mediumNewPreparation tags: Google, Amazon, MicrosoftUpdated Nov 2026

Domain-validated certificates prove one thing: whoever asked could answer a challenge on that domain at that moment. Every attack on issuance is a way to answer that challenge without owning the domain.

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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

A team automates TLS certificates with ACME for hundreds of internal and public hostnames.

  1. How does ACME issuance work?
  2. Which challenge types exist?
  3. What is the risk with DNS-01 automation?
  4. What about the ACME account key?
  5. What should internal hostnames use?
  6. How would you run ACME at scale?

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 should mutual TLS terminate (app, load balancer, sidecar), and how does the client's identity reach the application safely?Next in this trackHow does an HSM actually protect keys: non-extractable attributes, PKCS#11 sessions, FIPS 140-3 levels, key ceremonies and backup?Next in this trackCompare hardware roots of trust: TPM, TrustZone, SGX and confidential VMs, Secure Enclave, secure elements and HSMs. What does each stop?
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