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Interview question · Cryptography, PKI & TLS

What does forward secrecy give you, and how does Diffie-Hellman provide it?

The question behind the question: if your server's private key leaks next year, what happens to the traffic someone recorded today?

mediumEssentialNewPreparation tags: Google, Apple, AmazonUpdated Nov 2026

The question behind the question: if your server's private key leaks next year, what happens to the traffic someone recorded today?

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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

An attacker has been recording a company's encrypted traffic for two years and has just stolen the web server's private key.

  1. What is forward secrecy?
  2. How does ephemeral Diffie-Hellman provide it?
  3. What if the server used RSA key transport?
  4. What can the attacker still do with the stolen key?
  5. What about session resumption tickets?
  6. How would you assess the damage and harden?

Each rung has a nudge, a model answer, the principle that carries over and the weak answer to avoid. Sign in free to open it.

UP NEXT ON YOUR JOURNEY
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