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Interview question · Cryptography, PKI & TLS

What is crypto agility, how do downgrade attacks exploit it, and how would you plan a migration to post-quantum algorithms?

Being able to switch algorithms is essential. Letting an attacker choose which algorithm you switch to is a vulnerability with a long history.

hardNewPreparation tags: Google, Microsoft, AppleUpdated Nov 2026

Being able to switch algorithms is essential. Letting an attacker choose which algorithm you switch to is a vulnerability with a long history.

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 service still supports old protocol versions and cipher suites for compatibility with a few legacy clients.

  1. What is crypto agility?
  2. How does negotiation become an attack surface?
  3. How do protocols resist downgrade?
  4. What about the legacy clients?
  5. How do you know what is in use?
  6. What would you recommend?

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 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?Next in this trackWho decides which root CAs your devices trust? Explain root programs, trust stores per runtime, distrust events and root expiry.
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