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Interview question · Cryptography, PKI & TLS

Compress then encrypt, or encrypt then compress? Why does it matter for security?

Encryption hides content, not length. The scored answer shows how compression turns length into a leak when secrets and attacker text share a message, and where that bites today.

mediumNewPreparation tags: Google, Salesforce, MicrosoftUpdated Nov 2026

Encryption hides content, not length. The scored answer shows how compression turns length into a leak when secrets and attacker text share a message, and where that bites today.

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

An app compresses data that contains a session secret and attacker-supplied text before encrypting it, to save bandwidth.

  1. Why compress before encrypting?
  2. What does compression leak?
  3. What attacks used this?
  4. How do you prevent it?
  5. Where else might this pattern appear?
  6. How would you fix the design?

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 trackYou are reviewing code that implements its own encryption. What do you look for?Next in this trackWhere do randomness failures come from, and how have they broken real systems?Next in this trackWhen do you need a hardware security module rather than a cloud KMS, and what does each protect against?
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