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Interview question · Cryptography, PKI & TLS

What properties should a cryptographic hash have, and why does a 256-bit hash give only about 128 bits of collision resistance?

Preimage, second preimage and collision resistance sound alike and protect different things. The birthday bound explains why the last one costs half the bits.

easyEssentialNewPreparation tags: Google, Microsoft, AmazonUpdated Nov 2026

Preimage, second preimage and collision resistance sound alike and protect different things. The birthday bound explains why the last one costs half the bits.

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  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.

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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 truncates SHA-256 to its first 32 bits to use as a short ID for uploaded documents, and as the key in a deduplication cache.

  1. What properties should a cryptographic hash have?
  2. What is the birthday bound?
  3. What goes wrong in the deduplication cache?
  4. What length should IDs and keys use?
  5. Should a hash ID also grant access?
  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 trackDesign an internal PKI for tens of thousands of services, devices and developers: hierarchy, issuance, revocation and failure.Next in this trackWhat is crypto agility, how do downgrade attacks exploit it, and how would you plan a migration to post-quantum algorithms?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.
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