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Case AC9A26 · Cryptography · L3 Applied

Post-Quantum Cryptography

Practise as: Explain it · Interview · Deep dive

Interview questionWhat does quantum computing break in today's crypto, and how are we migrating?

  1. 01 What is it?
  2. 02 How is it abused?
  3. 03 How do you stop it?
Practise this card As Interview As Deep dive
What a strong answer covers

Try it out loud first. Then check yourself:

  1. Shor's algorithm breaks RSA, DH and ECC; Grover only halves symmetric strength, so AES-256 stays safe.
  2. NIST standardized ML-KEM (FIPS 203, Kyber), ML-DSA (FIPS 204, Dilithium) and SLH-DSA (FIPS 205, SPHINCS+).
  3. Harvest-now-decrypt-later makes key exchange the urgent priority, ahead of signatures.
  4. Hybrid key exchange (X25519MLKEM768) is already deployed in TLS by major browsers and CDNs.
  5. Migration requires crypto inventory and crypto agility; PQC keys and signatures are much larger.

If the interviewer pushes back

  • Why deploy hybrid classical+PQC key exchange instead of pure ML-KEM, and what does it cost in handshake size?

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