Key Takeaways: Chapter 30 — Quantum Cryptography and Post-Quantum Security
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Shor's algorithm breaks RSA and ECC by providing polynomial-time factoring and discrete logarithms. The question is when, not if, sufficiently large quantum computers will be built. Quantum is linear algebra, not magic — Shor's algorithm is a clever application of the quantum Fourier transform to period finding.
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"Harvest now, decrypt later" means data with long-term sensitivity is already at risk. Organizations must act now, not when quantum computers arrive.
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NIST has standardized post-quantum algorithms based on diverse mathematical foundations: lattices (Kyber, Dilithium), hashes (SPHINCS+), and codes (McEliece). The diversity is intentional — if one mathematical assumption falls, others may stand.
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Lattice-based cryptography (Learning With Errors) is the most versatile PQC family, offering both KEMs and signatures with reasonable key sizes. The worst-case to average-case reduction gives strong security guarantees.
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QKD provides information-theoretic security but has practical limitations: distance, key rate, authentication requirements, and hardware vulnerabilities. It complements, but does not replace, post-quantum cryptography.
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Hybrid cryptography (classical + post-quantum) is the pragmatic bridge during the transition, providing security as long as either scheme remains unbroken.
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The transition is urgent and massive. Every cryptographic system must be inventoried, prioritized, and migrated. Crypto-agility is essential. We're at the beginning — PQC deployment will be a multi-decade process.