Modern Cryptography · 6.4
⚛️ Post-Quantum Cryptography
Why quantum computers threaten RSA and ECC — and what replaces them⏱ ~3 min
The Quantum Threat
★ FactIn 1994, Peter Shor showed that a sufficiently large quantum computer could factor large integers exponentially faster than any classical algorithm. This means RSA, Diffie-Hellman, and ECC — which all rely on hard problems solvable by Shor's algorithm — would be broken by a large quantum computer.
| Algorithm | Classical Security | Quantum Security | Fate |
|---|---|---|---|
| AES-128 | 128 bits | ~64 bits (Grover's algorithm) | Upgrade to AES-256 for quantum resistance |
| AES-256 | 256 bits | ~128 bits | ✓ Sufficient — still secure |
| SHA-256 | 256 bits (collision: 128) | ~128 bits (Grover) | ✓ Still acceptable |
| RSA-2048 | ~112 bits | Broken (Shor's algorithm) | ❌ Must migrate |
| ECC P-256 | ~128 bits | Broken (Shor's algorithm) | ❌ Must migrate |
Harvest Now, Decrypt Later
⚠ Warning'Harvest Now, Decrypt Later' (HNDL): nation-state adversaries are recording encrypted internet traffic today, storing it, planning to decrypt it once quantum computers are large enough. Data that must remain secret for 10+ years (medical records, state secrets, long-term financial data) is at risk RIGHT NOW, even though practical quantum computers don't exist yet. This is why migration to post-quantum algorithms is urgent — not theoretical.
NIST Post-Quantum Cryptography Standards (2024)
In August 2024, NIST finalized the first post-quantum cryptography standards. These algorithms are based on mathematical problems believed to be hard for both classical and quantum computers:
| Standard | Algorithm | Type | Based On |
|---|---|---|---|
| FIPS 203 | ML-KEM (Kyber) | Key Encapsulation (replaces ECDH) | Module lattice learning-with-errors (LWE) |
| FIPS 204 | ML-DSA (Dilithium) | Digital Signature (replaces ECDSA) | Module lattice |
| FIPS 205 | SLH-DSA (SPHINCS+) | Digital Signature (hash-based) | Hash function security only |
🔒 SecurityTLS 1.3 already supports hybrid key exchange (X25519+Kyber768) — combining classical and post-quantum algorithms so that you need to break BOTH to decrypt the traffic. Chrome, Firefox, and Cloudflare have supported hybrid PQ TLS since 2023. If you're deploying new infrastructure today, choose libraries and configurations that support PQ-hybrid key exchange.
🔗NIST Post-Quantum Cryptography Standards
Official NIST PQC project — finalized standards, migration guidance, and algorithm details
https://csrc.nist.gov/projects/post-quantum-cryptography
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