| Security Type | Basis | Quantum computer breaks it? |
|---|
| RSA-2048 | Factoring | YES — Shor's algorithm |
| ECDSA P-256 | Discrete log | YES — Shor's algorithm |
| AES-256 | Brute force | REDUCED — Grover (128-bit eff.) |
| ML-KEM (Kyber) | Lattice | BELIEVED NO — unproven |
| BB84 QKD | Physics | NO — information-theoretic |
The no-cloning theorem (Wootters & Zurek, 1982) states that an unknown quantum state cannot be perfectly copied without disturbing the original.
Eve's only option: measure the photon and resend a new one.
But she does not know Alice's basis. If she guesses wrong:
• She collapses the photon to her basis
• She resends in her measured state
• Bob, using Alice's correct basis, gets a random result 50% of the time
• This produces a ~25% QBER when Eve intercepts all photons
Even with a quantum computer, Eve cannot do better.
The security is guaranteed by physics, not by computation.
REAL-WORLD LIMITATIONS
✗ Distance: ~100km max in fiber without quantum repeaters
✗ Infrastructure: requires single-photon sources and detectors
✗ Key distribution only: does not provide authentication or signatures
✗ Trusted nodes: long-distance networks require trusted relay points
which reintroduce classical vulnerabilities
✗ Side channels: timing, power, or implementation flaws can
leak information not protected by quantum mechanics
This simulation assumes a perfect implementation — the no-cloning
theorem protects the quantum channel, not the hardware around it.
QKD and post-quantum cryptography are complementary:
PQC for most infrastructure, QKD for highest-value point-to-point links.
DEPLOYED QKD NETWORKS (2026)
China: 2,000km Beijing–Shanghai ground network + Micius satellite
Europe: EuroQCI linking EU member states (in deployment)
Asia: Metropolitan networks in South Korea, Japan, Singapore