OE-4.5 Quantum Cryptography

What this is and why it exists

Quantum key distribution is this field's one genuinely deployed product. Not a demonstration — something sold and installed.

What makes it unusual is the kind of argument behind it. Its security is physical rather than computational. It does not rest on a problem being hard to solve; it rests on measurement disturbing what it measures, so an eavesdropper leaves evidence.

And one distinction must stay firm, because the names are similar and the things are not. Post-quantum cryptography is classical mathematics chosen to resist quantum attack. It is not this.

The vocabulary

• Private key cryptography — both parties share one secret key.
• Public key cryptography — a public key encrypts, a private one decrypts.
• Key distribution — the problem of both parties getting the same secret key.
• Quantum key distribution — using quantum states to establish that key.
• Eavesdropper — a third party listening to the channel.
• Disturbance — the change a measurement makes to a quantum state.
• Post-quantum cryptography — classical schemes chosen to resist quantum attack.

The mental model

Private key cryptography is fast and has one hard problem. Both parties need the same secret, and getting it to both of them securely is the whole difficulty. Public key cryptography solves that with a pair of keys. The cost is that it rests on a mathematical problem being hard, which is the assumption Shor's algorithm threatens.

Quantum key distribution attacks the original problem from a different direction. Rather than making the key hard to compute, it makes eavesdropping detectable. The sender transmits quantum states. Any measurement by a third party disturbs them. The two legitimate parties compare a sample of their results. If the error rate is above what the equipment explains, somebody listened. They discard the key and try again.

Notice what is being promised. It does not prevent eavesdropping. It reveals it, before the key is used for anything. That is a narrower promise than the popular account suggests, and it is still a genuinely useful one.

The implementations are real, over optical fibre and over free space. The limits are worth knowing: distance, rate, and the need to trust the equipment at each end. Security proofs describe the protocol, not the hardware somebody built, and attacks on real systems have targeted the hardware.

And the distinction, restated because it is the thing most worth leaving with. Quantum key distribution uses quantum physics to distribute keys. Post-quantum cryptography is ordinary mathematics on ordinary computers, chosen because no efficient quantum attack on it is known. The second needs no new hardware, and it is what most organisations are deploying.

What you should now be able to explain or do

Contrast private and public key cryptography and name the hard problem each has. Explain quantum key distribution as making eavesdropping detectable rather than impossible. Say what the parties compare and what an unexplained error rate means. State the real limits of deployed implementations, including the trusted-hardware assumption. Keep quantum key distribution and post-quantum cryptography apart, and say which needs new hardware.

Check yourself