Quantum key distribution

Steve's big cupboard of crime

Imagine you have a secret. Of course you do - you’re an enigmatic person. But, as everyone knows, a secret shared is a secret doubled, so you want to tell your friend, Bertha - but only her. You’d be mortified if anyone else found out.

 

You can’t just send it to her any-old-how. That would be madness; anyone could intercept it. You could use public key cryptography. Then your secret would look like gibberish to anyone who didn’t have the right decryption key. Sure, an attacker - let’s call him Steve - could break the encryption, but it would take him thousands of years, by which time you, Bertha, and Steve would be dead. So everything will be fine, right?

 

Wrong. Nothing will be fine, because quantum computers are coming. That means Steve may eventually be able to break some of the public key cryptography we rely on today far faster than we’d like and, unfortunately, you might live longer than that. Steve certainly will. He’s very hardy. So what can you do? Is it time to panic?

 

No, but keep that in your back pocket as a solid Plan B. The solution is Quantum Key Distribution (which will henceforth be known as QKD because distribution is a difficult word to type quickly). What’s that you say? What is QKD? Well, I’m glad you asked…

 

What is QKD?

Exactly what it sounds like - a method of distributing an encryption key in a quantum-y way. But not just any quantum-y way; the way is quantum-y such that if Steve sneaks a peek at your key, you will definitely know about it. Then, because you’re smart as well as enigmatic, you can discard that key and try again until you have one that Steve has never caught a whiff of.

 

So, there you have it. Problem solved. Why? Well, using typical public key cryptography, you would encrypt your scandalous secret using a digital encryption algorithm and Bertha’s public key. Bertha would then decrypt it using her private key - unless she’s lost it, but that’s unlikely because Bertha’s a very organised person - and then she’d gasp in shock.

 

The thing about this encryption algorithm is that it’s tricksy. It’s easy for you to encrypt your secret with Bertha’s public key, and Bertha has no trouble decrypting with her private key. She doesn’t even break a sweat. But Steve isn’t Bertha (gosh, can you imagine?) and Steve doesn’t know her private key, only her public key (she has it printed on all her t-shirts). Because the algorithm is so tricksy, even knowing the algorithm, the encrypted secret, and Bertha’s public key, it would be very, very difficult for Steve to break the lock. That’s why it would take him thousands of years on a normal computer - he’s not lazy, it’s just difficult by design.

 

But Steve is betting on quantum computers to solve all his problems. He’s not worried that he can’t decrypt your secret right this second. He’s content to steal the encrypted version and shove it into the back of his big cupboard of crime until the time is right. Then, once he has a quantum computer ready and willing to do his dirty work, he’ll fish out your encrypted secret and break into it in a jiffy. He, too, will gasp in shock at gossip meant for Bertha’s ears only, and you will feel very embarrassed.

 

Here’s the crucial point: the real problem isn’t that Steve can’t read your message today - it’s that he can steal it today and read it later. With normal public key cryptography, you and Bertha agree a secret key using maths that’s hard for classical computers. But a powerful enough quantum computer could make that maths much less hard, which means Steve can steal the encrypted message now, shove it in his big cupboard of crime, and come back later with a quantum can-opener.

 

QKD dodges that by generating a fresh shared secret key using photons. If Steve tries to peek at the key while it’s being formed, quantum mechanics forces him to leave fingerprints — extra errors that you and Bertha can measure. If the error rate looks suspicious, you throw the key away and try again. If it’s low enough, you do a bit of error correction and privacy amplification wizardry, and you’re left with a key Steve essentially knows nothing about.

 

(Also, Bertha can keep printing her public key on t-shirts if she insists - public means public. The point is that thanks to QKD, you won’t be relying on ‘hard maths’ alone to agree your secret key in the first place.)

 

Sidebar: does quantum break all encryption?

Not exactly. The big existential dread is for some public key systems. Symmetric cryptography doesn’t fall over in the same way; it mostly needs bigger keys and sensible choices. But the ‘steal now, decrypt later’ threat is very real for public key key agreement and long-lived secrets.

 

So, that’s why QKD can help you share your secret with Bertha without Steve ever finding out. But, as well as smart and enigmatic, you’re also curious - so now you’re wondering about the how.

 

The how relies on three principles of quantum mechanics, which we’ll… what’s that? You already know all the principles of quantum mechanics? Oh. Well, fine. Just skip the next instalment, I guess. (Don’t though -  it has cake.)

 

Key takeaways

• QKD isn’t about encrypting messages directly; it’s about establishing a fresh shared secret key.
• The main modern threat is ‘harvest now, decrypt later’: steal ciphertext today, decrypt when capabilities improve.
• QKD’s promise is tamper evidence: if Steve eavesdrops during key establishment, he leaves detectable errors.
• Real QKD pairs quantum key establishment with classical post-processing (error correction and privacy amplification).

 

For more information, please contact Tom Mahon. 

 

This briefing is for general information purposes only and should not be used as a substitute for legal advice relating to your particular circumstances. We can discuss specific issues and facts on an individual basis. Please note that the law may have changed since the day this was first published in June 2026.