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Comment by xgstation | Hacker News Reader

We will just need to use quantum to beat the quantum. QKD (quantum key distribution) is much more mature comparing with quantum computer which is still far away for cracking crypto algorithms from real practical application standpoint.

Twin-field quantum key distribution over 830-km fibre https://www.nature.com/articles/s41566-021-00928-2

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What is the problem that QKD is solving? On the one hand you need a totally separate point-to-point network for the quantum connection. On the other hand you get relatively short symmetric keys on both ends. I’ve yet to see a proposal that wouldn’t be better served by transferring the key material via DVD/USB-stick/whatever and an armed guard. I’m certain I’m missing something rather obvious.. but I don’t see what

I do not think you are missing anything that would change your cost-benefit analysis, but here are two things that might be of interest:

- A QKD link would be much lower latency than transmitting a physical token over an authenticated channel (same type of advantage as with asymmetric key encryption, but without the drawback of relying on assumptions about computational complexity)

- It does not need to be point-to-point if you have a network of quantum memories/repeaters (which are probably much easier to build than quantum computers).

Thanks for the reply. I’ll give you latency, but that’s almost never a problem in the first place. You need to (re)authenticate anyhow. I don’t think your second point holds though. Even if we assume memories/repeaters to exist (iiuc this should contradict the no-cloning-theorem) you’d need to trust them not to listen in so you’d be back to square one with electronic key distribution schemes?

There might be an argument that one is unable to secure the two sets of key material (at least on one end and at least long term) and the destination is hard to reach (e.g. James Web or so). But at that point I’d also not trust that organization to implement their end of QKD correctly..

> Even if we assume memories/repeaters to exist (iiuc this should contradict the no-cloning-theorem) you’d need to trust them not to listen in

The whole point of having repeaters is that it's impossible for them to listen in. For the same reason why it's impossible to just "listen in" on a fiber transmitting the QKD quantum signals. Repeaters don't contradict non-cloning.

QKD isn't really that suitable for the internet as it is today. You also still have to authenticate the server somehow.

The utility of a cryptosystem is the extent it force-multiplies compute in the defensive direction. Crypto that depends on equally computationally leveraged parties is broken crypto

But how far away are we from hardware for stable quantum key generation/storage that would fit on a tabletop, much less a home laptop or smartphone? Almost certain that consumer devices will stay in classical computing and use PQE.

QKD is not battle-tested. Like RSA, a naive textbook implementation would be worthless: messy physical implementations will leak information through side-channels.

If quantum cryptography is the solution, it won't arrive immediately, or for free.

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