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Comment by tschumacher | Hacker News Reader

Some post-quantum signatures like CRYSTALS-Dilithium are based on lattices. Makes me think that quantum key distribution (what I've been working on for the past 6 months) has a chance to actually become useful instead of being only of interest to academics and to a few companies that sell overpriced solutions to paranoids.

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QKD does not solve the problem that quantum computers create, and cannot replace public key cryptography. That's a common misconception that the marketing departments of QKD research tries to keep alive.

Even under ideal conditions (whether these can exist is debatable), the best QKD gives you is a securely encrypted channel only when you already have a securely authenticated channel. The latter is extremely important, makes the whole thing mostly useless, and is often omitted by QKD advocates.

If you don't have an authenticated channel, you are susceptible to a MITM attack which makes any asymmetric crypto useless. Thus I think there is an implicit assumption in any asymmetric crypto that you already have an authenticated channel. Or did I miss something?

Grossly simplifying, Alice and Bob may establish an authenticated channel either by physical means (a wire) or by some combination of certificates/passwords and out-of-band authentication. Most of the time, QKD implicitly assumes the former - a line-of-sight connection or a fiber-optics cable. In these circumstances the parties might as well exchange flash drives with one-time pads, similarly to how the Kremlin-White House hotline was protected.

I'm not a huge fan of QKD, but there is a potential use case for it. Basically, for digital signatures we have schemes like SPHINCS+, and perhaps also PICNIC and FAEST, which don't require "mathematically structured" assumptions like other public-key crypto, but instead are secure based on not much more than one-way functions. If (and it's a big if) quantum computers can break all those structured assumptions but not AES/SHA, then we would still have secure public-key signatures, certificates etc but not KEMs.

But QKD can, in principle, securely distribute keys if you have a way to exchange quantum state (e.g. line-of-sight or some sort of currently-nonexistent quantum router) and a classical authenticated channel. SPHINCS+ could provide that authenticated channel. In that case QKD would enable secure key exchange even between parties who don't have a pre-shared secret.

Of course right now, all of that is science fiction.

Code based systems are still in, and classic McEliece could be extended to ~50 MiB for a keypair and still be way more practical than QKD. Just run the max current classic McEliece spec hybrid post quantum with X448.

NSA is that you?

please explain?

OP recommended McElice, not DUAL_EC_DRDBG. Is there something I should know about the former?

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