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Comment by whysosadge | Hacker News Reader

I’m exaggerating a lot here, but one concern people have with quantum computing is what if it breaks all cryptography, but doesn’t help us solve “useful” problems.

The what if is pulling a lot of weight here but I hope it makes my point.

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Quantum computing is extremely useful for accurate computational chemistry, which impacts everything from materials science to medicine.

That statement alone does not paint the whole picture. It also has to be better in terms of some metric (e.g. accuracy, computation time, size of molecules simulatable) when compared to classical ones.

It makes accurate simulation of quantum mechanical systems of relevance (e.g. molecules) even possible. DFT simulations are full of approximations just to get them to be able to run on classical supercomputers. Nobody does a full solution of Schrödinger's equation. With a sufficiently sized quantum computer, you could.

I guess that would be about asymmetrical encryption. Which is today indispensable for a lot of infrastructure. Still, there's symmetrical encryption, which is mostly untouched by Shor's algorithm.

Symmetrical encryption requires key distribution, which presents new areas of attack.

Though there is also quantum key distribution, which will probably be solved as soon as quantum algorithms can break classical encryption.

QKD also needs key distribution to prevent MITM attacks. It behaves almost exactly like a classical stream cypher, except that it cannot be package-routed, is many orders of magnitude slower, and is prone to side-channel attacks.

QKD is essentially a layer 1 protocol. It's not in the same realm as symmetric key establishment.

That said, there are quantum-resistant key establishment algorithms out there, and more likely one of them will be selected.

Depending on the use case. Symmetric crypto varies. For example, if your source only has 100 bits of classical entropy. For example, fuzzy extractors.

It becomes manageable to break it now with a QC.

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