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The same way we create and vet conventional cryptography: withstanding public scrutiny of experts until we are reasonably confident that there are no major issues. Most attacks are theoretical and require only novel analysis and a new bound on computational difficulty, not a proof of concept (i.e., algorithms are abandoned by cryptographers long before attacks become feasible).

Post quantum algorithms are simply algorithms designed and analyzed against a stronger threat model. Some of the attacks and techniques are still being developed, so no one knows if new quantum algorithms will be invented tomorrow that trivialize certain problems but the same danger is present to some extent for conventional cryptography today.