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Not only quantum computers. Every cryptographic method relies on fact that there is no easy way to decide NP problems. And we don't really know whether P!=NP (in fact, according to polls, about a quarter of researchers disagree), or, in case there actually is a polynomial algorithm, its exponent will make it untractable in practice.

So there might be DOOMSDAY in the future, where all cryptography will cease to work, because somebody just figures a way to decide NP problems quickly enough.