To clarify a bit further, only a few cryptosystems (certain public key systems) are really threatened by quantum computers. Theoretically, literature indicates

To clarify a bit further, only a few cryptosystems (certain public key systems) are really threatened by quantum computers. Theoretically, literature indicates that quantum computers are really good at factoring large numbers [1], and they are really good at solving discrete log problems [2]. So yes, QCs are theoretically able to crack in linear time some systems whose security depends on those problems being hard (eg ElGamal, RSA). But systems that don't rely on large primes/discrete logs (eg symmetric algorithms like AES) are not vulnerable to quantum computers. (At least not vulnerable in the "throw them out, they're useless now" sense.) In fact, there's even asymmetric cryptosystems that aren't threatened by quantum computers [3]. Again, any system that doesn't rely on large primes, or the difficulty of the discrete log problem, is not significantly threatened. [1]http://arxiv.org/abs/quant-ph/9503007 http://arxiv.org/abs/quant-ph/9503007 [2]http://epubs.siam.org/doi/abs/10.1137/S0097539795293172?journalCode=smjcat http://epubs.siam.org/doi/abs/10.1137/S0097539795293172?jour... [3]http://link.springer.com/chapter/10.1007%2F3-540-44586-2_2?LI=true http://link.springer.com/chapter/10.1007%2F3-540-44586-2_2?L...