[citation][nom]jellico[/nom]If and when they do crack the quantum computing nut, it will be a great day for computer technology and a black day for privacy and security as it would render all currently known and utilized forms of cryptography obsolete.[/citation]
Public key cyphers (e.g. RSA, Diffie-Hellman, Elliptic Curve) are screwed if this makes it to functionality and scale. The security is based on the impossibility of factoring multiples of large primes, but with quantum computers the feat is the same order of complexity of using the key for a single encryption.
Symmetric cyphers (e.g. AES, BlowFish) don't allow for a publicly known key and are much more secure. Quantum computing only reduces the complexity by an equivalent of halving the key length. Simple brute forcing AES-128 is currently computationally infeasible. If we developed 1 watt, 3 GHz cpu cores, 2^128 cpu core cycles would still take more energy than required to boil off the oceans, plus any electricity used by the rest of the computer. AES was designed to support 256 bit keys so that a quantum computing attack would still have the same complexity as a traditional attack on AES-128. As long as the keys can be properly handled the data is still safe.
Unfortunately, secret keys for symmetric cyphers are often exchanged using some sort of public key encryption. A quantum attack would make this insecure. Pre-shared keys (e.g. copied by thumb drive to both computers) don't have to worry about public exchange, but because of their generally longer use, are more vulnerable to side-channel attacks, which are actually feasible. While this doesn't affect being able to securely store a secured bank file on your computer, it makes it difficult to do online banking or shopping or anything on an accessible network without a changing list of pre-shared keys.
Public key exchange doesn't really have a good solution at this point anyway. It's often a matter of "secure enough," so we can just add this to a list of impending security problems. In the end, it's still easier for a waiter to steal your credit card number than for someone over unencrypted Internet transmissions. Just think how often you let a stranger with a pocket camera run off with your credit card at a restaurant (e.g. the waiter with a cell phone).
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