A quantum computer might be able to decrypt messages but what a lot of people seem worried about, which is silly, is password cracking. The computer could run through nearly infinite combinations of passwords in a second and find the right sequence of letters and numbers.
The good thing about password or access cracking is that there's no way for the quantum computer to know the right answer - it would have to send the password to whatever program it was trying to access, and you can only do that so fast. Shoot ten quadrillion possible passwords at the CIA's computer system and see what happens. It'll crash, and the men in black will start looking for you.
The solution to encryption cracking, I think, isn't better encryption but preventing the criminal with the quantum computer from getting an encrypted message in the first place. That's where the interesting technology challenges will be.
For going through the interface it would never work, especially since accounts are usually locked out or logins suspended for a short time after only a few tries.
This comes into play when someone has the password file with the hashes of the passwords. A quantum computer can compute the correct inputs very quickly.
Sub-Space relays
You're obviously not up on computer security issues, otherwise you would see that your point is totally irrelevant. Yes, quantum computers would not be useful for cracking passwords in the manner you suggest. But it turns out that doesn't matter in the least.
Consider that secure html transmissions could be cracked quite easily, since they depend on public key cryptography and you can crack the prime number private key. Now, you can see in clear text everything transpiring between a person and his or her bank account. Who needs the password, you've got all the account info. You could even insert some transactions while you're at it, since you can appear to be the person who is logged in.
As an added bonus, you can intercept pages where a customer changes the password, or sets up a new account and chooses an initial password.
It would be devastating for a terrorist or just plain criminal to get ahold of a quantum computer. All ecommerce in the world would have to cease immediately, in a manner similar to how they grounded all air traffic during 9/11.
Only this time, you would have to stop all ecommerce until every computer in the entire world was upgraded to have quantum encryption (or at least those that are used for any kind of ecommerce). It would be the end of the world as we know it. At the very least a massive worldwide depression would ensue as every company and individual in the industrialized world cut back over to paper and mail for all transactions and supply chain operations. Productivity rates would be set back 20 years, causing massive hyper-inflation, severe labor shortages, etc.
The interesting point is, if quantum computers are ever invented, we have this problem, since it will be impossible to cut ever computer over to use that kind of encryption overnight. I call this the "quantum computing bootstrap problem." The only way to avoid it is the possibility that they are never invented in the first place.