From what I understand, stealing money with a quantum computer works exactly the same way as with a "normal" computer:
- You pretend to be someone else (a bank customer or the bank itself) and instruct the bank to transfer money to your own bank account. or
- You manipulate a valid money transfer to your own account and change the amount of money from 10€ to 10.000€
Both strategies are currently prevented by strongly encrypted data transfer. If your online banking app sends the following instruction:
transfer 10€ from A to B
It adds a signature to the messge saying more or less
I am OnlineBankingApp, this message is exactly 24 characters long and the check sum of this message is 1B.
Then it applies a secret encryption key (a very long string of random letters and numbers) and turns the message into something like this
The bank has a decryption key (called public key) to decrypt the message back to its original content. When doing so, it checks that the information in the signature match up with the content of the message (if they don't match, the message is invalid and ignored). A hacker has the chance to intercept this message and decrypt (read) it just like the bank would. But changing the content of the message is currently impossible:
- If you change the amount of money, the length of the message is wrong
- If you change the target bank account, the checksum is wrong
You would need to recreate the signature after changing the messages content. To do so, you would need to try to guess the secret (and very long) encryption key and try as many random keys as you can. With our current technology there is no chance of success. Our computers are simply too slow to try enough random keys to find the right one.
A quantum computer is simply faster and has a chance of guessing the right key before the message reaches the bank and is processed.
This was written from memory and might not be up-to-date anymore. If I made a mistake or am completely wrong, please tell me.