I'm writing about whistle blower who discovers that his employer, huge profitable corporation is just a front operation. All their diverse products and services are a sham, and the company is actually making their money from development of a quantum computer. They keep the machine secret and use it to steal money from the banking system.

Assuming some company develops a quantum computer, and are able to keep it secret, could they use it to steal money?

My knowledge about quantum computer is limited. From the articles I've read it uses superposition to break the encryption using Shor's algorithm used by the banks.

Here's the article from IBM

I'm not very knowledgeable about computers, I'm just looking for a plausible setting.

closed as off-topic by Mołot, elemtilas, Vincent, Frostfyre, L.Dutch Nov 16 at 16:00

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  • Explain how a quantum computer works? You can't assume that we know since it isn't really a 'thing' yet. – kingledion Nov 16 at 13:26
  • Welcome to SE! Even if quantum computers work, they are mostly doing what ordinary computers do only faster. Please explain how using this quantum machine helps them - apart from saving a few milliseconds of time which may not be worth the development costs. – chasly from UK Nov 16 at 13:28
  • Doesn't IBM's article answer your question? They likely know more about quantum computers than anyone here. I'm sorry, I'm just having trouble thinking of what you want the answer to tell you. – kingledion Nov 16 at 13:33
  • @chaslyfromUK From I read , they break the Prime Numbers used in cryptography. How that works I really don't know – yaxig Nov 16 at 13:34
  • @kingledion I would like somebody knowledgeable about communication to answer. IBM is known for touting their own horn many times, and they have a solution to sell. – yaxig Nov 16 at 13:35

For the purposes of this answer I'm going to treat your hypothetical quantum computer's prime benefit as being it's capability to perform conventional computational mathematics in a way that's billions of times faster than a traditional machine. (So you can do things like crack encryption such as RSA very quickly etc)

The fundamental problem here is that if someone had a functioning quantum computer they wouldn't need to steal money.

Not only would it take phenomenal amounts of money to design, develop and operate such a computer but once you had it stealing money from the banking system would be chump change compared to what you could earn (legitimately or otherwise) through using it.

I imagine all sorts of government agencies would pay exorbitant amounts of money for computational time on it (breaking encryption, analyzing data sets and so on), similar for more conventional military.

Academic and commercial research organisations would be falling over themselves to give you money for computational time on it.

Using it to steal a few billion here and there from the banking system would be like inventing a perfect invisibility cloak and using it to steal penny chews from the local sweet shop.

  • AES (and most other symmetric encryption schemes) aren't affected nearly as much as asymmetric encryption. The work factor for those is reduced to the square root of that on a conventional computer. (I don't recall off hand whose work that is, but it should be easy to dig out if you are interested.) Doubling the key length (128 to 256 bits, say) solves that problem. The real killer is Shor's algorithm applied to semiprimes, which breaks RSA and, I think, any discrete-log problem. – a CVn Nov 16 at 14:26
  • @aCVn good point.. you're correct of course, I was actually meaning to say RSA but was musing about whether the Related Key attack on AES-256 would be feasible given a quantum computer in my head and it sort of slipped out! I blame the lack of caffiene – motosubatsu Nov 16 at 14:36
  • @aCVn I've edited to (hopefully) make myself sound less idiotic :) – motosubatsu Nov 16 at 14:38

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.

  • TLS works by deriving a symmetric session key, which is used with a symmetric cipher such as AES. In perfect forward secrecy capable ciphersuites, the key is never transmitted; only data that either side needs to generate the same key as the other side (but never enough data to allow a man-in-the-middle to generate the same key). – a CVn Nov 16 at 14:29
  • 1
    if we, like @motosubatsu assume this computer had MUCH more computing speed, then you are mistaken. the public/private keys you are thingking of can be cracked(and have been) it just currently takes somewhere on the scale of years to do and the keys are very easy to replace so once the bank realized it had been cracked it would be easy for them to make you start all over. If however, you can crack the keys in seconds then you can easily defeat the bank security – Supamee Nov 16 at 16:09

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