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Current Bank security is based on encryption, once quantum computing or another system to break security is invented all banks are unable to securely transfer money digitally.

Assuming banks are unable to find a new form of digital security, digital inter-bank transactions (a vital part of a modern economy) will cease, and all digital currency (including BitCoin) will effectively fall to zero value.

When this happens, what will be the new currency in the world? Gold? Cash? Barter system? If the old currencies were to persist, how would transactions be handled?

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    $\begingroup$ Really not seeing why this question has a negative score. It seems like a perfectly valid question to me. $\endgroup$ – ArtOfCode Dec 29 '14 at 0:58
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    $\begingroup$ @ArtOfCode Current security has nothing to do with quantum computations, that probably isn't helping. $\endgroup$ – Tim B Dec 29 '14 at 9:17
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    $\begingroup$ The confusion probably comes from the fact that in theory quantum computers will be able to crack existing cryptography very fast should they be developed. So existing security is not quantum computing, but quantum computing may make it obsolete. $\endgroup$ – Tim B Dec 29 '14 at 11:16
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    $\begingroup$ You might refer to this answer over on security regarding the encryption implications of a quantum computer. Summary : Currently widely used public key encryption is totally broken, but potential alternatives already exist and would pretty much just require implementation. Symmetric encryption is weakened, but can be compensated for simply by increasing key sizes. Diffie-Hellman key exchange is totally broken. Hash algorithms are weakened, but still useful. $\endgroup$ – Compro01 Dec 29 '14 at 13:40
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    $\begingroup$ The international banking system worked perfectly fine in eras where the most advanced computing device was an abacus. So I don't see why the cryptocalypse would destroy the world finance system. $\endgroup$ – Philipp Dec 29 '14 at 15:36
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Money is chosen because money is convenient. Current prevailing opinions suggest a four major factors that affect the choice of a money:

  • Durability: The currency should avoid devaluing over time
  • Divisibility: It should be possible to use the currency for arbitrarily small transactions.
  • Transportability: It should be easy to transport a great deal of wealth
  • Noncounterfitability: It should be hard to create fake money.

In your scenario, the last hold-out of the banks, quantum cryptography, fell, leaving non-counterfitability no longer feasible. You want to know where it goes.

I'm assuming by your "apocalypse" tag that we should make this worse than the question actually asks for. Let's say QM crypto took a long time to break, and along the way we figured out how to make gold from carbon, etc. Lets drag the concept of currency all the way back to its basics.

Money is funny in that its primary purpose is trade. However, there is no reason its ONLY purpose is trade. Several cultures in the Americas used Cacao as currency. Yes, in that era, money actually did grow on trees. In some cultures, it was even so valuable that the peasants could not afford to drink hot chocolate -- it was a noble's drink simply because it cost too much to prepare!

This shows the simplest layer of money: an object that can be used by a sufficiently large portion of the population as to develop value. This is known as "commodity money." Cigarettes in POW camps is another common example of this sort of money. Even those who did not smoke accepted cigarettes as currency because they knew they could use them to trade with those who did smoke.

In this incredibly perverse apocalyptic world, there is one thing which should still hold value: energy. If we have enough energy capabilities to uproot Gold from its platform, we have certainly learned a thing or two about storing energy. Large powerful batteries would easily form the basis for a energy based currency. They pass all of the standards:

  • Durability - Large batteries in this highly technological world would not deplete easily.
  • Divisibility - Limited only by quantum particle limitations, energy is virtually perfectly dividable across any number of stores.
  • Transportability - We're handwaving this until the next argument.
  • Non-counterfitability - Nobody has figured out how to counterfit a Joule yet.

But what if energy is hard to transport? It would be a strange apocalypse where energy is hard to transport but gold is forgable: this might even qualify as a utopia to some people. Accordingly, I'm going to assume that either batteries or gold is a valid form of storage.

Once you have a physical good like a battery or gold, banks can come back into play. Right now we accept that we can put a dollar into bank A and withdraw it a thousand miles away in bank B. Crush all of our crypto algorithms, and this is no longer a safe assumption. Accordingly, each bank would have to be responsible for its own reserves, gold or energy alike. It would issue a certificate for each deposit. Bank deposits are tied to their physical location, and have to be moved by bank order if you wish to withdraw them elsewhere.

These certificates are safe, even in the presence of QM crypto breaking capabilities. They can function as a shared secret, with a random string of characters identifying each certificate. This shared secret is invulnerable to QM because owners of these certificates are advised to never place the strings in a hackable storage medium -- a physical safe would be best. Both the owner and the bank know the shared secret, but this secret is never transmitted until the withdraw is made. No cryptography can break such a system.


The resurgence of trust frees banks from this physical good trap. To make a withdaw from a far-off bank, you would have to give the local bank your certificate, and have a new one issued which says "in X days, when the banks re-distribute goods, this certificate is good for Y energy/gold." You then have to sit and trust the banks... but we rather trusted them already.

There is a funny aspect to these certificates, which is that they are easy to transfer with a little trust. If you trust that a certificate is not counterfitted, you can simply assume ownership of the certificate, and withdraw the goods at a future date. If you trust that a bank will actually transfer the resources, you can even accept one of those futures certificates used to tranfer resources.

This will rapidly lead to a trust-based economy, where it is possible to forge a certificate, but it is rare for this to occur because the entire culture values the simplicity of being able to trust certificates. With this step, you have boot-strapped the gold-standard, for you will have pieces of paper which have a promised value in gold or energy.


Think that utopian ideal is unlikely? Guess again. We humans are far more creative than most recognize. Let's shave a little off of the durability of the certificates every time we trade them: let's say 0.11% of the transaction cost. In exchange for that durability, we can fund a community whose sole job is managing the currency for us. When a transaction is fraudulent due to a poor certificate, they will investigate. The merchant will usually be held responsible, because they have the most ability to cope with such a burden, but sometimes the community absorbs the cost. Consumers can feel safe because they know that, even though the merchant may hold a set of numbers which can make any arbitrary charge they please, merchants don't. If they did, the community would find out, and revoke their membership in the community.

  • Durability: The value of the transaction only depreciates 0.11% each transaction
  • Divisibility: The value is represented as a number which is written down, so it is arbitrarily divisible.
  • Transportability: Only a few numbers are needed on one's person to have access to their entire life worth
  • Noncounterfitability: There is an entire community dedicated to ensuring this is not an issue. It happens from time to time, but the system absorbs such costs.

Sound familiar?

This system is called Visa, or MasterCard. It is in use today.

Welcome to the apocalypse.

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    $\begingroup$ It is perfectly possible to counterfeit (I suggest changing the misspelled word in your answer) batteries. If you have ever tried shopping for lithium ion batteries online, you will find that most of them have less than half of their advertised capacity. $\endgroup$ – March Ho Dec 29 '14 at 10:46
  • $\begingroup$ @March Ho: That is an interesting and true statement. When I was writing this, I was thinking of the energy being the valuable part, and that is easily verifiable by using charging one battery with another and measuring the energy transferred. You are right that the physical battery capacity is much harder to measure. In my opinion, trusting the "marked battery capacity" on the label on the battery fits in better with the phase where you are trying to learn to trust certificates, because arguably that's what such a label is. $\endgroup$ – Cort Ammon Dec 29 '14 at 17:57
  • $\begingroup$ In popular American media, cigarettes are still used as currency in prisons. I don't know if this is really the case or not. $\endgroup$ – TylerH Dec 29 '14 at 20:41
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Your question is based on a flawed premise, so the answer is: nothing will change, except for technical details that you don't understand.

Bank security is not based on a computer's inability to solve quantum computations in a reasonable time. First of all, cryptography is only one aspect of bank security. Second, quantum computers (if they are at all possible in a form that would affect common cryptographic algorithms, which is not a given) would not invalidate the principle of cryptography; they would only require new algorithms that people are already working on.

If, for some reason, there was an (extremely unlikely) breakthrough in our understanding of computation which led to invalidating the very principle of computations that are easy to verify but hard to carry out without knowing a secret, then much of cryptography would become impossible. However, one-time pads have provable security: their security is a fact of mathematics, unlike other cryptographic algorithms whose security relies on the fact that none of the world's smartest cryptographers have been able to break it after years of trying really hard. In this event, securing and transporting one-time pads would become an important industry. Another consequence is that technologies ensuring communication confidentiality and integrity through physical rather than mechanical means would flourish — not only fast courriers but fiber optic channels where eavesdropping is detectable.

A collapse of the banking system as such would require significant social upheaval, not just a scientific or technological breakthrough. It's impossible to predict the consequences because they depend on what that social upheaval is about. Should it involve a collapse of the fiat currencies that are represented by bits in computer memories backed to some extent by physical objects with a fiat value shadowing their production cost (gold and silver), we would likely turn back to the previous system of fiat currencies that are represented by writing on paper backed to some extent by the same physical objects.

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    $\begingroup$ However, OTPs would also be impossible to implement widely, in the manner that PGP or similar techniques vulnerable to quantum computers are. $\endgroup$ – March Ho Dec 29 '14 at 10:49
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    $\begingroup$ I wish I could upvote this more. Realistically quantum computers are not magic problem solving devices. they do certain type of math faster yes, but they by no means invalidate cryptography. $\endgroup$ – dsollen Dec 30 '14 at 15:49
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Quantum computing won't be "solved" in a single stroke. It will become possible, for a small number of bits, to solve problems in parallel. So an 8-bit quantum computer will be able to test 256 solutions simultaneously. As technology improves, the number of bits will increase, but there will always be a limit where reliability becomes the limiting factor, and the number of useful bits cannot be increased without improvements in technology.

Encryption has always been easier than encryption-cracking, and always will be. The same quantum computers that can crack 256-bit encryption in 1ms could be used to create 100,000-bit encryption and come up with million-digit prime numbers.

So as computing capabilities improve, encryption and encryption cracking will improve at a similar pace. This is what has always happened in the past. From my experience in the industry, the limiting factor will be the banks! When "known encryption systems become obsolete" the banks will be the slowest to respond

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  • $\begingroup$ How come I only get the really really good first posts in the review queue? +1 and welcome to Worldbuilding! Hope you stick around and enjoy the site :) $\endgroup$ – Shokhet Dec 30 '14 at 3:05
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The only portion of bank security that is dependent on cryptography is online access. If cryptography fell without replacement, then banks would no longer be able to offer online access. This would be true regardless of currency. I.e. even if we switched to commodity money, we still wouldn't be able to do online transfers.

Bitcoins are an interesting area in regards to this. As a cryptographic currency, they would be impacted. The immediate result would be that the rest of the Bitcoins would be mined. Instead of a growing money supply, there would be a static amount. This is something that would happen eventually anyway, so not that big a deal. More importantly, all Bitcoin transactions use cryptography to authenticate transactions. They'd have to find another way to do that, since cryptography would no longer be secure.

So how would we do things like shop online? We'd have to come up with a process that put those portions that need to be secure offline. For example, perhaps you shop on Amazon.com and get a cart ID. You then call your bank on the telephone (not VOIP) and tell them that you authorize that cart ID. The bank phones Amazon.com with the authorization and other private information (e.g. your address) which then fetches your order based on the cart ID and processes it. Note that under this system, Amazon.com has two networks: a public insecure network and a private disconnected network. Because private information is only available on the private network, this is secure without cryptography. Well--at least as secure as phone ordering ever was.

Credit card transactions could no longer be processed online. We'd be back to processing them by phone. Phone processing would not be impacted. Physical transfers of paper money would not be impacted.

Presuming we're only talking about public key cryptography, parties that communicate online frequently could still manage. They'd just have to do the initial handshake offline so that they can exchange private keys rather than public keys. This would be suitable for long running connections where the increased cost can be amortized over time. It wouldn't work well for short term or intermittent connections, e.g. online shopping by individuals.

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    $\begingroup$ It'd become easy to spend bitcoins that aren't yours, but mining would not become trivial (just easier). AFAIK, the only quantum attack on SHA-256 that is known is the generic quadratic speedup, as opposed to ECDSA where there's an exponential speedup. They'd become worthless, but because they'd be easy to steal, not easy to mine. $\endgroup$ – cpast Dec 31 '14 at 0:20
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Quantum computers will make some public key algorithms insufficient and cast doubt on others. It doesn't help much for symmetrical encryption: By existing estimates, bruteforcing 256-bit AES key on quantum computer is equal to bruteforcing 128-bit AES on a classic computer, so security levels remain very high.

However, the same technology offers new abilities on the side of the secret keepers, too. Quantum encryption relies on the no cloning theorem to prevent eavesdropping. The first description of quantum cryptography ideas was an example using currency! If I recall, it proposed using 20 quantum traps on a banknote as a way of preventing counterfeiting. Maybe that's enough detail to find the original paper?

More generally, you can do weird and wonderful things like produce the correct answer only if you forget the question, providing for tamper-proof decoding machines that can't be looked into to peek at the workings.

Quantum computers are not an oracle even for classical algorithms. The class of problems is now understood and called BQP.

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current public/private key cryptography is based on the fact you have a massive number (256, 512, 1024.. bits ) which is your public key you give out and publish openly and a private key which are the two numbers that multiply together to make this number (and which you keep really safe!)

A New form of math that could factorise this public key into its two multipliers in a reasonable time using current state of the art computers (including quantum computers) would either mean we have to go to much bigger numbers (128K, 256K.. bits) which would take longer to factorise (and longer to process), some new cryptography or as stated above another method entirely.

I personally think Peter Hamilton got it close when the AI took over the banking and removed humans from the equation as lets face it we are the weak link in any transaction system and always will be.

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Money will never disappear. Neither will banks. They may change and take on a different guise, but as long as there is human society there will ba money. Banks and encryption are simply the latest in a very very long line of technologies used to allow human beings to spend there wealth.

If the entire digital banking system were to collapse, it would be a surprisingly short period of time before something else emerged.

I would give people a week before they start bartering goods and services & products of universal value such as jewellery.

Barter is the most fundamental system and is used all over the world today anytime currencies become unstable, such as the present day in Russia

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