Can Quantum Computers Kill Bitcoin?

“What if quantum computers are designed and then Bitcoin gets hacked? Bitcoin will be dead!”

The first part of this criticism: “What if QCs are designed?” –  This is a BIG “if”. Experts I have heard speak on the matter say, in terms of scientific breakthroughs, we are orders of magnitude away from technically being able to create a general-purpose QC.

But let’s assume it was done. Then the question is, “when?” 

Let’s be outrageously generous and say within 10 years. At that point, Bitcoin adoption would be further along (and you would have missed out while worrying about Quantum Computers).

There are 2 broad ways it could go:

  1. QCs are designed in secret, with progressive advancements made unknown to the public.
  2. QCs are designed with public awareness. Progressive discoveries are published in, say, Nature, and individuals bask in glory and receive further funding/investment to keep the research going. This way, the stepwise progress to a QC threat is observed by Bitcoin developers, and anticipated.

The more dangerous scenario (scenario number 1) is less likely to occur as it goes against human nature, and the economics of progress – you need funding to keep the work going, and you need many people. In this scenario, someone or some group has created a QC – what are they going to do? It is not just Bitcoin that can be exploited but the whole world – all institutions, (including banks) – all encryption! 

Bitcoin in the long run will be safe, even if attention is turned to Bitcoin with a secret QC. “Safe” as in it won’t be stopped. It will be hurt, absolutely, but Bitcoin is CODE, and code can be changed to resist new attacks.

With such an attack, the most important thing for Bitcoin to preserve is the distribution of coins – ie, who owns what. If this is disrupted going forward, it does not mean the historical record is destroyed. Bitcoin will have to pause, get fixed, resist QCs with a new algorithm, and once designed, however long it takes, the ledger will continue from the point in time before it was disrupted. This is a huge nuisance but doesn’t kill Bitcoin. It should also be considered just how unlikely it is for events to play out in this way.

The second scenario, where stepwise progress is made, allows Bitcoin developers plenty of time to upgrade the code of Bitcoin in order for there to be no disruption should a QC ever be invented. At the moment, more important work on Bitcoin is required, but if there was any sort of real threat, attention gets diverted as needed. This is the more likely of the two scenarios, but remember, even the second scenario is incredibly unlikely to happen.

What can a QC potentially do?

1. Break cryptograph private keys

This puts peoples’ personal holdings at risk and allows them to be stolen. New methods resistant to QCs will have to be used. Once released, people would have to move their coins to new, more secure, addresses. Lost coins, and even Satoshi’s coins, will be available for grabs. This would increase the supply of Bitcoin, and yes it would hurt the price in the medium term… but doesn’t kill Bitcoin.

2. Mining risk (breaking SHA256)

With a QC, not just the security of private keys, but the mining of Bitcoin may also be disrupted. An attacker may actually overpower the network with a QC, and either earn income (the threat of which incentivises others to invent QCs for the same purpose), or try to damage Bitcoin. But Bitcoin mining is competitive – as computers get better, they can both attack OR defend Bitcoin. 

The game theory that Bitcoin is designed around doesn’t change just because computers get better. We have already seen this. Bitcoin used to be mined on personal computers, but then the technology got better and ASICS were invented (computers specifically designed to mine Bitcoin, which are much more efficient than regular computers). Did this kill Bitcoin? No. It made Bitcoin even harder to attack!

Allowing the market to sort this out, and let QCs fight as honest players is how the game theory predicts it will go. But there is also the alternative to change the proof-of-work algorithm to resist QCs. The very last “nuclear” resort would be to move away from proof-of-work, which would cause a bit mess.

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