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The difficulty in Bitcoin is recalculated every 14 days. What if immediately after this difficulty adjustment someone targets Bitcoin nodes with a supercomputer and start creating new blocks?

Such attack can create some many bitcoin to complete the series to the 21 million bitcoins before the 14 days pass by. Then the difficulty will be recalculated and it is going to result sky high, because of so many blocks were generated in the previous cycle.

But after that the super computer is disconnected, and the Bitcoin network (all nodes that were mining prior the attack) will have no power to create another block. The network will stall. The interest in mining Bitcoin will disappear and nobody would be able to withdraw anything, since you would need a supercomputer to create the next block, or wait for hundreds of years for the next block to come.

Is there any error in my reasoning? Is this scenario possible?

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"TL;DR: Bitcoin mining is virtually immune to someone attacking it with a supercomputer, because the mining market is already flooded with supercomputers custom tailored to the job at the hardware level."
(via Cort Ammon in comments)

First of all, the difficulty reset happens after 2016 blocks. That's only after about 14 days if the hashrate is stable. When there is a steep increase in mining power the difficulty will therefore adjust much more quickly. The difficulty change is limited to a factor of four, so e.g. if the hashrate increased by a factor ten (at the start of a difficulty period), the difficulty would increase by a factor four after 1.4 days, and then again by a factor of 2.5 after 5.6 days, so that it would have adjusted a factor of 10 after only one week. Since the adjustment is an exponential function, the network can similarly adjust to even bigger jumps. You are correct however, that if the hashpower left at that point, the network could get stuck.

Luckily, supercomputers are not effective at producing Bitcoin blocks. For Bitcoin blocks, the only operation that needs to be churned endlessly is a double SHA-256 hash. And that's exactly what Bitcoin ASIC miner hardware does, their chips have the necessary algorithm implemented in hardware and are therefore exclusively designed to calculate Bitcoin blocks. This is what makes even a single contemporary ASIC faster than thousands of computers.

The window for supercomputers to take over Bitcoin mining closed some time in 2012 or 2013. Today, the Bitcoin network produces about 10 exahashes/s. To my knowledge, the most cost effective miners come at about $100 per TH/s which translates to a magnitude of one billion USD invested in Bitcoin mining hardware alone.

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    there are GPU supercomputers. or, you could also make your own ASICs to attack bitcoin, how much would you need? around 100k USD, that is not much, considering that all discovered blocks would pay for the ASICs during the period you would be attacking it.
    – Nulik
    Oct 2, 2017 at 4:55
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    You severely underestimate the mining power of the Bitcoin network. ;) I've amended my answer to address your comment.
    – Murch ♦
    Oct 2, 2017 at 5:06
  • Does that cost factor in the amount of power consumed by those miners?
    – Shadur
    Oct 2, 2017 at 7:22
  • No just a guesstimate for the hardware acquisition cost. Although, miners producing for themselves probably would have lower costs.
    – Murch ♦
    Oct 2, 2017 at 7:26
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    Tl/Dr: Bitcoin mining is virtually immune to someone attacking it with a supercomputer, because the mining market is already flooded with supercomputers custom tailored to the job at the hardware level. (similar argument: what prevents someone with a gun from going onto an airforce base and taking a modern F-22 fight? Answer: the Airforce base has more guns than he does)
    – Cort Ammon
    Oct 2, 2017 at 17:37
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It's not done every 14 days - it's done every 2016 blocks, which will happen in 14 days if hashpower stays the same. If hashpower goes up, then the retarget happens sooner.

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  • Also supercomputers wouldn't make a dent compared to today's hashrate. Likely, even the top 1000 most powerful general purpose computers together wouldn't even.
    – Pieter Wuille
    Oct 2, 2017 at 1:20
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    @NickODell , I am exploring the possibility for the governments to attack bitcoin. They can use supercomputers to drive the difficulty so high, that the currency will be unusable, nobody would be able to withdraw or deposit because there will be no new block. Creating new block would take months, which means, bitcoin has failed its purpose.
    – Nulik
    Oct 2, 2017 at 4:52
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    You also need a LOT of power to do that. If that happens you can rather destroy bitcoin by reverting transactions. See, in order to "stop transactions for hours" you need to run like 20 or 50 times the current hashpower - otherwise you are just "inconveniently" slowing down the network, not disrupting it. That is a surreal amount of specialized hardware. You really need something that can hammer down like 2015 blocks in - half a day or so ;)
    – TomTom
    Oct 2, 2017 at 16:21
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    @Joshua That wouldn't work. Your chain (the one with the block you just found) is way shorter than the chain they were on previously (because it has a few weeks of additional blocks in it). The longest valid chain wins and your chain would be way shorter. (It's slightly more complicated than that, but that's the basic idea.)
    – David Schwartz
    Oct 2, 2017 at 16:43
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    The system is designed so that attempting attacks in simplistic ways like re mining an old block from a few weeks ago would not work. If this was possible bitcoin would be effectively useless as a reliable store of value, and this form of attack would most likely have occurred by now.
    – DanielSon
    Oct 2, 2017 at 19:45
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It would take the power out put of a 30GW power plant for 3months and 49 qibits running of GPU to start from the gen block Jan 3rd 2009 up to date giving 2 months of mining the chain into oblivion. Thus collapsing the bitcoin.

"IBM has come up with a way to simulate quantum computers that have 56 quantum bits, or qubits, on a non-quantum supercomputer – a task previously thought to be impossible."

Set back and watch for a time and price a GOV backed attack takes place and evaporates the bitcoin barter business. In return causing a market dump of all blockchain and etherum platted trade.

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