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Comment by hnolable | Hacker News Reader

Also wrong, 'Can the government shut it down?'

Should be: Q. Who can shut it down? A. Anyone who is willing to pay 51% of the monthly operating cost of all miners. Plus an initial investment to manufacture the hardware. Neither of these numbers are out of the range of Governments or Banks. And they probably won't be for a long time. This is the death of Bitcoin scenario that worries me most. And it seems no one is really worrying about it or taking it seriously.

edit: Here are the numbers someone calculated at the height of the first mining bubble [1]. We're currently at less than double the hashing power of then [2]. So a current rough estimate would be to double all those numbers.

[1] http://bitcoin.stackexchange.com/a/1094

[2] http://bitcoin.sipa.be/speed-lin-ever.png

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Controlling only 51% of the current operating power of the miners would take over eventually but it would take a very long time. Since each client trusts the longest chain, you would have to recompute a new chain that's longer than the entire chain so far. If you only pay for an extra 1% over the "legitimate" chain, it would take several years to make a longer chain.

You're thinking of the wrong scenario. If you have 51% of the power you start mining a new block off the latest legit block but you include 0 or no "real" transactions in it. After you find one, you only compute new blocks off of _your_ "bad" last block. Since you have 51% you will control the entire blockchain from this point on. And you can basically shut down all transactions.

Note that we aren't trying to reverse any transactions or steal people's bitcoin. We are just trying to stop all transactions. This is what bitcoin is all about, so do this and you've destroyed bitcoin.

51% doesn't mean you always "win" a block, just that you win 51% of the time.

Ok I'll bite and explain it.

You're right, sometimes someone else will find a block before you and will add it onto the longest existing chain. IF you always appended your bad blocks to the longest chain then the blockchain would look something like bad->bad->good->bad->bad->good->bad->bad. BUT you WONT be adding your blocks to the longest chain. You will always add your blocks to YOUR OWN last bad block. So you will always be working on a blockchain that looks like ... ->bad->bad->bad->bad. And since you have 51% of the power your blockchain will eventually always be longer than the other chain(s) miners are working on. For a while the two chains (legitimate and your bad one) will go back and forth between which is longer. But at some point yours will get long enough and outpace the legitimate one.

If you don't understand this you don't understand how bitcoin works. I don't say this to be a dick, but I say it because this is a real concern that a lot of people don't realize or think about.

I hadn't thought about only working on your own blockchain like that.

A piece I'm missing is the heuristic most clients use to determine "longest". Depending on how that works, this plan could be really easy or really hard.

I believe the way longest chain is determined is the sum of the difficulty of all the blocks. This basically boils down to just the chain with the most cpu power that went into it. So as long as you control the majority of the cpu power, you can control the longest chain.

Why do you need to recompute the entire block chain. At the moment, the block chain is in some state. If you control the majority of the processing power, you can devote it all to expanding the chain from that state. Because you have more CPU, your version of the block chain would replace the old version when you choice to release it. This would not let you re-write history from before you started mining, but gives you complete control of events after you started.

"Controlling" would only give you the power to block legit transactions and not to change the ownership of any coins, correct?

If so, could people could react by making btc nodes that consider that rude behavior and begin blacklisting rude nodes wrt which block chains they consider accepted?

Correct, it would only block legit transactions.

This is one of the proposed solutions. Basically centralize bitcoin by somehow whitelisting good blocks or blacklisting bad ones. The problem is this would require a change to _all_ bitcoin software. Meaning EVERYONE has to upgrade to this new centralized approach. Plus the decentralization of bitcoin is lost.

Pretty sure that solution would spell certain doom for bitcoin.

Gavin offered a solution which _might_ work [1]

[1] http://gavintech.blogspot.com/2012/05/neutralizing-51-attack...

On a side note, I recall hearing that almost all of the TOR traffic is routed through about 12 exit nodes. That seems like the government is investing the resources needed to weaken that system. Of course, I have no idea how large the TOR network is relative to bitcoin.

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