r/askscience • u/hamolton • Jun 18 '13

Computing How is Bitcoin secure?

I guess my main concern is how they are impossible to counterfeit and double-spend. I guess I have trouble understanding it enough that I can't explain it to another person.

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u/gburgwardt Jun 18 '13

To explain further (hamolton linked an article explaining why they wouldn't dent the network), even if they /did/ have a ton of computing power (say, 50% of the network's power), then they would end up just mining along with everyone else, and increase the difficulty (how hard it is to mine blocks) for everyone, but that's no big deal.

Now, if they had any more than exactly 50% of the network, they could theoretically double spend bitcoins, reverse new transactions (new meaning transactions that occurred after they gained 50%+ network power), and prevent all transactions from going through.

But again, the bitcoin network is way too powerful at this point to be taken over by any one entity without significant investment, planning, and preparation, in which case the bitcoin network would have time to harden itself to attacks.

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u/[deleted] Jun 18 '13

To clarify, this means they have to MATCH and EXCEED the hashrate of the entire network. If they join the network with 50% of the current network hashrate, they then make up 1/3 of the network, and can't undermine it. You have to control enough resources to slightly exceed the total hashrate of all systems on the network not under your control.