5. The 51% Attack

5

The 51% Attack

IN THIS POST, Satoshi addresses an argument concerning the socalled 51% attack. In this scenario, a miner or group of miners could gain a majority of hash generation power (i.e., the proof-ofwork) in order to initiate and then reverse transactions and so double spend, to prevent some transactions from being confirmed or to prevent some or all other miners from mining valid blocks.

Re: Bitcoin P2P e-cash paper

Satoshi Nakamoto Mon, 03 Nov 2008 11:45:580800

John Levine wrote:

Satoshi Nakamoto wrote:

As long as honest nodes control the most CPU power on the network, they can generate the longest chain and outpace any attackers.

But they don’t. Bad guys routinely control zombie farms of 100,000 machines or more. People I know who run a blacklist of spam sending zombies tell me they often see a million new zombies a day.

This is the same reason that hashcash can’t work on today’s Internet—the good guys have vastly less computational firepower than the bad guys.

Thanks for bringing up that point.

I didn’t really make that statement as strong as I could have. The requirement is that the good guys collectively have more CPU power than any single attacker.

There would be many smaller zombie farms that are not big enough to overpower the network, and they could still make money by generating bitcoins. The smaller farms are then the “honest nodes”. (I need a better term than “honest”) The more smaller farms resort to generating bitcoins, the highe the bar gets to overpower the network, making larger farms also too smallto overpower it so that they may as well generate bitcoins too. According to the “long tail” theory, the small, medium and merely large farms put together should add up to a lot more than the biggest zombie farm.

Even if a bad guy does overpower the network, it’s not like he’s instantly rich. All he can accomplish is to take back money he himself spent, like bouncing a check. To exploit it, he would have to buy something from a merchant, wait till it ships, then overpower the network and try to take his money back. I don’t think he could make as much money trying to pull a cardingscheme like that as he could by generating bitcoins. With a zombie farm that big, he could generate more bitcoins than everyone elsecombined.

The Bitcoin network might actually reduce spam by diverting zombie farms to generating bitcoins instead.

Satoshi Nakamoto

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