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I was under the impression that the mining percentage would give you the same percentage chance to cook the books. 51% means you are more likely to succeed tha
by Lerc 12y ago
I was under the impression that the mining percentage would give you the same percentage chance to cook the books. 51% means you are more likely to succeed than fail in an attempt. Much like buying 51 percent of lottery tickets gives you a slightly better than even chance of winning the big prize.
in that respect, wouldn't 51% be only marginally different to 49%. Both would be a bit of a concern, but neither would be the "position to exercise complete control over which transactions appear on the blockchain" that this article refers to.
Is there some mechanism I'm missing that makes 51% be vastly more powerful than 49%?
- aosmith 12y agoFrom the bitcoin wiki: An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to: Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain. Prevent some or all transactions from gaining any confirmations Prevent some or all other miners from mining any valid blocks The attacker can't: Reverse other people's transactions Prevent transactions from being sent at all (they'll show as 0/unconfirmed) Change the number of coins generated per block Create coins out of thin air Send coins that never belonged to him
- Lerc 12y agoWhat does "in control" mean in this sense? Is that an equivalent expression to "greater than 50%"? Also the "Some or all" doesn't seem very specific. What is the factor that makes some become all?
- jdmichal 12y agoI believe the intention in the phrasing is that they could selectively or universally exercise that action. Not that there is some spectrum of control.
- im3w1l 12y agoYou could theoretically mine your own chain from the genesis block right?
- wmf 12y agoThat won't work due to checkpoints and even if it did work it would be the equivalent of nuking the entire Bitcoin ecosystem. It would be more profitable to do something like extending the current blockchain but charge 1% transaction fees.
- im3w1l 12y agoInteresting. For the benefit of others unfamiliar with this. There is a hardcoded list of checkpoint blocks in the Bitcoin client. Any new chain starting before the last checkpoint will be rejected. https://bitcoin.stackexchange.com/questions/1061/can-a-51-attack-be-detected-and-dealt-with/1065#1065 https://bitcoin.stackexchange.com/questions/1061/can-a-51-at... https://bitcoin.stackexchange.com/questions/3114/which-blocks-get-to-be-checkpoints https://bitcoin.stackexchange.com/questions/3114/which-block...
- TheLoneWolfling 12y agoSo in other words if a 51% attack were to mine starting before a checkpoint block, it would end up "just" hard-forking Bitcoin.
- GhotiFish 12y agoWait. If the attacker did any of that while it was in control, then wouldn't it fail validation for every other bitcoin miner? If they lose their 50% control then they generate invalid blocks. Particularly the part regarding the number of coins generated per block. That one will obviously be wrong.
- tinkerrr 12y agoNo, 51% is vastly more powerful than 49%. With 51%, you essentially control the entire Blockchain because you can always create a new Blockchain that would be accepted by the network, given enough time. Always. With 49%, you can only get away with it a few times, and it's less likely you will mine the next 6 blocks. Essentially, as time progresses, with 49% you lose out, with 51%, you keep winning.
- Lerc 12y agoI think I understand now. That would still be a bit of a tricky position because it isn't so much 51% when you commit the fraud that is important it is the period following the fraud. How detectable would such an action be? Wouldn't other systems be able look at the block and say "it's verified, but it don't look right to me"
- tinkerrr 12y agoNot really. There are always lots of 'versions' of the blockchain floating around. The network only keeps track of the longest chain (broadly speaking). This means the network with 51% can determine which transactions get into the blockchain. For example, if the pool owner doesn't like you, he can essentially 'blacklist' your account, which means your Bitcoins can become unspendable, basically. The longer they have the 51% power, the more damage they can do. GHash already performed what you describe as your '49% attack' against a gambling site that accepted 0 confirmation deposits. There has been no known instance of a 51% attack yet (e.g. double spend after 6 confirmations).
- wmf 12y agoYou'd see that every single block was mined by the same pool.
- tinkerrr 12y agoThat doesn't mean a 51% attack happened, it just means someone got 51% of the network hash rate.
- dmclain 12y agoThe bitcoin protocol defines the longest chain as the correct/canonical chain. At 51% you have more hashing power than the rest of the network combined, so you can start mining blocks on your own and create your own chain with the knowledge that eventually your chain will be longer than the 49% chain everyone else is working on. When that happens, the 49% will abandon their chain and start working on yours.