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Just having 51% of the network doesn't mean checkmate. It allows you to perform something called a double spend. I could buy something from you with bitcoin, while at the same time I attempted to send the same funds back to myself. If I controlled 51% of the network, there is a slim chance that would work, because the other 49% of the hash power is still validating the transaction where I sent you the funds, not the secret one I sent myself. If this were a big transaction, I might choose to wait for more than one confirmation before completing my end of the transaction. The longer I wait, the more expensive it becomes for you to conduct your 51% attack. It really is prohibitively expensive, to conduct such an attack, when you consider a few other factors. I could double spend the money to myself, but after news of such an attack came out, the price likely could crash, reducing the prize. Not to mention opportunity cost. If you have the means to conduct a 51% attack it means you have a lot of capital and expertise. You could put those two together to make a giant pile of money through legitimate means, like operating a hedge fund. The risk is a lot less. The only group that would be motivated enough and capable enough to perpetrate such an attack would be a state, and why would they bother when, like was mentioned elsewhere, they could attack bitcoin with the stroke of a pen, via regulation etc. It's waaay less effort. |