> The way they could take over the network is by crafting a new blockchain, forging fake transactions or even claiming that they own all bitcoins. If they solve

> The way they could take over the network is by crafting a new blockchain, forging fake transactions or even claiming that they own all bitcoins. If they solve blocks faster than the rest of the network, soon their malicious chain would become longer and therefore accepted by all clients! Clients always trust the longest chain because it takes the most work to make, so you would have to control most of the mining power to fake anything for long. This is incorrect. Having 51% or more of the computing power of the network doesn't allow you to replace the whole blockchain (there are checkpoints, and it gets more difficult as you want to replace a bigger part of it). You also can't forge new transactions, since transactions need to be signed by the sender. Basically the only practical attack (assuming no serious bugs in the clients) is a double spend: they could pay out of their own wallet a sum X to a third party, the third party checks that the transaction has Y confirmations (remember, as Y grows, it gets more difficult to pull this off), they receive whatever they've bough from the third party, and then they start pushing a different and longer block not including the transaction.