> If a miner has 51% hash rate, they can in theory steal your coins by rewriting history to replace the transaction that granted you the coins with a different

> If a miner has 51% hash rate, they can in theory steal your coins by rewriting history to replace the transaction that granted you the coins with a different transaction that sends the coins somewhere else. Nope. Unless they have your private key, they can't sign your coins no matter how much hashing power they have. What they can do is unspend their own coins. So if they paid you BTC$1,000 and you gave them a yacht they can go back and send that money to someone else (double spending). But even that is limited: You can think of it like a race. With 51% of the network you're slightly faster than the other half. That means you can probably win disagreements going forward. But in order to change past transactions you have to go backwards in the race and then catch up again. Your ability to do that is proportional to how far back you are going and how much above 50% you have. If I want to go back one block (ten minutes) and I am going at 1.1x the rest of the network, it will take me 100 minutes to catch up (I can do 11 blocks to their 10). If I want to go back 2 blocks (20 minutes), it will take me 3 hours to catch up. You can see how this gets out of hand quite quickly.