Proof of work

Proof of work in more detail

Across the network, mining machines make hundreds of quintillions of these guesses every second. The first miner to find a valid hash broadcasts the block, and every other node can check it with a single calculation. Hard to produce and easy to check: that is proof of work.

To rewrite history, an attacker must redo the proof of work for the block they change and for every block after it. They must then overtake the honest chain, which takes more computing power than the rest of the network combined. This is often called a 51% attack.