What is proof of work?

To add a block, a miner has to find a version of it whose fingerprint, its hash, begins with enough zeros. The only way is to keep trying, so miners make enormous numbers of attempts every second. Anyone can check a winning answer with a single try.

That effort is what protects Bitcoin’s history. Nodes follow the valid chain with the most accumulated work, so to change an old block an attacker would have to redo its work and the work of every block after it, and overtake everyone else. The white paper calls proof of work “essentially one-CPU-one-vote”.

The idea came from hashcash, which Adam Back invented in 1997 to make each email cost its sender a little computing effort, to deter spam. The Bitcoin white paper builds on “a proof-of-work system similar to Adam Back’s Hashcash”.

The same race meters out new bitcoin: whichever miner wins a block claims its reward.