Proof of work

Proof of work requires miners to expend real computation — and therefore energy — to produce each block. The winning hash is trivial for anyone to verify but statistically expensive to find, which makes block production costly and history rewriting costlier still.

Its genius is anchoring a purely digital system to physical scarcity: an attacker needs not passwords but power plants. Its energy use is the most debated externality in Bitcoin — and its security model.