Proof of work
The energy-burning puzzle that makes rewriting Bitcoin's history practically impossible.
In plain words
To add a block, a miner must find a rare number that makes the block's fingerprint start with enough zeros. Finding it costs quadrillions of guesses. Checking it costs anyone an instant. That gap is the invention: blocks expensive to make, free to verify. Rewriting history means redoing the guessing faster than the whole network — a bill no attacker can pay.
Why it matters
- History expensive to fake, cheap to check.
- Security tied to real cost: attacking pays worse than honesty.
- Strangers agree on one history with no referee.
Numbers that matter
- ~10 minutes — the target time between blocks.
- 2016 blocks — how often difficulty adjusts, about every two weeks.
- SHA-256 — the hash function, run twice per guess.
Not to be confused with
- Proof of stake — other networks secure themselves with locked-up coins. Bitcoin deliberately doesn't.
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Updated 2026-07-29