Proof of Work (PoW)

In PoW a miner iterates a value (nonce) and recomputes the hash of the block header until the result falls below a target set by the network. Finding it requires massive trial and error, yet verifying it is trivial, a single hash operation. Bitcoin uses double SHA-256, and difficulty re-adjusts every 2016 blocks, roughly two weeks, to keep the average block time near 10 minutes regardless of how much power joins or leaves the network.

Security is economic and physical. Reverting history would require redoing the work of the affected blocks faster than the rest of the network, hence the 51% attack concept, controlling more than half of the hashrate. That hardware and electricity cost is what anchors the chain. PoW was the first system to solve double spending without a central authority or identities, using physical cost (Sybil resistance), and it remains Bitcoin's original consensus since January 2009.

Bitcoin uses SHA-256, Litecoin uses Scrypt and Ethereum Classic keeps PoW with Etchash. Ethereum ran on PoW (the Ethash algorithm) until The Merge, on 15 September 2022, when it moved to Proof of Stake. The main debate around PoW is its energy consumption, the central reason for that migration. On finality, PoW offers probabilistic guarantees, never deterministic ones, a nuance that matters when deciding how many confirmations to require before treating an operation as settled.