Proof of Work (PoW)

Proof of work secures a blockchain by making block production expensive. To add a block, a miner must find a hash below a target value, which can only be done by brute-force guessing. Checking the answer is instant, but finding it takes enormous computation. The protocol adjusts the target every 2,016 blocks on Bitcoin, about two weeks, so that blocks keep arriving roughly every 10 minutes regardless of how much hash power joins.

The security argument is economic. An attacker who wants to reverse a transaction must redo the work of every block since it and outpace the honest network at the same time. With the Bitcoin network's hash rate in the hundreds of exahashes per second, that would require billions of dollars of hardware and electricity, and the attack would undermine the value of the coins the attacker is trying to steal.

Proof of work was introduced by Bitcoin in 2009 and used by Ethereum until 2022. It remains the mechanism behind Bitcoin, Litecoin, Dogecoin, and Monero. Its main criticism is energy consumption; its main defense is that the energy cost is exactly what makes the ledger expensive to attack.