PoW

Proof of Work (PoW) is a consensus mechanism used by some distributed systems, most notably public blockchains. In PoW networks, participants called miners compete to demonstrate that they have expended computational effort by solving a cryptographic puzzle. The winner appends the next block to the chain and receives a reward, while the work also serves as proof that the block is valid.

How it works: Miners assemble a block of transactions and repeatedly hash the block header with different nonces until the hash meets the network's difficulty target. The probability of success is proportional to hash power. A valid block is broadcast and added after verification; difficulty adjusts to target roughly constant block times.

Cryptographic hash functions with preimage resistance underpin PoW. Popular implementations include Bitcoin's SHA-256 and Litecoin's Scrypt; Ethereum used Ethash PoW before transitioning to Proof of Stake in 2022, with many other PoW networks existing.

Security and incentives: PoW deters double-spending by requiring substantial work to alter history. It can be secured by majority hash power, but is vulnerable to 51% attacks. Centralization can arise from mining pools and hardware manufacturers, reducing decentralization.

Criticism and alternatives: PoW's energy use has drawn scrutiny for environmental impact and potential waste. Some networks adopt alternative consensus mechanisms such as Proof of Stake, which typically uses far less energy. Ethereum completed its transition to PoS in 2022.

In practice, PoW remains a widely deployed consensus mechanism for public blockchains, valued for security and simplicity, though many projects are moving toward PoS or other models.