Proof of work

Proof of work is a consensus mechanism in which participants expend computational effort to produce proofs that satisfy a network’s rules. Its costly proofs help order transactions and make rewriting history expensive.

Hashcash: A proof-of-work system that uses computational puzzles to deter email spam and denial-of-service attacks. Bitcoin adapted Hashcash’s puzzle structure for selecting who may propose a block.
Distributed consensus: A process by which independent computers agree on a shared state despite failures or conflicting messages. Proof of work is one method for reaching agreement without a central coordinator.
Proof of stake: A consensus mechanism that selects or weighs validators according to cryptocurrency they lock as stake. It replaces computational expenditure with economic collateral as the basis for influence.
Double-spending: The use of the same digital currency unit in more than one transaction. Rewriting proof-of-work history can enable a spender to reverse a payment and spend again.
Cryptographic hash function: A function that maps arbitrary data to a fixed-size output designed to resist inversion and collisions. Mining repeatedly hashes candidate block data until an output meets the target.
Blockchain: A replicated ledger organized as blocks linked by cryptographic references. Proof of work commonly secures blockchains by making alternative histories costly.
Proof of authority: A consensus method in which approved validators produce blocks under an identity-based trust model. It avoids mining costs by relying on a limited set of recognized block producers.
51% attack: An attack in which an entity controls most of a proof-of-work network’s mining power. Majority hash power can reorganize recent blocks, though it cannot forge arbitrary signatures.
Difficulty adjustment: A rule that changes the work required to produce valid proofs as computational capacity changes. Bitcoin adjusts its target so blocks arrive near a predictable interval.
Merkle tree: A tree of hashes that commits to a set of data and supports compact proofs of inclusion. Block headers use Merkle roots to commit to transactions while keeping mining inputs compact.

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