Bitcoin and Proof of Work
How Bitcoin achieves consensus without a central authority: hash chains, mining difficulty, and the energy cost that secures double-spend resistance.
Bitcoin is the first widely deployed solution to the double-spending problem — how to prevent digital money from being spent twice without a trusted ledger-keeper. Its answer, published under the pseudonym Satoshi Nakamoto in 2008, combines a public transaction ledger (the blockchain), public key cryptography for ownership, and proof of work for ordering.
How proof of work works
Miners bundle pending transactions into a block, whose header includes the hash of the previous block. Mining is the search for a block header whose SHA-256 hash is below a difficulty target — effectively a lottery over hash outputs, since good hashes cannot be predicted from inputs. The winner broadcasts the block and collects newly minted coins plus fees. Each block's header commits to the previous block's hash, so the chain is a linked list where every historical entry is sealed by every subsequent one.
The difficulty self-adjusts every 2,016 blocks (roughly two weeks) so that blocks arrive every ~10 minutes on average regardless of how much hashing power joins or leaves. A reorganization of the past requires redoing the work of every block being replaced, faster than the honest network extends it — probabilistically infeasible past a few confirmations.
Costs and properties
The security model is economic: the cost of rewriting history grows with the network's total hash rate, which is why the system's energy consumption (on the order of a mid-sized country's electricity use) is intrinsic, not incidental. It buys a shared, append-only ledger with no administrator — and pays for it in throughput (a handful of transactions per second at the base layer) and energy. Related reading: hash functions for the core primitive, and supply and demand for why a fixed-supply asset still needs demand.
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bitcoin blockchain cryptography money