Proof of Work (PoW)
Consensus secured by verifiable expended energy — Bitcoin's original design, where rewriting history costs more than it pays.
Proof of Work requires miners to burn real computation finding valid block hashes. Verification is instant, production is expensive — so the longest chain embodies the most spent energy, and rewriting history means redoing that work faster than the honest network extends it (see 51% attack).
The design’s genius is anchoring digital consensus to physical cost: attacking Bitcoin requires industrial hardware and power at nation-scale, visible and slow to assemble. The cost is the point — and the controversy. PoW’s energy budget (comparable to mid-size countries) is either “waste” or “the price of neutral, unforgeable money,” and that argument defines the design’s politics in 2026.