All terms
Securing a chain by making block production computationally expensive.
Miners race to find a number that makes a block hash fall below a target. Finding it takes enormous computation and verifying it takes almost none, which is the useful asymmetry.
Rewriting history means redoing all that work faster than the honest network can extend it, which is prohibitively expensive at scale.
Bitcoin adjusts difficulty every 2016 blocks to keep block times near ten minutes regardless of how much mining power joins.
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