is an economic measure to deter denial of service attacks and other service abuses such as spam on a network by requiring some work from the service requester, usually meaning processing time by a computer. ! Computational puzzles were first invented to prevent spam emails by Cynthia Dwork and Mani Naor in 1992 ! In 1997 Adam Back invented hashcash with the proof of work algorithm to control spam ! SHA-256 output is effectively random, getting zero bits = same as flipping a coin and getting N heads in a row