Hashcash

Hashcash is a proof-of-work system used to limit email spam and denial-of-service attacks. Hashcash was proposed in 1997 by Adam Back^[1] and described more formally in Back's 2002 paper "Hashcash - A Denial of Service Counter-Measure".^[2] In Hashcash the client has to concatenate a random number with a string several times and hash this new string. It then has to do so over and over until a hash beginning with a certain number of zeros is found.^[3]