https://en.wikipedia.org/wiki/Hashcash
> Hashcash was proposed in 1997 by Adam Back[1] and described more formally in Back's paper "Hashcash - A Denial of Service Counter-Measure".[2]
> Hashcash was proposed in 1997 by Adam Back[1] and described more formally in Back's paper "Hashcash - A Denial of Service Counter-Measure".[2]