Comment by johnisgood

Agreed, it indeed is Hashcash. I love it. So simple yet effective.

http://www.hashcash.org

https://en.bitcoin.it/wiki/Hashcash

https://en.wikipedia.org/wiki/Hashcash

C implementation (feature-rich): https://github.com/hashcash-org/hashcash/tree/master/c

A Factor (Forth-like language) implementation of it: https://github.com/factor/factor/blob/master/extra/hashcash/...