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A comment on Bot or Human? Creating the Invisible Turing Test for the Internet. Parent comment
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/...
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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/...