Nice way to do privacy-respecting captca. This reminds me of the 2002 paper "Hashcash - A Denial of Service Counter-Measure" https://en.wikipedia.org/wiki/Hash

Nice way to do privacy-respecting captca. This reminds me of the 2002 paper "Hashcash - A Denial of Service Counter-Measure" https://en.wikipedia.org/wiki/Hashcash https://en.wikipedia.org/wiki/Hashcash) used for things like anti-spam email, whose Proof-of-Work was an inspiration for and applied to bitcoin.