hashcash for blocking bots

[komuw]

• https://en.wikipedia.org/wiki/Hashcash
• https://blog.cachix.org/posts/2023-02-19-downtime-due-to-signup-spam/ : nix/cachix uses it to block spam
• https://gist.github.com/i3visio/388ef5154052ed8173df4b7b9eda541b
• https://github.com/antbob/hashcash-python/blob/master/hashcash.py
• Apple uses hashcash as of 2023;
  • https://news.ycombinator.com/item?id=38724753
  • [spaceship] implements new hashcash header in auth flow fastlane/fastlane#21073

Unfortunately, realworld data from a large ISP shows that these difficulty levels would mean
that significant numbers of senders of legitimate email would be unable to
continue their current levels of activity. We conclude that proof-of-work
will not be a solution to the problem of spam

• https://www.cl.cam.ac.uk/~rnc1/proofwork.pdf

[komuw]

Related; #222
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