open source kaptcha alternative

[kaguy4]

Kokonotsuba (especially in textboard mode) is weak against spam. Heyuri's /lounge/ is using Kolyma's property Kaptcha as a module, but that can't be made public for reasons.

Other instances of live kokonotsuba would be totally vulnerable to bot spam, so an alternative is required. I would also use it on Heyuri