I think this is a great solution! I have been thinking over the problems with captchas and also came to the conclusion that a proof-of-work puzzle is a fair, pr

I think this is a great solution! I have been thinking over the problems with captchas and also came to the conclusion that a proof-of-work puzzle is a fair, private, and hopefully-effective solution. You can look around for "useful" work, similar to how recaptcha was originally about transcription. If you can find some problems of the right difficulty that people want solved (e.g. I dunno, protein folding or something), then the electricity isn't wasted and you might even be able to sell the solutions.