Captcha-solving services charge about $0.003 per reCaptcha solve currently, so the computation cost to an attacker (not an average user) would have to be comparable to that. Certainly this makes SHA-x PoW impractical, as it would be far cheaper for an attacker using GPUs/ASICs and special code vs an ordinary user solving it via JavaScript on a CPU
Oh, really? If a thousand users logs in, you pay $3 to Google. There are many users, you pay too much!
This thread is about POW-mining of new ASIC/GPU-resistant coins, not Bitcoin! Algos like yescrypt, yescryptr16, yescryptr32 are mineable by CPU only. No way for attackers to use ASICs.
Futhermore, POW with mining coins like
Koto,
Yenten,
WAVI will give the forum some coins! With my idea implemented you will yield coins instead of spending them.
Someone could right now write a userscript which integrates the forum captcha with a captcha-solving site. Some of them allow you to purchase captcha solves, solve captchas in exchange for credits (eg. solve on your computer and then use the credits on a mobile device), or transfer credits between accounts (eg. buy credits from other users). This is still not ideal from a privacy standpoint, of course.
So we need:
1. Javascript client-side miner for CPU-only algos. This depends mostly on coins' developers and communities.
2. Server-side scripts. This depends on server-side software.
I am not a professional Javascript or PHP coder but I hope this idea will attract the proper specialists.