reCAPTCHA is the only effective one. I just looked at FunCAPTCHA, and the rotating-pictures thing would be pretty easy to train a neural network to solve. Also, the language on the front page makes it look like it's targeted toward enterprises who'd have no problem blocking Tor. I bet they'll start blanket-banning Tor once they start losing the arms race against attackers.
I use Tor a lot, and while reCAPTCHA is annoying when used via Tor, it only takes a few minutes. On the forum, you can do it twice and then get a bypass code.
Is there no way to make people mine monero for 5 minutes when they log in instead? I'd pick that over everythiing else, even just as an option (funds can be added to donations for the forum). Assume people have a 5GHz clock speed...
I've looked into that sort of thing, but there is no good solution as far as I've seen. Most solutions are focused entirely on maximizing profit, so they do eg. SHA256 mining which isn't good as an anti-spam PoW, and they use some central pool which has serious privacy implications. (For bitcointalk.org, the revenue from a mining-captcha could be discarded for all I care.) A good solution would:
- Use a memory-hard PoW, or even better, one designed specifically for JavaScript.
- Have an excellent API.
- At least allow the site to proxy mining traffic to protect user privacy.
Also, web-miners are always detected as malware by antivirus software.
Anti-spam PoWs are always asymmetrical vs an attacker, which might make the whole idea unworkable. If it takes an ordinary user 1 minute to solve the PoW, then an attacker is always going to be able to improve on that using specialized hardware and software. If they can bring it down from 1 minute to 0.6 second, then the service is going to have to increase the difficulty so that ordinary users are spending 15+ minutes solving the PoW. For an anti-spam PoW to be a good idea, you need to be able to reduce this asymmetry to at worst something like 10:1, and this has to
last, not just be a result of obscurity. I'm not sure that it's possible.