I looked into this research area as well. In fact, I considered writing a thesis about CAPTCHA. The problem is really that there is little to no formal theory a

I looked into this research area as well. In fact, I considered writing a thesis about CAPTCHA. The problem is really that there is little to no formal theory about this subject. CAPTCHAs remind me of how cryptography used to work: Someone invented a cipher, someone else broke it and so the cipher had to be improved to prevent this attack (well, it still works kinda like that, but the theoretical background is much better now). Much more an art than a science. What is a strong CAPTCHA? Again, almost no theory behind this. It's strong if programmers have a hard time figuring out how to break it with software. It is a frustrating situation, really, and computers aren't getting any dumber so CAPTCHAS must get harder to the point where they're not recognizable by humans.