CAPTCHAs that ask users to read distorted text are fine for sighted people, but a terrible barrier for those who are blind or have other disabilities. Audio alternatives are occasionally available but still don't help those who are both deaf and blind and can be hard to use with a screenreader (which is already reading words to you).
There exist a couple of solutions that use humans to solve the CAPTCHA on behalf of the user, such as WebVisium and Solona, but these rely on the availability of volunteer operators (for example, Solona apparently has just one volunteer so you have to hope he is awake when you want help).
It occurs to me that the volume of CAPTCHA solutions needed by blind people is very low - I'd guess less than a few hundred per day in a populous country like the UK. This means that unlike the bad folks who want to perform an action many times in a short period, a CAPTCHA assistance service for blind people could afford to devote considerable computational resource - for example, a cloud of computers in Amazon EC2 - to identifying the presented text.
My question is this: assuming you don't care about speed very much, and you have lots of computers available, are there algorithms that let you solve the text-distortion CAPTCHAs that are common today, such as those used by reCaptcha? Or are these problems really intractable even with lots of resource and time?
A few notes:
At this point, my question is just theoretical, but clearly any such service would have to carefully control access to keep spammers out. Perhaps only registered blind people would be allowed to use it.
I am aware that an old Yahoo CAPTCHA was broken a few years ago using an algorithm that runs in seconds on a single computer. I am asking whether modern CAPTCHAs can be broken, perhaps more slowly and with more resource.
I am aware that some new CAPTCHA types are appearing, which ask users to identify kittens or orient a picture. These aren't widespread yet, so I'm just asking about text-distortion for now.
Basically solving a text distortion CAPTCHA consists of three individual steps:
The only problem that's left which is pretty hard for computers is the second one. The first usually isn't very hard, unless you happen to stumble upon the CAPTCHA from hell. And the third gets solved by computers with a much better success rate than by humans.
An interesting site for learning how CAPTCHAs are broken is the one by the OCR Research Team.