I want to include a CAPTCHA in my web application, but I also want to make sure that my web application is accessible to individuals with impairments or disabilities. How should I go about doing that?
For instance, does anyone know of any resources on how to make CAPTCHAs accessible, or how to accomplish similar security goals without sacrificing accessibility? Are there any best practices on accessibility and use of CAPTCHAs? Does anyone know of any readings on how to comply with accessibility requirements for government web sites, and ensure that a web site is usable by individuals with impairments or disabilities, when using a CAPTCHA?
I've seen that many sites offer an alternative audio CAPTCHA, which folks with visual impairments (or who otherwise cannot read the CAPTCHA) can use instead. For instance, I know that reCAPTCHA offers an audio CAPTCHA as an alternative to their visual CAPTCHA. However beyond merely noticing the existence of these alternatives, I'm totally ignorant about the right way to provide accessibility. Is an audio CAPTCHA a good way to provide accessibility? Are audio CAPTCHAs supported on all browsers (without additional plugins) and usable by all users with disabilities? Or, are there better approaches you would recommend? Are there existing solutions I can outsource to?
Update: I realize there are alternatives to CAPTCHAs that may be sufficient for many common situations, like fighting spam. I have investigated many standard alternatives, but I've found that they are not suitable in my particular situation. Therefore, I'm hoping you'll focus on what can be done for accessibility assuming I do need to use a CAPTCHA. (Listing alternatives to CAPTCHAs is not likely to be useful to me.)