CAPTCHA stands for Completely Automated Public Turing Test it is the process of trying to insure that a human is filling out a web form. It is also, unless an accessible alternative is provided, quite possibly illegal in Australia and the United States and other nations with disability rights legislation. CAPTCHAs were created to deal with the problem of automated submissions to websites. Their effectiveness in this regard has, of late, been called into questions with reports that the CAPTCHAs employed by both Google and Yahoo being cracked in recent months.
An example of a CAPTCHA from Yahoo.
The problems with CAPTCHAs are so numerous it is hard to know where to start a few issues are outlined below.
The last of these items is perhaps the greatest problem and the one that presents the a legal challenge to those employing CAPTCHAs in commercial and governmental settings is that CAPTCHAs are not accessible to the blind and print disabled as they are often presented as images only which can not be read by screen readers. There are attempts to deal with this by having recording of the CAPTCHA play. The problem with this approach is that in many cases the audio is so distorted as to be difficult for anyone to understand. This solution also can not deal with the deaf-blind user. Below is a sample of such an audio CAPTCHA from the recaptcha service which demonstrates this issue.
Audio CAPTCHA from recpatcha
A simple and accessible solution to this is to offer the user math problems which are written out such that a human would find them simple to solve but a commuter would not. Below is such and example:
The full PHP source code for this approch, including the function that transforms numbers to words can be found in the mathproblem.zip file.
Greg Kearney, February 2008
