The Inaccessibility of CAPTCHA: Alternatives to Visual Turing Tests on the Web

A common method of limiting access to services made available over the Web is visual verification of a bitmapped image. This presents a major problem to users who are blind, have low vision, or have a learning disability such as dyslexia. This document examines a number of potential solutions that allow systems to test for human users while preserving access by users with disabilities.

W3C (2005). Design>Web Design>Accessibility>Security

Captcha Usability Revisited: Google Inaccessible to Blind People

An online petition is being circulated to all Internet users for the purpose of collecting signatures showing support for Google to make its word verification scheme accessible to the blind and visually impaired.

Rønn-Jensen, Jesper. Just Add Water (2006). Articles>Web Design>Accessibility>Security

CAPTCHAs, CAPTCHAs Everywhere

My business and passion is accessibility and there is obviously a huge problem with these visual CAPTCHAs. If you used alt-text on this image, alt="e3TJ6Jdp", that would be fine and very welcome for blind visitors. It would also be welcome for any computer system seeking to sign up for lots of emails. Using alt-text on the image does not solve the problem! The visual image CAPTCHA is fundamentally inaccessible. For the example above, this means very simply that Yahoo excludes people who are blind (or vision impaired) from signing up for Yahoo email accounts.

Thatcher, Jim. JimThatcher.com (2009). Articles>Accessibility>Security>Web Design

Evaluating Existing Audio CAPTCHAs and an Interface Optimized for Non-Visual Use  

Audio CAPTCHAs were introduced as an accessible alternative for those unable to use the more common visual CAPTCHAs, but anecdotal accounts have suggested that they may be more difficult to solve. This paper demonstrates in a large study of more than 150 participants that existing audio CAPTCHAs are clearly more difficult and time-consuming to complete as compared to visual CAPTCHAs for both blind and sighted users. In order to address this concern, we developed and evaluated a new interface for solving CAPTCHAs optimized for non-visual use that can be added in-place to existing audio CAPTCHAs. In a subsequent study, the optimized interface increased the success rate of blind participants by 59% on audio CAPTCHAs, illustrating a broadly applicable principle of accessible design: the most usable audio interfaces are often not direct translations of existing visual interfaces.

Bigham, Jeffrey P. and Anna C. Cavender. University of Washington-Seattle (2008). Articles>Accessibility>Security>Web Design