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Home > Web Services > Resources > You are here Articles

Wrestling With the CAPTCHA Arm-Lock

You’ve all seen them –those images of a series of distorted letters and numbers that you need to decipher as part of the sign-up process to a website or online service. They are so prevalent that, if you haven’t yet had to contend with one of these images, you probably haven’t been doing a lot of surfing in the last half decade or so.

If you can’t accurately decipher the code and correctly type it into a text box you are unable to register - and the vast majority of sites provide no alternative. Unless you can crack the code you are barred from using that website.

CAPTCHA is an acronym standing for ‘Completely Automated Public Turing Test To Tell Computers and Humans Apart’, and was first coined in 2000 by von Ahn, Blum, Hopper and Langford of Carnegie Mellon University.

A CAPTCHA is a process that protects websites against automatic (and usually malicious) programs that roam the internet and try to set up spam accounts on websites - they comprise tests that humans can pass but current computer programs cannot.

The above distorted image of a sequence of characters is the most commonly encountered form. A character recognition program trying to recognise the image cannot do it accurately enough due to the distortion of the characters.

The critical problem with CAPTCHA is that many humans cannot decipher the code either. If you have a vision impairment, dyslexia or learning disability (amongst many others) you too may not be able to crack the code and complete the registration process. If you can’t see a CAPTCHA image, or decipher its contents, then you are in big trouble.

Moreover many users with disabilities are also using technologies that interpret the screen - such as screen reading software that speaks out the text. These technologies are also unable to decipher the image for the same reasons that malicious software is prevented from doing so. The machine readable alternative text (or tooltip) that blind users usually rely on to know what an image contains are, of course, absent from CAPTCHA images.

As a result many disabled users must either wait for assistance in registering with a particular site, or else give up and go looking for an alternative site that offers similar services or functionality and which does not require CAPTCHA.

This catch-22, or accessibility arm-lock, that CAPTCHA represents is an ever-present problem for many millions of users world-wide. Several alternatives have been suggested to help alleviate the problem – some of which are less of a challenge than the standard approach but none of which entirely solve the problem for all users regardless of disability or technology they use.

The most common alternative offered is an audio version of the graphical image. Sites such as Google and Microsoft, but not Yahoo or YouTube (to take four sites at random) provide a second chance at deciphering the code – this time using an audio version that speaks out the characters .

The need to make it indecipherable by malicious programs, however, still applies. Thus the audio is also significantly distorted – in this case using background noise to prevent voice recognition software being able to interpret the characters. This level of distortion actually makes the code almost impossible to hear – even for people with no hearing impairment. Often numerous replaying, or refreshing of the page to serve up a new code, is required before a successful interpretation can be made.

Due to the intractable barrier that CAPTCHA represents, the best minds have been hard at work trying to develop technology for the disabled community that can crack the visual or audio codes. But as soon as they do, and site owners get wind of it, they make sure that the images they use become even more distorted and the audio even harder to hear.

Is there any way out of this crippling arm-lock for disabled users?

There are several alternatives to the distorted code approach that are either in use or in prototype and which do alleviate the difficulties for many users.

These include an approach advanced by Carnegie Mellon University that involves being presented with a number of images and being asked to choose from a large drop-down list the word that best describes what they have in common (e.g. ‘Airplane’, ‘Apple’, ‘Aunt’ etc) – see http://www.captcha.net/cgi-bin/esp-pix.

The chaps at the Google research labs are also offering a proposed alternative. They select a number of images from their database, randomly rotate them, and then require that the user indicate which way is ‘up’. If a picture chosen is rotated differently by different users then it is discarded as being unsuitable for use in this process - as ‘up’ is evidently not obvious in this case. More information at http://googleresearch.blogspot.com/2009/04/socially-adjusted-captchas.html.

Whilst these approaches avoid having to read and interpret distorted characters, they still require the user to have some useful vision and the ability to interpret the subject of the images. Better than distorted codes – but still not accessible for all humans.

Others are working on alternatives based upon logic problems - which are in text form and hence can be read by screen reading software. These questions are too complex to be answered by malicious software, but by the same token are often too difficult for users with cognitive or learning disabilities to answer too.

Some other ‘get-round’ solutions take the form of browser plug-ins such as WebVisum (http://www.webvisum.com) which grabs the CAPTCHA code image from the page and processes it remotely for you. It then provides you with the code as text – voilá! How this is done is necessarily a closely guarded secret. A very useful tool – but it doesn’t work on every CAPTCHA image and requires that you use Firefox as your browser.

CAPTCHAs are here to stay – at least for the foreseeable future. And whatever form they take they will always present issues for some (and possibly many) users. The only totally failsafe solution is for a human to wrestle with the CAPTCHA test on your behalf. If you have assistance then all well and good – but for the millions worldwide who do not the human will need to be provided by the site in question.

Google is the only site I know of that offers this option – and admittedly only for blind screen reader users. To register for a Google account you can use the CAPTCHA image, here an audio alternative or, crucially, follow a hidden link to a form where you can request customer services complete the registration process for you.

For any non-screen reader users out there wanting to take advantage of this option, here’s the link direct to the page with the required form - http://www.google.com/support/accounts/bin/answer.py?answer=33974. More power to your elbow!

Robin Christopherson
Head of Accessibility
17 June 2009

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