The Captcha Question? Article By Adrian Rayfield
Added: 02.01.2008
Text versus image captcha's
Captcha is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”. The purpose of a captcha is to allow genuine users the ability to submit a form, whereas spammers, robots or automated methods should fail to be able to send a form because they are unable to work out the answer.
There are a few types of captcha's around, the most common are images with words and numbers or both, that are distorted in some way, maybe stretched or have coloured background images. Many large sites such as Google, and social networking sites such as Facebook and Bebo use this type.
Visual captcha's such as the image ones mentioned above rely on the user seeing the image, working out what the word says and typing in their answer. This sounds easy enough except this method is highly inaccessible, they can even be hard for non-disabled people, and I have had trouble working them out myself.
There have been methods introduced to aid users that give auditory captcha's, by this I mean they ‘read’ out the letters or numbers so the user can type them in. This however isn't an ideal solution either.
Here, at Rayfields we use a captcha on our article comment forms. You will find an example at the end of this article. The reason why we decided to use a type of captcha was I guess the same as everyone else, SPAM!
Our article forms were getting a lot of spam so we decided to take action. After some research we decided not to go the traditional route and use the image captcha, but to go for a simple maths question captcha.
A simple sum is randomly and dynamically generated for the user, this is displayed as text, not an image and can be read by assistive devices such as a screen reader. If the user doesn't successfully answer the sum or doesn't supply an answer the form isn't sent, the user is then informed of this, they are then asked to go back and re-enter or give an answer.
We have provided a help page giving instructions to the user should they need any assistance, we also provide an email in which they can contact us giving them another option. Our code works with and without JavaScript ensuring all fields are filled that are needed and is not device dependant.
We also use client-side form validation as well as server side validation to ensure the form is filled in correctly and that certain words or phrases are not sent in the form thus stopping the form being sent and adding addition protection from being spammed.
We are aware this isn't a foolproof answer to spam, but this we feel is the more accessible captcha because of it's device independence, and is accessible via assistive devices as the captcha is text rather than image based, coupled with the other server and client side features included gives us the most protection. In time no doubt the spammers will beat this captcha but by then I'm sure there will be more sophisticated and accessible solutions.
Oh, I'm sure your wondering how successful this captcha has been? Well from the day we implemented this feature the article forms haven't received a single spam form, so I guess you could say it works, for now.
After the success of our trial period we may consider implementing this on some other of our sites and forms. If you would like to try the captcha below, please feel free, we would like to hear your comments.
Some useful links:
- W3C Document about the inaccessibility of visual captcha's
- The official Captcha site
- ReCaptcha
- Captcha on Wikipedia
Article written and copyright held by Adrian Rayfield of Rayfields Accessibility © 2008.
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For more information on Rayfields Accessibility Consultancy and Web Development Services please contact us by telephone on 0845 0037 508 or email : accessibility@rayfields.co.uk.
