Tuesday, February 01, 2005

What is a CAPTCHA?

CAPTCHA is an acronym for Computer Aided Public Turing test to tell Computers and Humans Apart. You have probably seen a CAPTCHA in action on a website as a colorful image with distored text in inside it.

CAPTCHA's are used to prevent automated-bots from signing up for various web services like email or for taking part in online polls etc. Previously companies like Yahoo realized that online bots are signing up for thousands of email accounts and then using these email addresses to send out massive spam. These days CAPTCHA is used extensively by the blogging community to reduce comment spam.

The reason CAPTCHA's are used, is because its easy for humans to read the distorted text (as above), but this cannot be done by computers. (although this claim has been proven false. See here). The user has to type in the word as shown in the CAPTCHA while signing up for service. If the users input matches with the text embedded in the image, then the sign-up is allowed.

CAPTCHAs pose serious accessibility problems. Because CAPTCHAs are designed to be unreadable by machines, common assistive technology tools such as screen readers cannot interpret them.

This noble idea of using CAPTCHA to reduce spam, is being thwarted by spammers. Next time, I'll describe how this is done.


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