CAPTCHA

A CAPTCHA is a software tool developed at Carnegie Mellon University  that is used primarily by Web sites to filter out automated Internet bots and only allow genuine human users to access specific services on the Web site.

CAPTCHA is an acronym that stands for "completely automated public Turing test  to tell computers and humans apart."

As its name says, CAPTCHA works on a principle inspirited by the Turing test, a computer science experiment that aims to tell humans and computers apart by asking questions. In CAPTCHA, a series of numbers and letters are displayed in a distorted form as an image. The user is challenged to recognize the characters in the image and type them out in a textbox provided. The characters are distorted in such a random way that a human user can distinguish them but automated computer algorithms cannot. Internet bots are restricted from accessing critical sections of the Web site.

The above image is an example of the distortion that is characteristic of the CAPTCHA tool.

A few such services protected using CAPTCHAs are Web site registration forms, Internet polls, forums and review boards.

By filtering out automated bots, Web sites aim to prevent the following suspicious and criminal activities:

• False creation of free accounts
• Spam and advertisement material posts on blogs, forums and review boards
• Worms in emails forwarded automatically
• Sometimes, to control or decrease the rate at which a user may post requests on a Web site