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(Completely Automated Public Turing test to tell Computers and Humans Apart) A category of technologies used to ensure that a human is making an online transaction rather than a computer. Developed at Carnegie Mellon University, random words or letters are displayed in a camouflaged and distorted fashion so that they can be deciphered by people, but not by software. Users are asked to type in the text they see to verify they are human.

CAPTCHAs were created in response to bots (software agents) that automatically fill in Web forms as if they were individual users. Bots are used to overload opinion polls, steal passwords (see dictionary attack) and, most popular, to register thousands of free email accounts to be used for sending spam. CAPTCHAs were designed to circumvent non-humans from performing such transactions.

The Battle of the Bots and CAPTCHAs
After CAPTCHAs were deployed in 2001, the felonious bots were updated to analyze the distorted text, enter the correct text and thereby render many CAPTCHA styles ineffective. In an ongoing battle between the bots and the CAPTCHAs, the CAPTCHA text is increasingly more distorted and camouflaged, often making it difficult for humans to decode.

Other approaches have been incorporated to validate humanness; for example, displaying several images and asking what object is common among them, such as a tree or dog. Or, a phrase might be displayed and the user is asked to re-type a word; for example, "Enter the second word in the phrase." See reCAPTCHA, dictionary attack and Turing test.

Type the Word You See
In this early CAPTCHA example from Carnegie Mellon, a random word is camouflaged, and users are asked to type what they see. (Image courtesy of Carnegie Mellon School of Computer Science,

More Obtuse, More Random
CAPTCHAs are increasingly more distorted in order to fool the bots, and real words have given way to random letters and digits. However, just like virus writers, who learn to code their programs more effectively, so do the bot writers... a fun-loving, creative bunch.
References in periodicals archive ?
Recognition of CAPTCHA operates with a common sense concept of the equivalence of an external object with an idea already contained in the mind of a singular individual.
See Nobles, supra note 19 (explaining filter seeks to eliminate mass-produced e-mail); see also Brittney Pescatore, CAPTCHA .
CAPTCHA acts as an authentication tool (Datta, Ritendra, Jia Li, and James Z.
We're excited to be the first in the market to announce we have taken the initiative to implement both CAPTCHA and DNSSEC into our routers, thus providing yet another layer of security, and we'll continue to provide our users with the latest in advanced security technologies," said Harrison Albert, Director of D-Link Middle East & Africa.
Podec passes CAPTCHA by redirecting the CAPTCHA processor to an online image-to-text recognition service, Antigate.
The first known practical CAPTCHA was the one designed by Broder (though they did not use the term CAPTCHA in their work) to prevent automatic URL submission in Alta Vista search queries [1].
CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a challenge response test which ensures that a response is not computer-generated and instead entered by human hand.
Dr Yan said: "There were suggestions that cheap labour was behind this increase and that CAPTCHA security was good enough, but low-paid people in developing countries were being hired to decode it manually.
Until now, bloggers had to either enable comment moderation or use CAPTCHA.
Google introduced a no CAPTCHA CAPTCHA API in December 2014.
Provision of CAPTCHA in the booking page of e-ticket/i-ticket on the IRCTC website to check use of scripting tools by unscrupulous elements for cornering of tickets.