Also found in: Dictionary, Thesaurus, Medical, Legal, Financial, Acronyms, Wikipedia.


(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, I Am Not a Robot, 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.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
References in periodicals archive ?
Turner, and Cliff Changchun Zou, iCAPTCHA: the next generation of CAPTCHA designed to defend against 3rd party human attacks, IEEE International Conference on Communications, 2011.
(1) Automation and gradability: Above all, CAPTCHA challenges must be generated and graded automatically by computers [8,10,16].
Vicarious set its cognition algorithms to work on solving Captchas as a way of testing its approach.
This attack process is known as CAPTCHA relay attack.
to tell Computers and Humans Apart (CAPTCHA) into its home and small office routers to stop worms designed to access a router and alter the victim's DNS records to divert traffic toward the attacker's network.
You are familiar with CAPTCHA even if you do not recognize the name.
Moneeeb, another IT expert said that some websites ask to sign-up that requires fees and these websites give easy work of data entry and CAPTCHA entries.
CAPTCHA technology requiring users to identify images of cars or street signs -- something humans can do better than machines (for now, at least) -- can also limit automated sign-ups and bot activity.
Captcha technology requiring users to identify images of cars or street signs-something humans can do better than machines (for now, at least)-can also limit automated signups and bot activity.
CAPTCHA model and instead uses a new "No CAPTCHA reCAPTCHA
- Select 'my home' from the options list, then enter your email address and fill in the CAPTCHA verification box, before pressing submit