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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, 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.
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References in periodicals archive ?
In the simplest form of image-based CAPTCHAs, test images were manually constructed by human administrators, and thus, only a small number of test images were available.
Vicarious set its cognition algorithms to work on solving Captchas as a way of testing its approach.
In tests IT experts said they have developed software that beats audio Captchas up to 89 per cent of the time.
This strategy greatly reduces the relative cost to bots of solving CAPTCHAs, and is largely immune to technological advances in CAPTCHAs.
To date, willing volunteers have already been recruited by cybercriminals to help crack CAPTCHAs (Completely Automated Turing Test To Tell Computers and Humans Apart), according to a study conducted by TrendLabs, the global threat research and support organisation of Trend Micro.
Software features comparison Software Spam prevention Page User- Document Name access customizabl renaming control e interface DokuWiki Yes, blacklist Yes, Partial-- Yes, optional CSS, PHP; plugin heavily documented PHP API MediaWiki Yes, URL Yes Partial-- Yes, old blacklist, word many page blacklist, IP features are becomes a blocking, user- redirect captchas (as customizabl used on e, templates wikinews) MoinMoin Yes, BadContent Yes Yes-- Yes, old filtering via Themes, page can Regular templates, be a Expressions CSS, XSLT redirect PmWiki Yes, word/URL Yes Yes, themes, Yes, block (addon, per page/per module auto-upd.
Most of the tasks are similar to CAPTCHAs: transcribing texts, labeling images, and so on.
Moreover, the constant need to update CAPTCHAs to retain filter efficiency will exacerbate the overall costs of maintaining the system.
Accessible alternatives to the visual CAPTCHAs (Completely Automated Public Turing Test to Tell Computers and Humans Apart) that are used to provide secure online forms are already available for consumers with visual impairments who visit the web site.
CAPTCHAs, short for Completely Automated Public Turing tests to tell Computers and Humans Apart, are used by websites as a defense mechanism against those evil, spam-generating bots.
Key to the new project is assigning a new, dual use to existing technology: CAPTCHAs, the distorted-letter tests found at the bottom of registration forms on Yahoo, Hotmail, PayPal, Wikipedia and hundreds of other sites worldwide.