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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 ?
Both playing Go and deciphering Captchas are clear examples of what we call narrow AI, which is different from Artificial General Intelligence (AGI) - the stuff of science fiction.
While there are a few AIs that can solve Captchas, they needed to be trained on millions of images and text combinations.
Exploring the usability of captchas on smartphones: Comparisons and recommendations.
Recognizing Objects in Adversarial Clutter: Breaking a Visual CAPTCHA.
One such solution makes use of CAPTCHAs (Completely Automated Public Turing Test to Tell Computer and Humans Apart) to tell whether the host which is accessing the system is human/bot.
However, as optical character recognition algorithms have developed, text-based CAPTCHAs have become less secure [9].
According to Andrew Berkowitz, a longtime Web developer and the chief product officer at TeamSnap, adding CAPTCHAs to a web-based form is relatively easy these days.
Vicarious set its cognition algorithms to work on solving Captchas as a way of testing its approach.
Future Ad Labs of London will pitch their 'PlayCaptcha', an alternative to Captchas - hard to read words at the bottom of online forms - replacing them with short fun mini-games, while The Good Night Lamp, also of London, will pitch their family of internet-connected lamps.
Entering the kilometric ID, promo code, product key and annoying captchas was enough to discombobulate me.
Captchas used for security purposes can also be difficult for the blind to overcome, he added.
Put simply, CAPTCHAs are automated systems used to distinguish other automated systems from flesh-and-blood ones.