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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 e-mail 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 ?
By incorporating both DNSSEC and CAPTCHA initiatives, D-Link(R) routers now facilitate strong security thus protecting Internet users against man-in-the-middle, cache poisoning and other cyber attacks to ward off web hacking and phishing.
CAPTCHA Response 1 Replied: The Web server submits the response to the CAPTCHA agent who verifies the result with its store true value.
Users complete these little games faster than other CAPTCHAs, with no frustrating failures and no typing.
com), a startup developing artificial intelligence software, today announced that its algorithms can now reliably solve modern CAPTCHAs, including Google's reCAPTCHA, the world's most widely used test of a machine's ability to act human.
A randomly chosen segmented CAPTCHA character appears in each round, and shoots towards the target only when both players correctly identify it before their opponents.
Both humans and robotic programs were observed using the CAPTCHA.
We saw an opportunity to make the Internet even more secure while making the CAPTCHA test easier for genuine human users.
It's a big step in the right direction for the CAPTCHA industry, which has seen little movement away from hard to read text and images since its creation over a decade ago.
NEW YORK -- Solve Media today announced the launch of its TYPE-IN[TM] advertising platform, a unique solution which leverages CAPTCHA authentication for brand messaging to bring unprecedented consumer engagement to online advertising.
Google introduced a no CAPTCHA CAPTCHA API in December 2014.
Also enforces compliance and identity management via two-factor authentication with LDAP and RSA SecurID/Radius, IP Reputation and CAPTCHA.
As cybercriminals exploit new ways to bypass CAPTCHA technologies, instant messenger (IM) attacks will grow in popularity.