CAPTCHA - Computer Definition
In 2000, Luis von Ahn, Manuel Blum, and Nicholas J. Hopper, affiliates of Carnegie Mellon University, and IBM’s John Langford coined this term, which stands for “Completely Automated Public Turing test to tell Computers and Humans Apart.” The test, administered by a computer, is different from the original Turing test, which is typically administered by a human. It is a kind of challenge-response test whose purpose is to ascertain whether a particular user is a human. The test is frequently used to identify human users and block computerized applications when signing up for some forms of Internet accounts. An example of this use is to block spammers from automatically setting up email accounts with free, public email services. The test involves the recognition of a distorted image of letters, often with the inclusion of some obscure sequence of numbers or letters.
(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.