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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 ?
To properly de-energize the shovel, first power must be disconnected from the main source, and then there must be visual confirmation that the conductors are separated, or the line must be tested to ensure the lack of electricity.
Couplings provide visual confirmation of proper installation and there is no need for X-ray testing.
I needed better SA of the original two aircraft while on downwind, either by visual confirmation or by radio calls.
A Navy F/A-18 fighter jet later "made visual confirmation of the crash," a statement said.
The company said the DF4 Connector System helps simplify implantation procedures via a redesigned port and provides additional reliability by giving physicians visual confirmation to the lead connection to the device.
This permits visual confirmation of the remaining adjustment thread, and allows the leveling-pad assembly to be removed from the leg and replaced, without lifting the stand.
A JR official said Japanese shinkansen systems have been equipped with automatic train control since they were inaugurated in 1964 because of the understanding that it is difficult for the driver to rely on visual confirmation in applying the brake on a high-speed service.
During the curing process, the blue color of the adhesive transitions to clear, and provides an obvious, visual confirmation that a full cure has been achieved.
Designed to match compact devices with USB ports, such as netbooks and notebook PCs, Microvault Style displays a bright yellow-green LED, on top of the drive, giving visual confirmation during file transfers.
Except for the visual confirmation that the waste was indeed human, unfortunately, we couldn't find proof and therefore couldn't find the source from where it came," said Kariolou yesterday.
One of these assets, an Air Force autonomous unmanned vehicle (AUV) (read more about the AUV in the article: Full Spectrum: Unmanning Unmanned Systems in this issue), provides visual confirmation of the threat and targeting information to the network.
In addition, the coupling provides visual confirmation of proper assembly -- assembly is complete when housings are metal-to-metal and torque requirements have been met -- an added benefit that helps to ensure proper assembly.