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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 ?
Conveyance and visual confirmation that the package is protecting something good has much to do with a perception about what is happening with the "behind the scenes" function of the package.
coli to glow rhythmically in a stunning visual confirmation of the transplant's success.
As the product cures with sufficient exposure to UV light, its blue color transitions to colorless and provides visual confirmation that the adhesive is fully cured and the bond site secure.
Five other big objects have been found on the floor of the ocean, though no visual confirmation has been obtained yet.
We used the LabVIEW Vision Development Module to implement features such as debayering and displaying the image data in real time, which gave a very quick visual confirmation that the hardware was operating correctly," Salter says.
The gasket's integral central leg serves as a pipe stop, ensuring full pipe insertion, and metal-to-metal bolt-pad contact provides visual confirmation of proper assembly.
As the product cures with sufficient exposure to UV light, its blue color transitions to colorless and provides obvious visual confirmation that the adhesive is fully cured and the bond site secure.
As the product cures under UV light, it loses its color, providing visual confirmation that the adhesive is fully cured.
Those who rely on the hadith use visual confirmation per region and for this reason consistent variations of the start of Ramadan have existed since the time of Prophet Mohammad (PBUH.
Tapered BHT[R] fittings for surge bins and hoppers provide quick visual confirmation of product flow and volume.
These products are also the only catheters with the double-flash SurFlash feature that gives visual confirmation of when both the needle and catheter gain entry into a vessel, minimizing the risk for failed attempts.
Visual confirmation is another option, and the use of low-power LEDs may be a viable alternative, though in certain situations, it may also be a hindrance.