A CAPTCHA is a type of test used to protects website services against use by automated programs that might abuse or otherwise take advantage of the service. The idea is to have a simple question that is easy for a human to answer, but difficult for a computer program. The standard technique is to ask users to type in text shown in an image. Typical applications include protecting registration and comment forms.

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3DCaptcha
Open source system that uses spatial cognition abilities of humans to differentiate them from machines.
Captcha Creator
PHP Captcha Script that generates unique captcha images to stop automated submissions, highly customizable, easy to install and configure.
CAPTCHA: Telling Humans and Computers Apart Automatically
Includes demonstrations, research papers, and news.
Inaccessibility of CAPTCHA
A W3C Working Group Note explaining how CAPTCHAs can present a major obstacle to users who have limited vision or a learning disability such as dyslexia. This document examines a number of potential solutions and alternative systems.
JCaptcha
An open source Java framework for captcha definition and site integration. Project information, documentation, issue tracker, and download.
reCAPTCHA
A captcha system that uses successful decodings to helps digitise books for online use. Background, and details on how to incorporate into a site. Source code in several languages, and plugins for popular blogging and CMS software.
Securimage
Open source PHP script for generating image CAPTCHAs with audio alternatives. Includes documentation, a gallery and an online demonstration. [LGPL]
Wikipedia: CAPTCHA
Explains how captcha features can keep spammers from signing up for certain accounts online, such as email or weblogs. Explains how the system can be circumvented as well.

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Last update:
November 12, 2015 at 23:05:04 UTC
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