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Completely automated public Turing test to tell computers and humans apart, better known as CAPTCHA, is a test to ensure responses through a human versus a computer program.
CAPTCHA was developed at Carnegie Mellon University by Nicholas J. Hopper, John Langford, Luis von Ahn and Manuel Blum. CAPTCHA automatically generates response challenges by providing a problem which can only be solved by humans, automatically preventing access to system software and requesting a typed character series.
A computer administers CAPTCHA to a human, whereas a human administers the Turing test to a machine.
Websites utilize CAPTCHA to prevent quality of service (QoS) degradation by bots or other automated programs by using a test only understood by humans. CAPTCHA enhances security by using background noise, which appears as character letters or links to actual letters. All online systems are constantly vulnerable to hacking. Including a CAPTCHA element on a registration form can help avoid brute-force hacking attempts. This is simply one element of security, as getting around a CAPTCHA element on a page is extremely simple for a human.
To the average user, the CAPTCHA element is "that annoying box at the bottom of a registration form." While it is simple to implement from a Web development perspective, a web designer needs to decide whether user annoyance is worth the added security.