CAPTCHA, an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart,” is one of the most common solutions to the problem of proving that a user is a human—that is, to check whether the user trying to enter a website is a real person or an automated piece of software, usually referred to as a bot and often used for malicious purposes. Various types of CAPTCHA tests have been developed to address accessibility and security issues. This entry discusses the development of CAPTCHA tests and their use for cybersecurity, the various types of these tests, and the difficulties these tests can pose for Internet users.

Security Versus Accessibility

Malicious automated programs try to access websites for many reasons, for example, for industrial espionage ...

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