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CAPTCHA stands for "Completely Automated Public Turing Test to Tell Computers and Humans Apart." The P for Public means that the code and the data used by a CAPTCHA should be publicly available. This is not an open source requirement, but a security guarantee: it should be difficult for someone to write a computer program that can pass the tests generated by a CAPTCHA even if they know exactly how the CAPTCHA works (the only hidden information is a small amount of randomness utilized to generate the tests). The T for "Turing Test to Tell" is because CAPTCHAs are like Turing Tests . In the original Turing Test, a human judge was allowed to ask a series of questions to two players, one of which was a computer and the other a human. Both players pretended to be the human, and the judge had to distinguish between them. CAPTCHAs are similar to the Turing Test in that they distinguish humans from computers, but they differ in that the judge is now a computer. A CAPTCHA is an Automated Turing Test. We deliberately avoid using the term Reverse Turing Test (or even worse, RTT) because it can be misleading Reverse Turing Test has been used to refer to a form of the Turing Test in which both players pretend to be a computer

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