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The tag, however, doesn't guarantee that bots won't read a web page; it only serves to say "no bots, please." Search engine bots, since they usually belong to large companies, respect web pages that don't want to allow them in.
(2) Systems where a solution to the same CAPTCHA can be used multiple times (this makes the CAPTCHA vulnerable to so-called "replay attacks").Any implementation of a CAPTCHA should allow blind users to get around the barrier, for example, by permitting users to opt for an audio or sound CAPTCHA. CAPTCHA images of text should be distorted randomly before being presented to the user.Many implementations of CAPTCHAs use undistorted text, or text with only minor distortions. In addition to making the images unreadable by computers, the system should ensure that there are no easy ways around it at the script level.For example, humans can read distorted text as the one shown below, but current computer programs can't: The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University.A free, secure and accessible CAPTCHA implementation is available from the re CAPTCHA project.