CAPTCHA (captcha, captcha) and reCaptcha
CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart - Automated Turing test to distinguish between robots and humans). The term appeared in 2000.
CAPTCHA - a trademark of Carnegie Mellon University, computer-based test used to determine the system user. The main idea of the test - to offer the user a task that is easy to solve the people, but that is impossible (or extremely difficult) to learn how to solve your computer. Basically it is the task of recognizing patterns. I must say that the standard character sets used in the CAPTCHA, has successfully broken systems, optical character recognition.
The next step of protection against bots become the newest system, called ReCaptca. She not only successfully fight spam, but also serves a good cause digitizing books. The fact that ReCaptca used text optical character recognition (Optical character recognition, OCR) could not bite through. Creators ReCaptca Manuel Blum, Ben Maurer and Mike Crawford, engaged in digitizing printed for the Internet Archive under the auspices of the nonprofit Open Content Alliance. Brewster Kahle, Internet Archive director, believes a good idea to use the Internet to fix bugs OCR.
When poderzhkoy Intel Corp., Team Luis von Ahn developed a free Web service that allows webmasters to individually install reCAPTCHA to protect their sites. Individuals can also use the service to protect their own email addresses. In the case of some commercial Web sites with heavy traffic, reCAPTCHA may charge for additional bandwidth.
To ensure that the user is not a robot, the system prompts you to enter two words, one of which she already knows, and the second is unknown. This leads to two problems: the introduction of the famous words of the user proves that he is a man, and deciphering the second helps to correct the errors of machine recognition. If multiple users enter the same transcript, this option is considered valid.
Also available is the audio version of the access, which, however, is difficult to use because of specially conducted disturbances and fuzzy pronunciation of numbers.
Details on reCAPTCHA 's official website.