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 user of the system. 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 to solve your computer. Basically it is the task of recognizing patterns. It should be noted that the standard character sets used in the CAPTCHA, has successfully broken optical character recognition systems.
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 system (Optical character recognition, OCR) could not see through. The creators of 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 Kale, director of the Internet Archive, said a good idea to use the Internet to fix bugs OCR.
When poderzhku Intel Corp., The team Luis von Ahn developed a free web service that allows individual webmasters to 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 verify 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 the second decoding helps to correct 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 the specifically induced noise and fuzzy pronunciation of numbers.
Details on reCAPTCHA on the official site.