CAPTCHA - «Completely Automated Public Turing test to tell Computers and Humans Apart» [1]. Any problem that can be easily solved by a human, but cannot be solved by a computer (or which requires excessive computation), can be used as a Turing test. To protect against auto registration, mass mailing in forums and guestbooks, a text-based CAPTCHA was chosen (we will reference it as text-CAPTCHA below). Initially text-CAPTCHA was used in 1997 by AltaVista search system to protect against automated URLs (Uniformed Resource Locator) submitting. An example of a such CAPTCHA is shown on Figure 1. Since then, this type of protection was adopted by other resources: free mail services, blogs, social networks, etc.
