Completely Automated Public Turing Test to Tell Computers and Human Apart (CAPTCHA) is designed for simple problems that can be quickly solved by human, but are difficult for computers to solve [5]. It can work as a defeating CAPTCHA and make countless number of registrations on the websites. The methods in the existing system are complicated for people but, on the other hand, they are easier for bots. Also they provide only designing principles for captcha and captcha generation using some random function. In this paper a new unpredictable captcha algorithm have been proposed to generate captcha using LCG Algorithm. The generated random characters are converted into ASCII character. This method generates captcha in symbols, alphabets and numbers. As this is a robust text based scheme, it is difficult to identify the characters by bots. This paper describes in detail the CAPTCHA technology principle, method of implementation, examples, comparison of the analysis of safety and security risks.
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