A Comparative Study of Techniques used in Breaking CAPTCHA

Completely Automated Public Turing test to tell Computers and Human Apart (CAPTCHA) is a type of challenge response test used to differentiate computers and human being. Nowadays, many harmful computer programs attempts on attacking security and availability. CAPTCHA is the solution for such kind of problems. Breaking CAPTCHA can be considered as advantageous as well as disadvantageous, if someone is capable of implementing a technique to break difficult CAPTCHAs their solution can be used in solving unsolved hard AI problems. Most online payment and transaction systems use CAPTCHA in differentiating bots and humans. Different types of CAPTCHA includes text based, image based, audio and motion based CAPTCHAs. In this paper, we have discussed various techniques such as IMAGINAION, CNN, Naive attack used in solving a CAPTCHA. Over a decade there are many algorithms proposed by researchers to break CAPTCHA.