Systematic approach to measure strength of Text based CAPTCHA

There is a substantial emphasis on security design of Text based CAPTCHA by way of providing the security at pre-processing layer, segmentation layer and character recognition layer. The strength to protect the system from security threats defines the robustness of any given system and which in turn depends on number of attempts the malicious program, which does not have direct access, will need to defend it. This paper proposes a systematic approach to measure the qualitative strength of the Text based CAPTCHA depending on type of TBC, Length of TBC, Character recognition rate and its response time. Applying our systematic approach to different CAPTCHA providers and users, we found that approximately 80% CAPTCHAs are weak and susceptible to our attacks. Along with the attacks leading to weakness of text based CAPTCHA, we have provided the distinguishing features of different CAPTCHA Segmenter which play an important role in defending anti-segmentation techniques.