SECURITY ANALYSIS OF TEXT BASED CAPTCHAs

Abstract

Completely Automated Public Turing Test to Tell Computers and Humans Apart, also known as CAPTCHA is a program which generate tests which can be solved by humans but cannot be solved by other computer programs known as "bots". CAPTCHAs plays a very important role by providing practical security in various areas and tasks. There are different CAPTCHA schemes available. The most encountered CAPTCHA is text based CAPTCHA, in which user has to identify the text to pass the test. However, most of these CAPTCHAs are breakable by using present techniques which raise a serious concern regarding the security provided by these programs. The objective of this work is to understand the present attacks and analyze the security provided by text based CAPTCHAs. Also, a generalized CAPTCHA breaker is proposed to understand strength and weakness of different text based CAPTCHA schemes. Finally, a new security level in present text based CAPTCHAs is proposed to make them more secure.