A Survey on Different CAPTCHA Techniques

Security researchers devised many mechanisms to prevent adversaries from conducting automated network attacks. One of the mechanism to prevent the network attacks are CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart). There are different types of CAPTCHAs we can see in the websites now a days. Every CAPTCHA have its own techniques to provide security. In this paper we compare and evaluate the performance of different types of CAPTCHAs based on their security and usability