CAPTCHAs are automated tests that are there to avoid misuse of computing and information resources by bots. Typical text-based CAPTCHAs are proven to be vulnerable against malicious automated programs. In this paper, we present an image-based CAPTCHA scheme using easily identifiable human appearance characteristics that overcomes the weaknesses of current text-based schemes. We propose and evaluate two applications for our scheme involving 25 participants. Both applications use same characteristics but different classes against those characteristics. Application 1 is optimized for security while application 2 is optimized for usability. Experimental evaluation shows promising results having 83% human success rate with Application 2 as compared to 62% with Application 1.

Abstract-A two-factor remote authentication scheme was presented by Chun-Ta Li et al. in 2010. We present the framework of an impersonation attack against their scheme if the smart card gets stolen. We show that it is easy for an attacker to compute password of a user by using information extracted from the stolen smart card. We also propose a simple and easy solution to fix this problem.

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