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A CAPTCHA, elaborated as a Completely Automated Public Turing Test to Tell Computers & Humans Apart is a popular software used to prevent automated bots from engaging the web applications and hogging resources. All the available CAPTCHA formats remain insufficiently resistant to bots, especially when combined with a relay attack. We propose a new type of dynamic CAPTCHA that is resistant to automated as well as relay attacks due to its dynamic nature. In our CAPTCHA, the user needs to identify a moving object as the target, from among a number of randomly moving decoy objects and trace that target with the mouse cursor. The user passes the test when they are able to trace the target for a certain amount of time. The latency introduced to the remote solver makes it difficult to break the CAPTCHA since the target object moves dynamically. It is also difficult for a bot to track the target using image processing because there are a number of similar looking objects. With the CAPTHCA‟s parameters set to suitable value, a relay attack cannot be established economically and false acceptance rate with bots are minimized without affecting human success rate.
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