| dc.description.abstract | Many websites and applications rely on CAPTCHA to protect them from bot attacks. Otherwise, users and businesses will be exposed to risks. Although several different CAPTCHA systems have been proposed, the development of deep learning algorithms allows attackers to create more efficient and accurate attack methods. Many studies have shown that existing CAPTCHA systems are no longer safe, especially text-based CAPTCHA. To resolve this issue, a simple, secure, and effective annuli CAPTCHA system is proposed in this thesis, In the proposed system, the annuli CAPTCHA image containing the overlapping of circles and ovals is randomly generated. The user wishing to gain access to the system is required to answer correctly the total number of circles and ovals in the image to prove that he/she is not a bot. The security of our proposed CAPTCHA system is verified by three attack methods. The usability survey of our CAPTCHA system conducted by anonymous questionnaires shows that our system is user friendly. In other words, the proposed system maintains a high level of usability under the premise of high security. In addition, we identify the ＂indistinguishable region＂ and propose a reliable method to further improve the level of security for the proposed CAPTCHA system. The result shows the security of the proposed annuli CAPTCHA system has been significantly improved. Compared with the existing CAPTCHA system, our CAPTCHA system is significantly better in terms of security, usability and ease of implementation. | en_US |