許多網站和應用程序都依賴CAPTCHA來保護它們免受殭屍程序攻擊。否則用戶和企業將面臨風險。儘管已經提出了幾種不同的CAPTCHA系統，但深度學習算法的發展使攻擊者能夠創建更有效，更準確的攻擊方法。許多研究表明，現有的驗證碼系統不再安全，尤其是基於文本的驗證碼系統。為了解決這個問題，本文提出了一種簡單，安全，有效的環形驗證碼系統。在該系統中，隨機生成了包含圓形和橢圓形重疊的環形驗證碼圖像。希望訪問該系統的用戶需要正確回答圖像中圓圈和橢圓的總數，以證明他/她不是機器人。我們提出的CAPTCHA系統的安全性通過三種攻擊方法來驗證。通過匿名問卷對我們的CAPTCHA系統進行的可用性調查表明，我們的系統是用戶友好的。換句話說，所提出的系統在高安全性的前提下保持了高可用性。此外我們發現並驗證了“無法區分的區域”，以此為基礎提出了一種可靠的方法來進一步提高所提議的CAPTCHA系統的安全級別。結果表明，所提議的環形CAPTCHA系統的安全性得到了顯著提高。在現有的CAPTCHA系統中，我們的CAPTCHA系統在安全性，可用性和易於實施方面都明顯更好。;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.
