A secure circular text-based CAPTCHA

To prevent malicious activities and automated programmes from infiltrating and attacking websites or systems, a secure circular text-based CAPTCHA is designed based on a multi-secret visual cryptography. In this paper, multiple circular secret images are randomly generated by the server- side are encrypted into two circular share images, one of the share images is saved while the other is distributed to the user. Random characters are selected from each secret image to dynamically generate a circular CAPTCHA, with enhancing the authentication function for legitimate users and providing more effective resistance to phishing attacks. From the evaluation and recognition of the quality of the CAPTCHA, the circular text-based CAPTCHA has achieved a balance between usability and security at a certain extent.