1. reCAPTCHA: Human-Based Character Recognition via Web Security Measures
2. A Review on Various Interactive CAPTCHA Techniques Concerning Web Security;Rajpal;IJCSMC,2018
3. A Comparative Study of Audio and Visual CAPTCHA Accessibility;Smith;Journal of Accessibility Studies,2019
4. The Impact of Distorted Fonts on Text-Based CAPTCHA Usability;Garcia;Journal of Usability Studies,2015
5. A New 3D CAPTCHA Resistant to Machine Learning;Yan;IEEE Transactions on Information Forensics and Security,2008