1. ABBYY FineReader;ABBYY,2013
2. The robustness of a new CAPTCHA;Ahmad,2010
3. CAPTCHA design: color, usability, and security;Ahmad;IEEE Internet Computing,2012
4. Breaking reCAPTCHA: a holistic approach via shape recognition;Baecher,2011
5. PessimalPrint: a reverse turing test;Baird;IJDAR,2003