1. Verification of a human in the loop or Identification via the Turing Test;Naor,1996
2. CAPTCHA: using hard AI problems for security;Ahn,2003
3. Enhanced CAPTCHAs : using animation to tell humans and computers apart;Athanasopoulos;Ifip Int Fed Inf Process,2006
4. Dynamic cognitive game CAPTCHA usability and detection of streaming-based farming;Mohamed,2014
5. Breaking visual CAPTCHAs with naive pattern recognition algorithms;Yan,2007