1. L von Ahn, M Blum, J Langford, CAPTCHA: Using Hard AI Problems for Security (2003).
2. Knowledge-based System;Swain,2013
3. Text-Based CAPTCHA Strengths and Weaknesses;Bursztein,2011
4. Ian J. Goodfellow, Yaroslav Bulatov, Julian Ibarz, Sacha Arnoud, and Vinay D. Shet. Multi-digit Number Recognition from Street View Imagery using Deep Convolutional Neural Networks. CoRR abs/1312.6082 (2014).
5. Towards Evaluating the Security of Real-World Deployed Image CAPTCHAs;Zhao,2018