1. Yan, J., El Ahmad, A.S.: Breaking Visual CAPTCHAs with Naïve Pattern Recognition Algorithms. In: 23rd Annual Computer Security Applications Conf., pp. 279–291 (2007)
2. Huang, S., Lee, Y., Bell, G., Ou, Z.: A Projection-based Segmentation Algorithm for Breaking MSN and YAHOO CAPTCHAs. In: Proc. World Congress Eng., vol. 1 (2008)
3. Chen, Y., Wang, Y.: Segmentation of Single- or Multiple-Touching Handwritten Numeral String Using Background and Foreground Analysis. IEEE Trans. Pattern Analysis Machine Intelligence 22(11), 1304–1317 (2000)
4. Simard, P., Szeliski, R., Benaloh, J., Couvreur, J., Calinov, I.: Using Character Recognition and Segmentation to Tell Computers from Humans. In: Int’l Conference on Document Analysis and Recogntion, ICDAR (2003)
5. Von Ahn, L., Blum, M., Langford, J.: Telling Humans and Computer Apart Automatically. CACM 47(2) (2004)