1. Yan, J., El Ahmad, A.S.: Usability of CAPTCHAs or usability issues in CAPTCHA design. In: Proceedings of ACM SOUPS 2008, pp. 44–52 (2008)
2. Burzstein, E., Martin, M., Mitchell, J.C.: Text-based CAPTCHA Strengths and Weaknesses. In: Proceedings of ACM CSS 2011, pp. 125–138 (2011)
3. El Ahmad, A.S., Yan, J., Tayara, M.: The Robustness of Google CAPTCHAs. Newcastle University Tech. Report CS-TR-1278 (September 2011)
4. Matthews, P., Zou, C.C.: Scene tagging: image-based CAPTCHA using image composition and object relationships. In: Proc. of the 5th ACM Symposium on Information, Computer and Communications Security (CCS 2005), pp. 345–350. ACM, New York (2005)
5. He, P., Sun, Y., Zheng, W., Wen, X.: Filtering short message spam of group sending using CAPTCHA. In: Proceedings of the First International Workshop on Knowledge Discovery and Data Mining (WKDD 2008), pp. 558–561 (2008)