Haichang Gao, Wei Wang, Jiao Qi, Xuqin Wang, Xiyang Liu, Jeff Yan. The robustness of hollow CAPTCHAs. In Ahmad-Reza Sadeghi, Virgil D. Gligor, Moti Yung, editors, 2013 ACM SIGSAC Conference on Computer and Communications Security, CCS'13, Berlin, Germany, November 4-8, 2013. pages 1075-1086, ACM, 2013. [doi]
Abstract is missing.