Zhu, B. B., Yan, J., Li, Q., Yang, C., Liu, J., Xu, N., Yi, M. & Cai, K. (2010). Attacks and design of image recognition CAPTCHAs. CCS '10: Proceedings of the 17th ACM conference on Computer and communications security (p./pp. 187--200), October, New York, NY, USA: ACM. ISBN: 978-1-4503-0245-6