[SSK+12] Emmanouela Stachtiari, Yannis Soupionis, Panagiotis Katsaros, Anakreontas Mentis and Dimitris Gritzalis. Probabilistic model checking of CAPTCHA admission control for DoS resistant anti-SPIT protection. In Proc. 7th International Conference on Critical Information Infrastructures Security (CRITIS'12). 2012. [Studies the effectiveness of admission control policies for DoS-resistanct CAPTCHA schemes using probabilistic model checking and PRISM.]