Aimilia Tasidou, Pavlos S. Efraimidis, Yannis Soupionis, Lilian Mitrou, Vasilios Katos. Privacy-preserving, user-centric VoIP CAPTCHA challenges: An integrated solution in the SIP environment. Inf. Manag. Comput. Security, 24(1):2-19, 2016. [doi]
Abstract is missing.