Text CAPTCHA has been an effective means to protect online systems from spams and abuses caused by automatic scripts which pretend to be human beings. However, nearly all the Text CAPTCHA designs in nowadays are based on English characters, which may not be the most userfriendly option for non-English speakers. Therefore, under the background of globalization, there is an increasing interest in...

Over the last years, most websites where users can register (e.g., email providers and social networks) adopted CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart) as a countermeasure for automated attacks. The battle of wits between designers and attackers of CAPTCHAs led to current ones being annoying and hard to resolve for users, while still being vulnerabl...