Text CAPTCHAs are widely deployed to protect online systems from spams and abuses. However, nearly all the Text CAPTCHA designs in nowadays are based on English characters, which may not be user-friendly for non-English speakers. Therefore, designing CAPTCHAs based on local languages has been an important topic. Previously, researchers mainly focused on the algorithms of generating localized CAPTCHAs, while the usability of such designs has hardly been systematically evaluated. Particularly, a universal procedure for the designing of usable localized CAPTCHAs is still unavailable.
Taking the usability investigation of Chinese CAPTCHAs as a case study, we analyzed the cognition process of solving English and Chinese CAPTCHAs, compared the usability of CAPTCHAs based on both languages, explored the intrinsic factors that may affect the usability of Chinese CAPTCHAs, and finally proposed a generalized procedure for the designing of usable CAPTCHAs based on any other local languages. These studies not only provide guidelines for developing user-friendly Chinese CAPTCHAs, but also shine a light on the designing of usable CAPTCHAs that employ other languages.