A Proposal of Four-Panel Cartoon CAPTCHA: The Concept. - Takumi Yamamoto; Tokuichiro Suzuki; Masakatsu Nishigaki (NBiS, 2010)

A Proposal of Four-Panel Cartoon CAPTCHA. - Takumi Yamamoto; Tokuichiro Suzuki; Masakatsu Nishigaki (AINA, 2011)