A highly legible CAPTCHA that resists segmentation attacks

Abstract

A CAPTCHA which humans find to be highly legible and which is designed to resist automatic character-segmentation attacks is described. As first detailed in [BR05], these 'ScatterType' challenges are images of machine-print text whose characters have been pseudorandomly cut into pieces which have then been forced to drift apart. This scattering is designed to repel automatic segmentthen-recognize computer vision attacks. We report results from an analysis of data from a human legibility trial with 57 volunteers that yielded 4275 CAPTCHA challenges and responses. We have located an operating regime-ranges of the parameters that control cutting and scattering-within which human legibility is high (better than 95% correct) even though the degradations due to scattering remain severe. © Springer-Verlag Berlin Heidelberg 2005.