Association for Computing Machinery
Interactive websites use text-based captchas to prevent unauthorized automated interactions. These captchas must be easy for humans to decipher while being difficult to crack by automated means. In this paper the authors present a framework for the systematic study of captchas along these two competing objectives. They begin by abstracting a set of distortions that characterize current and past commercial text-based captchas. By means of user studies, they quantify the way human captcha solving performance varies with changes in these distortion parameters.