The Robustness of a New 3D CAPTCHA

Abstract

CAPTCHA is a standard security technology to tell humans and computers and the most widely used method is text based scheme. As many text schemes have been broken, 3D CAPTCHAs have emerged as one of the latest one. In this paper, we study the robustness of 3D text-based CAPTCHA adopted by Ku6 which is a leading website providing videos in China and provide the first analysis of 3D hollow CAPTCHA. The security of this CAPTCHA scheme relies on a novel segmentation resistance mechanism, which combines Crowding Character Together (CCT) strategy and side surfaces which form the 3D visual effect of characters and lead to a promising usability even under strong overlapping between characters. However, by exploiting the unique features of the 3D characters in hollow font, i.e. parallel boundaries, the different stroke width of side faces and front faces and relationships between them, we propose a technique that segments connected characters apart and repairs some overlapped apart. The success segmentation rate is 70%. With minor changes, our attack program works well on its two variations, the segmentation rate is 75% and 85% respectively.