Abstract
We introduce a new type of 3D text-based CAPTCHA, called DotCHA, which relies on human interaction. DotCHA asks users to rotate a 3D text model to identify the correct letters. The 3D text model is a twisted form of sequential 3D letters around a center pivot axis, and it shows different letters depending on the rotation angle. The model is not composed of a solid letter model, but a number of spheres to resist character segmentation attacks, and this is why DotCHA is classified as a scatter-type CAPTCHA. DotCHA is tolerant to machine learning attacks because each letter is only identified in each particular direction. We demonstrate that DotCHA, while maintaining usability, is resistant to existing types of attacks.
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