Annulus: A novel image-based CAPTCHA scheme

A new image-based CAPTCHA, Annulus, Which capitalizes on human ability of understanding the content of image, specially, counting the numbers of the annulus in a picture was presented in this paper. The Annulus CAPTCHA presents 1 to 8 annuluses in a picture randomly and asks a user input the number of annuluses. Preliminary experiments verified the usability and security of the proposed scheme. Compared with existing implementations, Annulus CAPTCHA achieves a human accuracy of 88% at least and is robust against common attacks. Besides, Annulus CAPTCHA is language independent and flexible for customization (both in terms of security and usability levels). These features make it a promising alternative image-based CAPTCHA to web service providers.