Oblivious CAPTCHA: A Fifth-Factor Technology for Practical CAPTCHA Use

抄録

The CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) idea is widely used as a HIP (Human Interactive Proof) for distinguishing between humans and computer programs. These are automated tests that humans can pass, but that current computer programs can't handle. Breaking a CAPTCHA generally involves solving a difficult Artificial Intelligence problem. There are demands for new technologies that are stronger against automatic attacks by machines, without making it too hard for humans to pass the tests. In this paper, we propose a concept called the Oblivious CAPTCHA, as a fifth-factor technology for a practical CAPTCHA. The Oblivious CAPTCHA uses tasks such as identifying the English alphabetic characters in a set of mixed alphabetic and non-alphabetic characters, or counting the English alphabetic characters in a string. In experiments we found that the Oblivious CAPTCHA was easy for users, because human beings can recognize images of alphabetic characters quickly and accurately, but this is difficult for computers, because OCR tech-niques tend to misrecognize non-alphabetic characters as though they were alphabetic. This shows our approach is practical. We also describe novel algorithms for enhancing the skill gap between humans and computers that can be used with many existing CAPTCHAs.