Currently, most authentication system requires users to answer the CAPTCHA (Completely Automated Public Turing Test to Tell Computer and Human Apart) before gaining the system access. CAPTCHA is a standard security technology for distinguish between human and computer program automatically. The problem of using CAPTCHA is the difficulty of reading the text-based presentation, or interpreting the image-based presentation. In addition, CAPTCHA cannot protect the third party attack. Therefore, this paper proposed a study of bio-detection function (BDF) of human based on the CAPTCHA system to identify the factors to be implemented into the CAPTCHA. The results can indicate that there are limitations in BDF of each person when uses CAPTCHA system and this limitation influences to the correctness of CAPTCHA typing.
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