A CAPTCHA with Tips Related to Alphabets Upper or Lower Case

Due to the ubiquity of web browsers and the dramatic development of dynamic web pages, web applications become the most popular computing model for providing services over the Internet. Unfortunately, this type of systems is vulnerable to the attacks issued by automated programs. CAPTCHA, a challenge-response test, is the most widely used mechanism to protect web application systems from attacks issued by computer programs. By considering the maintenance cost, most of the CAPTCHA systems employ the text-based challenge in which a string is represented as an image. A user is classified as a human by a text-based CAPTCHA if the user can respond a challenge with the exact string shown in the image. One drawback of a text-based CAPTCHA system is that the string is possibly recognized by a computer program equipped with OCR function. Obfuscating text images can strength the robustness of CAPTCHAs. But, it also degrades the usability of the systems. In this paper, we propose a new text-based CAPTCHA mechanism in which each challenge is associated with a tip providing humans enough information to recognize the alphabets in a highly distorted text image. Therefore, our system provides a balance between the effectiveness and human success rates of text-based CAPTCHAs.