Abstract
The CAPTCHA has become an important issue in multimedia security. Aimed at a commonly used text-based CAPTCHA, this paper outlines some typical methods and summarizes the technological progress in text-based CAPTCHA breaking. First, the paper presents a comprehensive review of recent developments in the text-based CAPTCHA breaking field. Second, a framework of text-based CAPTCHA breaking technique is proposed. And the framework mainly consists of preprocessing, segmentation, combination, recognition, postprocessing, and other modules. Third, the research progress of the technique involved in each module is introduced, and some typical methods of segmentation and recognition are compared and analyzed. Lastly, the paper discusses some problems worth further research.
Highlights
As a multimedia security mechanism, CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart [1]) called Human Interactive Proofs (HIP [2]), can protect multimedia privacy
CAPTCHA breaking can verify the security of existing CAPTCHAs, and it can promote the development of CAPTCHA design technique
The CAPTCHA is an integral part of artificial intelligence and an important prerequisite to actualize natural human-computer interaction
Summary
As a multimedia security mechanism, CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart [1]) called Human Interactive Proofs (HIP [2]), can protect multimedia privacy. In order to verify security and reliability of CAPTCHA, the breaking technology came into being. It involves image processing, pattern recognition, image understanding, artificial intelligence, computer vision, and many other disciplines. The research of breaking CAPTCHA constantly refreshes limits to Turing test, and can be applied in other fields such as digital paper-based media, speech recognition, and image labeling. Based on the research of textbased CAPTCHA breaking technique, this paper will review the relative research and prospect future trends. The remainder of this paper is organized as follows: Section 2 briefly introduces the text-based CAPTCHA.
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