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Security and Communication Networks
Volume 2017 (2017), Article ID 6898617, 15 pages
Review Article

A Survey on Breaking Technique of Text-Based CAPTCHA

1State Key Laboratory of Mathematical Engineering and Advanced Computing, Zhengzhou 450002, China
2Henan Institute of Science and Technology, Xinxiang 453003, China
3Dalian University of Technology, Dalian 116024, China

Correspondence should be addressed to Xiangyang Luo

Received 25 September 2017; Accepted 27 November 2017; Published 24 December 2017

Academic Editor: Zhenxing Qian

Copyright © 2017 Jun Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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.