Emerging-Image Motion CAPTCHAs: Vulnerabilities of Existing Designs, and Countermeasures
Emerging-Image Motion CAPTCHAs: Vulnerabilities of Existing Designs, and Countermeasures
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DOI：
10.1109/tdsc.2017.2719031
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发表时间：
2019-11-01
影响因子：
7.3
通讯作者：
Zhang, Chengcui
中科院分区：
文献类型：
--
作者：
Gao, Song;Mohamed, Manar;Zhang, Chengcui
基于“涌现”的概念，Xu等人（Usenix Security 2012; TDSC 2013）开发了使用2D空心对象（码字）的涌现图像移动对象（EIMO）CAPTCHA的第一个具体实例，该实例被证明是可用的，并且被认为是安全的。在本文中，我们强调了这种2D EIMO CAPTCHA设计的隐藏安全漏洞。一个关键的漏洞是2D对象上的摄像机投影是恒定的（与3D对象不同），这使得可以通过叠加和聚合从连续帧中提取的对象的时间分散部分来重建底层码字。我们设计并实现了一个自动攻击框架，使用图像处理技术来击败这种设计，并表明其识别移动码字的准确率高达89.2%，在不同的参数化。我们的框架可以广泛地用于破坏2D EIMO CAPTCHA的不同实例的安全性（不仅仅是Xu等人的当前最新技术），给出了广义的鲁棒的攻击后端理论，即从多个时间段对应的连续帧中定位码字、降低噪声和积累目标轮廓信息的方法。作为对策，我们提出了一个根本不同的设计EIMO验证码的基础上伪3D对象，并检查其安全性以及可用性。我们认为，这种设计可以抵御我们对二维EIMO验证码的攻击，虽然在成本降低可用性相比，现在不安全的二维EIMO验证码。
Based on the notion of "emergence", Xu et al. (Usenix Security 2012; TDSC 2013) developed the first concrete instantiation of emerging-image moving-object (EIMO) CAPTCHAs using 2D hollow objects (codewords), shown to be usable and believed to be secure. In this paper, we highlight the hidden security weaknesses of such a 2D EIMO CAPTCHA design. A key vulnerability is that the camera projection on 2D objects is constant (unlike 3D objects), making it possible to reconstruct the underlying codewords by superimposing and aggregating the temporally scattered parts of the object extracted from consecutive frames. We design and implement an automated attack framework to defeat this design using image processing techniques, and show that its accuracy in recognizing moving codewords is up to 89.2 percent, under different parameterizations. Our framework can be broadly used to undermine the security of different instances of 2D EIMO CAPTCHAs (not just the current state-of-the-art by Xu et al.), given the generalized and robust back-end theories in our attack, namely the methods to locate a codeword, reduce noises and accumulate objects' contour information from consecutive frames corresponding to multiple time periods. As a countermeasure, we propose a fundamentally different design of EIMO CAPTCHAs based on pseudo 3D objects, and examine its security as well as usability. We argue that this design can resist our attack against 2D EIMO CAPTCHAs, although at the cost of reduced usability compared to the - now insecure - 2D EIMO CAPTCHAs.