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.

Existing CAPTCHA solutions are a major source of user frustration on the Internet today, frequently forcing companies to lose customers and business. Game CAPTCHAs are a promising approach which may make CAPTCHA solving a fun activity for the user. One category of such CAPTCHAs – called Dynamic Cognitive Game (DCG) CAPTCHA – challenges the user to perform a game-like cognitive (or recognition) task interacting with a series of dynamic images. Specifically, it takes the form of many objects floating around within the images, and the user’s task is to match the objects corresponding to specific target(s), and drag/drop them to the target region(s). In this paper, we pursue a comprehensive analysis of DCG CAPTCHAs. We design and implement such CAPTCHAs, and dissect them across four broad but overlapping dimensions: (1) usability, (2) fully automated attacks, (3) human-solving relay attacks, and (4) hybrid attacks that combine the strengths of automated and relay attacks. Our study shows that DCG CAPTCHAs are highly usable, even on mobile devices and offer some resilience to relay attacks, but they are vulnerable to our proposed automated and hybrid attacks.

CAPTCHAs represent an important pillar in the web security domain. Yet, current CAPTCHAs do not fully meet the web security requirements. Many existing CAPTCHAs can be broken using automated attacks based on image processing and machine learning techniques. Moreover, most existing CAPTCHAs are completely vulnerable to human-solver relay attacks, whereby CAPTCHA challenges are simply outsourced to a remote human solver.

Body volume and body shape have been used in the estimation of body composition in clinical research. However, the determination of body volume typically requires sophisticated and expensive equipment. Similarly, the use of body shape to predict body composition is limited by rater biases as well as reproducibility. In this paper, we aim to introduce simple yet relatively accurate techniques for body volume and body shape representation that reduce limitations of traditional approaches. We propose an automated method to construct a 3D model of the body by accumulating ellipse-like slices formed by using the length and width features sampled from the back and side profile images. Body volume is represented in pixels by adding up the areas of the slices. Apart from representing body volume in pixels, we also aim to extract shape features from the 2D images and to create clusters of individuals according to their body shape. The body volume representation and the proposed shape features together with other meta-information including age, sex, race, height, and weight, could be effectively used in body composition prediction. Our study results indicate that the body volume calculated by the proposed method is reasonably accurate and the extracted shape clusters provide important information when estimating body composition.

Existing captcha solutions on the Internet are a major source of user frustration. Game captchas are an interesting and, to date, little-studied approach claiming to make captcha solving a fun activity for the users. One broad form of such captchas -- called Dynamic Cognitive Game (DCG) captchas -- challenge the user to perform a game-like cognitive task interacting with a series of dynamic images. We pursue a comprehensive analysis of a representative category of DCG captchas. We formalize, design and implement such captchas, and dissect them across: (1) fully automated attacks, (2) human-solver relay attacks, and (3) usability. Our results suggest that the studied DCG captchas exhibit high usability and, unlike other known captchas, offer some resistance to relay attacks, but they are also vulnerable to our novel dictionary-based automated attack.

Dynamic Cognitive Game (DCG) CAPTCHAs are a promising new generation of interactive CAPTCHAs aiming to provide improved security against automated and human-solver relay attacks. Unlike existing CAPTCHAs, defeating DCG CAPTCHAs using pure automated attacks or pure relay attacks may be challenging in practice due to the fundamental limitations of computer algorithms (semantic gap) and synchronization issues with solvers. To overcome this barrier, we propose two hybrid attack frameworks. which carefully combine the strengths of an automated program and offline/online human intelligence. These hybrid attacks require maintaining the synchronization only between the game and the bot similar to a pure automated attack, while solving the static AI problem (i.e., bridging the semantic gap) behind the game challenge similar to a pure relay attack. As a crucial component of our framework, we design a new DCG object tracking algorithm, based on color code histogram, and show that it is simpler, more efficient and more robust compared to several known tracking approaches. We demonstrate that both frameworks can effectively defeat a wide range of DCG CAPTCHAs.

Inaccurate image segmentation often has a negative impact on object-based image retrieval. Researchers have attempted to alleviate this problem by using hierarchical image representation. However, these attempts suffer from the inefficiency in building the hierarchical image representation and the high computational complexity in matching two hierarchically represented images. Existing approaches construct the hierarchical image representation in two steps. The first step is to perform segmentation at different image resolutions, and the second step is to construct a hierarchical representation of the image by associating segments from different resolutions. In this research, an innovative all-in-one run approach is proposed that concurrently performs image segmentation and hierarchical tree construction, producing a hierarchical region tree to represent the image. In addition, an efficient hierarchical region tree matching algorithm is proposed with a reasonably low time complexity and used in multiple object image retrieval. The experimental results demonstrate the efficacy and efficiency of the proposed approach.

In this paper we present a two-phase spam image clustering framework. The proposed framework performs a histogram based projective clustering on visual features in the first phase, followed by a text-based clustering in the second phase. There are several contributions in this study. First, we address the complex nature of spam image obfuscation techniques. Second, a multi-clue framework is developed to profile spam images of common spamming sources which provide evidence for tracking spam gangs. Third, projective clustering eliminates the need to choose among distance metrics for clustering analysis, while systematically exploring subspaces that correspond to clusters.