3,157 research outputs found
Discovering New Vulnerabilities in Computer Systems
Vulnerability research plays a key role in preventing and defending against malicious computer system exploitations. Driven by a multi-billion dollar underground economy, cyber criminals today tirelessly launch malicious exploitations, threatening every aspect of daily computing. to effectively protect computer systems from devastation, it is imperative to discover and mitigate vulnerabilities before they fall into the offensive parties\u27 hands. This dissertation is dedicated to the research and discovery of new design and deployment vulnerabilities in three very different types of computer systems.;The first vulnerability is found in the automatic malicious binary (malware) detection system. Binary analysis, a central piece of technology for malware detection, are divided into two classes, static analysis and dynamic analysis. State-of-the-art detection systems employ both classes of analyses to complement each other\u27s strengths and weaknesses for improved detection results. However, we found that the commonly seen design patterns may suffer from evasion attacks. We demonstrate attacks on the vulnerabilities by designing and implementing a novel binary obfuscation technique.;The second vulnerability is located in the design of server system power management. Technological advancements have improved server system power efficiency and facilitated energy proportional computing. However, the change of power profile makes the power consumption subjected to unaudited influences of remote parties, leaving the server systems vulnerable to energy-targeted malicious exploit. We demonstrate an energy abusing attack on a standalone open Web server, measure the extent of the damage, and present a preliminary defense strategy.;The third vulnerability is discovered in the application of server virtualization technologies. Server virtualization greatly benefits today\u27s data centers and brings pervasive cloud computing a step closer to the general public. However, the practice of physical co-hosting virtual machines with different security privileges risks introducing covert channels that seriously threaten the information security in the cloud. We study the construction of high-bandwidth covert channels via the memory sub-system, and show a practical exploit of cross-virtual-machine covert channels on virtualized x86 platforms
A New Transcoding Scheme for Scalable Video Coding to H.264/AVC
Requests from various video terminals push video servers to equip with scalability for video contents distribution in different ways. Scalable Video Coding (SVC) as the extension of H.264/AVC standard can provide the scalability for video servers by encoding videos into one base layer and several enhancement layers. To enable mobile devices without scalability receive videos at their best extent, converting bit-streams from SVC into H.264/AVC becomes the key technique. Bit-stream rewriting is the simplest way without quality loss. However, rewriting is not a real transcoding scheme, since it needs to modify SVC encoders. This paper proposes a novel transcoding approach to support spatial scalability by minimizing the distortions generated from re-encoding process. The proposed scheme keeps the input bit-streams’ information at maximum and adopts the hybrid upsampling method to do residue scaling, which can reduce the transcoding distortion into minimization. Experimental results demonstrate that the loss of the rate-distortion (RD) performance of the proposed transcoding scheme is better than Full Decoding Re-encoding (FDR) which can get the highest video quality in general sense, by achieving up to 0.9 dB Y-PSNR gain while saving 95%~97% processing time
Simulating gravitational waves passing through the spacetime of a black hole
We investigate how GWs pass through the spacetime of a Schwarzschild black
hole using time-domain numerical simulations. Our work is based on the
perturbed 3+1 Einstein's equations up to the linear order. We show explicitly
that our perturbation equations are covariant under infinitesimal coordinate
transformations. Then we solve a symmetric second-order hyperbolic wave
equation with a spatially varying wave speed. As the wave speed in our wave
equation vanishes at the horizon, our formalism can naturally avoid boundary
conditions at the horizon. Our formalism also does not contain coordinate
singularities and, therefore, does not need regularity conditions. Then, based
on our code, we simulate both finite and continuous initially plane-fronted
wave trains passing through the Schwarzschild black hole. We find that for the
finite wave train, the wave zone of GWs is wildly twisted by the black hole.
While for the continuous wave train, unlike geometric optics, GWs can not be
sheltered by the back hole. A strong beam and an interference pattern appear
behind the black hole along the optical axis. Moreover, we find that the
back-scattering due to the interaction between GWs and the background curvature
is strongly dependent on the direction of the propagation of the trailing
wavefront relative to the black hole.Comment: 24 pages, 9 figure
Government affiliation, real earnings management, and firm performance : the case of privately held firms
Using a moderated mediation model, we investigate the effects of government affiliation on the performance and real earnings management of privately held firms in China between 1998 and 2012. We find that politically affiliated firms tend to have superior accounting performance. The findings also suggest that politically affiliated firms are more likely than non-affiliated firms to engage in real activities to manipulate earnings. Furthermore, regional economic development moderates the relationships between political affiliation and real earnings management as well as firm performance. Finally, real earnings management mediates the effect of political affiliation on firm performance among privately held firms
A New Residual Distribution Hydrodynamics Solver for Astrophysical Simulations
A wide array of astrophysical systems can only be accurately modelled when
the behaviour of their baryonic gas components is well understood. The residual
distribution (RD) family of partial differential equation (PDE) solvers can be
used to solve the fluid equations, which govern this gas. We present a new
implementation of the RD method to do just this. The solver efficiently and
accurately calculates the evolution of the fluid, to second order accuracy in
both time and space, across an unstructured Delaunay triangulation, built from
an arbitrary distribution of vertices, in either 2D and 3D. We implement a
novel new variable time stepping routine, which applies a drifting mechanism to
greatly improve the computational efficiency of the method. We conduct
extensive testing of the new implementation, demonstrating its innate ability
to resolve complex fluid structures, even at very low resolution. Our
implementation can resolve complex fluid structures with as few as 3-5
resolution elements, demonstrated by Kelvin-Helmholtz and Sedov blast tests. It
includes three residual calculation modes, the LDA, N and blended schemes, each
designed for different scenarios. These range from smooth flows (LDA), to
extreme shocks (N), and scenarios where either may be encountered (blended). We
compare our RD solver results to state-of-the-art solvers used in other
astrophysical codes, demonstrating the competitiveness of the new approach,
particularly at low resolution. This is of particular interest in large scale
astrophysical simulations, where important structures, such as star forming gas
clouds, are often resolved by small numbers of fluid elements.Comment: 20 pages, 20 figure
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