Sharif Digital Repository

One of the main properties of digital watermarking methods is their security against attacks. In this paper, a novel attack against an adaptive semi-fragile image watermarking is proposed. In this attack, watermarking key and watermark are estimated by using several watermarked images. In order to improve the watermarking scheme against the proposed attack, the entropy of image blocks is utilized. Using entropy is compatible with Human Visual System (HVS); therefore it is suitable to determine the weight of watermark in image blocks. Since entropy is a sensitive feature, it is used to improve the watermarking method performance against the proposed attack. It is shown that this modification... 

One of the main properties of semi-fragile image watermarking methods is robustness against lossy compressions, e.g. JPEG conversion. In this paper, robustness of a semi-fragile watermarking scheme based on DWT (Discrete Wavelet Transform), coupled with SVD (Singular Value Decomposition), is improved. To achieve this goal a traditional, critically sampled wavelet transform is replaced by a redundant wavelet transform. The proposed approach is compared to the DWT-SVD based watermarking, introduced earlier, and significantly greater robustness under the JPEG compression is presented. ©2008 IEEE  

A5/1 is the strong version of the encryption algorithm on GSM (Global System for Mobile communications) used in many countries. It is constructed of a combination of three LFSRs (Linear Feedback Shift Registers) with irregular clocking manner. One of the most practical attacks against this algorithm is time-memory trade-off attack, which is based on birthday paradox. The goal of this attack is to find any intersection between precomputed LFSRs states set and set of states generating the output bits in the actual execution of the algorithm. In order to increase feasibility of this attack, the biased birthday attack was introduced. In this attack special states producing a specific pattern in... 

We study the volume averaging of inhomogeneous metrics within GR and discuss its shortcomings, such as gauge dependence, singular behavior as a result of caustics, and causality violations. To remedy these shortcomings, we suggest some modifications to this method. As a case study we focus on the inhomogeneous structured FRW model based on a flat LTB metric. The effect of averaging is then studied in terms of an effective backreaction fluid. It is shown that, contrary to the claims in the literature, the backreaction fluid behaves like a dark matter component, instead of dark energy, having a density of the order of 10-5 times the matter density, and, most importantly, it is gauge-dependent.... 

We present a study of the Fundamental Plane (FP) for a sample of 71 dwarf galaxies in the core of the Coma cluster in the magnitude range -21 < M I < -15. Taking advantage of the high-resolution DEIMOS spectrograph on Keck II for measuring the internal velocity dispersion of galaxies and high-resolution imaging of the Hubble Space Telescope (HST)/ACS, which allows an accurate surface brightness modelling, we extend the FP of galaxies to luminosities of ~1 mag fainter than all the previous studies of the FP in the Coma cluster. We find that the scatter about the FP depends on the faint-end luminosity cut-off, such that the scatter increases for fainter galaxies. The residual from the FP...