مجله علوم و فناوری های پدافند نوین
سال هشتم شماره 3 (پیاپی 29، پاییز 1396)

One of the most important methods for checking the resistant of a block cipher against linear and differential analysis is counting of minimum active s-boxes. According to this number, proportion of minimum active s-boxes to all used s-boxes can be obtained. In Feistel structure, left and right half XORing cause difference cancelation reducing this proportion. One method for reducing difference cancelation and improving this proportion is presented previously using multiple MDS matrix. However, this method is suitable for design of 128 bit block ciphers and hasn’t good efficiency in 256 bit block ciphers. In this paper, the problem of finding proper multiple diffusion layers for Switching Structure on big dimension and big field is firstly surveyed. Then, a search algorithm is presented, used for making several categories of Recursive Diffusion Layers. In the next section, by using this Recursive Diffusion Layers, a 256 bit block cipher is designed base on Switching Structure. We verify security and efficiency of this scheme is verified and it is concluded that this scheme is resistant to linear and differential attack showing impossible differential attack and also has a good efficiency compare to other 256 bit block cipher algorithm.

Feature selection is one of the key challenges in developing intrusion detection systems. Classification algorithms in intrusion detection systems may be inconvenient for problems having so many features, because the size of the search space grows exponentially in terms of the number of features. This is while most of the features may be either irrelevant or redundant. Therefore, considering only relevant features (i.e. feature selection) may have a significant impact on the performance of the classification algorithms. The Imperialist Competitive Algorithm (ICA) can be used as a feature selection method with a high convergence, but it sometimes gets trapped in a local optimum. On the contrary, the Genetic Algorithm (GA) is powerful enough in terms of search for solutions, but it suffers from late convergence. Therefore, using a combination of both algorithms for feature selection may result in a rapid convergence as well as in a high precision. In this paper, by applying the Assimilate operator of the ICA to the GA, we propose a new feature selection algorithm for intrusion detection systems. The proposed algorithm has been tested on the KDD99 dataset using the decision tree classification. The experimental results show that the proposed algorithm has improved the detection rate (95.03%), false alarm rate (1.46) and the speed of convergence (3.82 second).

Reinforced concrete structures are of the most common buildings exposed to vehicle bombs. In this study three typical structures were exposed to conventional and severe blasts. Three, six and ten story buildings with special moment frames designed according to earthquake were considered. Firstly, the local failure of first columns in front of explosion and the probability of progressive collapse were investigated and secondly, overall response of structures under explosion and earthquake was determined and compared. After analyses, it was found that explosions will create more severe consequences in low-rise structures than mid and high-rise structures, mostly due to the destruction of columns and the following progressive collapse. If columns are not damaged locally, there will be less severe responses in the frames due to explosion rather than earthquakes, containing displacement, drift ratio, base shear and plastic hinges. Therefore, major designer’s efforts should be in preventing damage to individual structural elements and preventing progressive collapset.

The selection of an appropriate cover to embedding a secret message is one of the main issues in the steganography. This leads to increasing the performance and decreasing the error from the point of view of passive defence. In this paper, preparation of a cover image with the suitable security level is described. For this porpose, some important features such as contrast, energy and the use of fuzzy logic with a selection threshold for Harris corners have been considered. Firstly, some images with different contrast were obtained but a fixed threshold for Harris corners from each cover image which extracting the features is done using Gray Levels Co-Occurrence Matrix (GLCM). Then, the security of images was graded using fuzzy logic. The images with different security levels can be stored in the different bank of images. According to the simulation which is done, selected images based on the proposed method, have an appropriate SSIM and PSNR in the steganography.

The reports have shown that the existed thermal I.R sensors are able to sense the temperature difference 2°C to 8 °C between buried spaces exhaust air and ambient with a good accuracy. In this paper, in order to minimize the probability of identifying safe buried structures by spy satellites and increasing their thermal camouflage, a plate heat exchanger for reducing the temperature of the exhaust air of buildings to ambient temperature is proposed. The technical specification of the proposed chevron type heat exchanger is determined based on the traditional design approaches. A MATLAB program code is developed for analysis of these heat exchangers and finding the optimized angle of grooves and estimating number of plates. The ANSYS software is also used to analysis the effects of plate materials on total heat transfer and their strength to deformation. Then the designed heat exchanger, under the same operating conditions with the analytical approach, is analyzed again by the ASPEN software. The analytical solutions and the numerical results were validated with approved results reported in the literature and a good consistency was observed. Finally as a case study, the groove angles, required number of plates and other geometrically data is calculated for a heat exchanger of a safe buried structure. The results showed that a plate heat exchanger, made by 161 stainless steel plates with 2m in height and 45 degrees groove angle, is needed for thermal camouflage of a safe buried building with 10000 CFM exhaust air at 22°C to an ambient at -20°C.

Today by the increasingly growth of internet users, many internet services such as electronic mails, search engines and social networks are presented. Due to developments in internet services, disabling the services through automatic programs such as internet robots and hackers has become a main problem. CAPTCHA is a mechanism that confirms whether the user is a human or a computer program. This task is performed through some tests. Performing these tests is easy for human but it is difficult for robots. CAPTCHA techniques are trying to increase the mentioned difficulties in automatic programs and it is performed through increasing the deviations and parasite. Then solving it will become difficult for human. In this research an image CHAPTCHA is presented and it is done through mathematical morphology the purpose of this solution to increase security and to defend against malicious attacks and hackers. In this method by the input image, a binary model is developed and worn. Then by making some changes, the image is presented to the user and he should recognize the early image. According to these estimations, 96% human have passed the face CAPTCHA and 90.5% human have passed the non-face CAPTCHA, and the average of required time to answer is 16.8 for non-face CAPTCHA and 7.6 second for face CAPTCHA.

Occurrence of large power deficits are the main cause of black outs in power systems. Usually, large power deficits occure as a result of large generating units or tie-lines outage. Considering the impact of electricity on economic growth of countries and daily life of people, triggering a blackout might be a goal of sabotage or military attacks. Underfrequency load shedding is commonly used to prevent blackout in case of large power defcits. In fact by curtailing part of the loads, load shedding schemes retain the power balance and keep the system stability. In this paper, an underfrequency load shedding method based on wide area monitoring system is proposed. In this load shedding scheme, the amount of load to be shed, in case of large power deficits, to preserve the stability of power system is determined based on the estimated minimum frequency. The effectiveness of the proposed method in different conditions, considering power system parameters variation, is proved using simulation studies.

An intrusion detection system in the cyber-networks is one of the most important lines of defense against the threats. Two main challenges in the field of intrusion detection systems are their ability to work in real-time domain and their attack detection accuracy. Elimination of non-critical features and discretization are two systematic ways to reduce the period of real-time processing and to increase the accuracy of the model. The main innovation of this paper is that eliminating of non-critical features and discretization are used simultaneously. In the proposed method, the pruned C4.5 algorithm is used as feature selection together with discretization algorithm in pre-processing phase. Experimental results on KDD cup 99 and NSL-KDD data sets, repectively showed that prediction accuracy of model in SVM, CART and Naïve Bayes algorithms after using the proposed method in the pre-processing phase, increases as 99.25% and 99.26%, 97.66% and 99.52%, 99.46% and 96.62% in that order. Also model construction time are reduced from 35.88, 0.08 and 6.64 seconds to 2.13 and 2.09, 0.01 and 0.01, 6.29 and 6.20 seconds, respectively. The results showed that the proposed system can effectively be used as a modern defense intrusion detection tool against cyber-attacks.

Different approaches have already been proposed for the analysis of the scattering field of targets. However, in practice, there are numerous targets for which, the calculation of the dispersion analysis is highly complex. To solve this problem, point target model is taken into account. This model aims to replace the target with a finite number of points that are dominant in their vicinity in terms of the radar cross section. These points, which are called scattering center or phase center, are local maximum in terms of the radar cross section and are effective in many practical applications such as radar imaging of targets, analysis of the dispersion, extraction of the target scattering mechanisms, Stealth technology, remote sensing, passive defence, etc. The present study is an attempt to extract the point target model for the complex, distributed targets using SAR image. For this purpose, image production and extraction of the dominating points have been conducted in two stages. All the simulations have been performed in MATLAB and CST software environments.

One of the modern issues in SAR processing is the use of the OFDM waveform instead of the conventional LFM signal. OFDM is a digital modulation scheme in which some orthogonal sub carriers are used simultaneously dividing the available bandwidth into several narrowband and parallel sub channels. The advantages of using OFDM waveform for synthetic Aperture radar (SAR) imaging include enhanced resistance against jamming, lower Doppler ambiguity and the possibility of extracting phase background of the target on the basis of phase estimation of the OFDM sub carriers. In this paper, the basic relations of OFDM waveform generation was reviewed firstly, and then the form of imaging in the range and azimuth by using this signal in the form of RDA algorithm was described.