Sharif Digital Repository

By employing the image theory the Er doped microsphere coupled to an external cavity made by a metallic mirror is simulated by a bimicrosphere laser. The symmetric structure causes the formation of odd and even modes corresponding to the TE and TM mode of the microsphere and splitting the oscillation frequencies. The oscillation frequencies are dependent on the coupling coefficient. The coupling coefficient is obtained near the mirror-microsphere distance. The ohmic mirror loss also is determined as a function of the mirror distance. The total optical loss experienced in these resonators is exceptionally low. In fact,quality factor as high as 108 − 1011 have already been demonstrated for... 

The Response modification factor R represents the behavior of the structure such as ductility, overstrength and the inherent redundancy. This factor shall be defined according to the type of lateral force resisting system. However, there are some complexities and/or uncertainties in codes related to the R factor according to the height, form of irregularities and structural systems. Therefore, from the scientific point of view as well as reliability, it is very clear that the R factor cannot be similar for all kinds of structures, categorized as a specific group. In this investigation, assuming a low-rise, vertically irregular structure with a heliport which has a dual structural system... 

Automotive brake pad is one of the most widely used composites. Commonly, more than 10 components are applied to make these materials and due to the large number of components and interactions between them, choice, explaination of the behavior and optimization of brake friction materials are some industry's challenges. Until now, some methods have been presented to design and optimization of these materials, but due to different complexities, there is lack of practical method that could be used for explaining the tribological behavior, engineering calculation and optimization of brake friction materials. Today, formulation of the current brake pads in the industry has mainly come from trial... 

Side channel attacks, where an attacker learns some physical information about the state of a device, are one of the ways in which cryptographic schemes are broken in practice. leakage resilient ryptography addressed this issue at a higher abstraction level, proposing models capturing such attacks. Auxiliary Input is one of these models. In this thesis, we study public-key encryption schemes in this model of leakage. we also propose a method to consider this leakage continuously (i.e., not just one observation of the adversary). at the end, we consider the leakage from both the secret key owner and the encryptor. This odel captures a wider class of side-channel attacks  

Voting is a detachable part of the democratic governments. In the era of modern computer systems, the need for electronic voting systems in no longer avoidable.Designing a secure electronic voting system is a difficult problem since both privacy and integrity must be achieved at the same time. This difficulty has cast the problem to the center of attention of many researchers.In this thesis we will consider the new generation of voting systems known as end-to-end that were designed to solve the problems in the previous generations of voting systems and to uphold the security of the system. Many of end-toend voting systems use concepts of cryptography in addition to machine tools.A central... 

Block ciphers are the central tool in the design of security protocols. GOST is a block cipher that several attacks have been published on it during the past 20 years. One of them is the reflection attack that was first introduced in 2007. This attack utilizes similarities of some round functions of both encryption and decryption processes. In addition, there are approaches that combines techniques of the reflection attack and variants of the meet-in-the-middle attack. In this thesis, we study reflection-based attacks on GOST. We first describe the GOST block cipher and the reflection attack. Then, we review previous reflectionbased attacks such as reflection-meet-in-the-middle attack and... 

In this thesis, first we will have a brief review on hash functions and related concepts.Then, we will descibe the hash function, Cubehash, which is one of the competitors in the competition held by NIST for selecting SHA-3 standard. Next, we will describe an attack based on linearization of differential paths in hash functions and apply this attack on Cubehash. At the end, we will discuss the results of this attack on Cubehash  

One of the main requirements in today’s computer networks such as the Internet is protecting the privacy of users. Hence, anonymity is a useful tool for privacy and provides user’s anonymity. So far, several protocols have been designed to provide anonymity but most of the times, attackers found a way to take advantage of weaknesses in the design and implementation of these protocols. There is a class of attacks,where attacker tries to relate clients of anonymity system to each other, regardless of the communication protocols they use, just by observing networks traffic.The MIX technique forms the basis of many popular ervices that offer anonymity of communication in open and shared networks... 

Photonic lattices can provide an inexpensive and fine tools to investigate some common phenomena in condensed matter physics, quantum optics, relativistic mechanics,atomic and molecular physics. These phenomena contain Anderson localization,Bloch oscillation, Dynamic localization, coherent and squeezed states, Relativistic Dirac equation, Rabi oscillation, ... .In this thesis, it was investigated three worthy phenomena of them, which at first are introduced in condensed matter physics for electronic systems, in photonic lattices. These phenomena are as follow: Transverse localization of light wave, Bloch oscillation and dynamic localization of light. It was also simulate classically, the... 

A hash function maps efficiently every finite length string to a fixed one. The output represents the entire content of the input, similar to digital fingerprint of input text. In order to be able to use a hash function in cryptography, it should be hard to find two distinct inputs with the same output (collision), because finding a collision in hash functions leads to malicious attacks on many security systems.We say a hash function is provably collision resistant, if finding a collision can be reduced to the known hard problems. Family of MD hash functions are one of the most famous applied hash functions which are considerably used in industrial applications. However, they are not... 

The Gaussian mixture (cardinalized) probability hypothesis density (GM-(C)PHD) filter is a closed form approximation of multi-target Bayes filter which can overcome most of multi-target tracking problems. Limited field of view, decreasing cost of cameras and its advances induce us to use large-scale camera networks. Increasing the size of camera networks make centralized networks practically inefficient. On the other hand, scalability, simplicity and low data transmission cost has made distributed networks a good replacement for centralized networks. However, data fusion in distributed network is sub-optimal due to unavailable cross-correlation.Among data fusion algorithms which deal with... 

In this thesis we will focus on Functional Encryption. At first a wide range of different types of requirements and applications which are not fulfilled by Public-Key Encryption are considered. These types of requirements finally led us to Functional Encryption. Then a breaf explanation of differrent types of Functional Encryption is presented. In the sequel we will exlain Functional Encryption more formally. Finally we will consider the latest achievemnets and open problems in this area  

Genome sequencing is one of the fundamental problems in today’s Biology. It has applications in significant problems such as finding association between an individual’s genome sequence and his phenotypes, discovering new genes, and finding evolutionary relations between organisms. Following the rapid advances in sequencing technologies and generation of a huge amount of short reads, efficient computational tools are needed for processing sequencing data. Shortness of reads is a factor that makes the task of reconstructing repetitive genomic regions complicated. In fact, the main challenge in both sequencing and resequencing problems is reconstruction of repeat regions. Common resequencing... 

Using “Searchable Encryption” enables us to encrypt the data, while preserving the possibility of running search queries. One of the most important applications of the mentioned is in Cloud Storage. As users do not trust the Cloud space, they are not inclined to store their data on the Could. The solution to this problem is of course, Cryptography. However, ordinary Cryptography methods, eliminate the data’s searchability. Hence, we need encryption schemes that code the data while retaining their searchability. So far, various schemes has been proposed that differ in their performance, security level, and usage. In this thesis, we aim to discuss and analyze these methods  

Designing efficient and secure electronic voting protocols without the presence of trusted authorities, which are known as decentralized voting protocols, is one of the most interesting and challenging problems in cryptography. In these protocols the outcome of the protocol is determined by voters collaborating with each other. Since voting is a significant concept in every society, the correctness and security of voting protocols are important. For security analysis of electronic voting protocols, an intuitive proof is not enough and we require a comprehensive approach that provides provable security. In this thesis, we use universal composition framework to analyze a decentralized e-voting... 

Increasing use of new data technologies have made data collection possible in large scales. Practicallity of the data relies upon the extraction of meaningful knowledge.Data mining is a solution to this problem. One of the new areas in data mining is consideration of the concern of privacy alongside the usefulness of the mining results.Main goal of privacy preserving data mining is to develop data mining models which only extract the useful knowledge. In recent years, many researches have been done in this area. Since the literature and notation of these published works vary, a survey would help to better understand these concepts. This thesis tries to explain, analyse,unify and categorize... 

Determining the value of optimal information ratio for a given access structure and finding the upper and lower bounds for the information ratio in secreet sharing is one of the main problems that has been studied over time. In this thesis, we have tried to introduce the tools used in this field, including polymatroids and Shannon’s inequalities, linear programming, decomposition and etc, examine the papers presented in this field and rewrite obtained results in a simpler way  

Objective of Secret Sharing is assigning some random variable which will be called shares, generated based on a random variable called secret, to some participant such that only specific subsets of participants could reconstruct the secret by polling their shares together. Constructing shares as small as possible is desired, hence notions like information ratio and average information ratio have been studied and considered to be a measure of efficiency of secret sharing schemes. In this research contribution vectors of access structures with five participants and graph based access structures with six participants have been studied. Small access structures have been considered shortly after... 

In recent years, different cryptographic tools have been introduced for a wide range of cloud computing applications that can be classified based on a trade-off between performance and security. In this thesis, we introduced the private set operation schemes, and at their heart, private set intersection schemes, in the cloud computing platform. These schemes are of particular importance because they can be used as basic cryptographic schemes for a wide range of functionalities in the cloud platform. Using these schemes, the user can securely store datasets on the cloud, run set queries remotely and receive the desired results. To this end, we first modeled the syntax and the security notions... 

A secret sharing scheme involves a dealer who has a secret and distribute that among a set of participants, in such a way that only authorized subsets can recover the secret value. The collection of authorized subsets called the access structure and a secret sharing scheme in which unauthorized subset cannot reveal any partial information about the secret is called a perfect secret sharing scheme. One of the problems in secret sharing schemes is finding the appropriate lower bound for a parameter called the information ratio and optimizing it. A perfect secret sharing scheme is ideal when it has an optimal information ratio. In other words, if all shares have the same length as the secret,...