Sharif Digital Repository

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  

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... 

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 many sciences, including computer science, generating random numbers have always been an interesting problem.Pseudo-random generators are one of the most important solutions to this problem and one of the primitives, used especially in cryptography and computer science, which have made it easier for us to generate random numbers.The main purpose of these generators is to increase the length of an n-bit string to an l-bit string where l>n.We need an initial value called Seed in this definition. We will present a new and stronger definition in this essay, called the Hinting Pseudorandom Generator, which has different outputs as well. Thus, our input is n bits and our output is n.l bits. We... 

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... 

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  

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  

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... 

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... 

In this thesis, in addition to reviewing the previous work done in the study of ideal access structures, we present the author's recent results in this field. As a first result, we introduce a new technique for reducing the size of the secret space in ideal homomorphic secret sharing schemes. The concept of decomposition of secret sharing schemes is formally introduced for the first time. In this regard, we show that ideal homomorphic and abelian secret sharing schemes are decomposable. We also examine the inherent group-characterizability of secret sharing schemes and show that an ideal secret sharing scheme is not necessarily inherently group-characterizable. Some weaker definitions of the... 

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  

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... 

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,... 

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... 

Correlation-intractability is a random-oracle-like property of hash functions, so one can expect that constructions that are secure in random-oracle-model, remain secure when implemented using a correlation-intractable hash function. This is true about Fiat-Shamir transform which is a method for eliminating interaction from public-coin interactive proofs. One can see that when this method is applied to a public-coin three-round honest-verifier zero-knowledge proof, using a correlation-intractable hash function that also satisfies some mild assumption, the result is a NIZK argument. This is the basis for the correlation-intractability framework for NIZK which recently has been used to... 

A secret sharing scheme is a way to share a secret between a group of people so that only authorized subsets can retrieve the original secret. Secret sharing schemes were first introduced by Shamir and Blakley for threshold access structures. In a threshold access structure, a subset of individuals is allowed if the number of members exceeds a predetermined threshold. After Shamir, the study of general access structures was considered, and Ito, Saito, and Nishizeki proposed a way to construct a secret sharing scheme for these structures. Reducing the length of party's share in secret-sharing schemes and finding the optimal scheme for access structures has long been the focus of researchers... 

Dynamic searchable encryption schemes enable keyword search query and update operations on encrypted data. One of their applications is to use them in cloud storage services that allow the client to store, search and update data on an external server. Due to the importance of efficiency in real-world applications, retrieving information in searchable schemes involves leakage of some information such as search and access patterns. However, with the addition of update operations (insertion and deletion), more leakages are considered for these schemes. In this regard, reducing the amount of information disclosed to the server is necessary. For this purpose, forward and backward security...