Sharif Digital Repository

Secret sharing scheme is a method for distributing the secret (secret information) among a set of participants in such a way that only the authorized sets can recover the secret and the unauthorized sets could not. In a perfect secret sharing scheme, unauthorized sets cannot get any additional (i.e. a posteriori) information about the possible value of the secret. In these schemes, to prevent information leakage and increase communication efficiency, the size of the share should be as close to the secret size as possible. In other words, finding the shares with reasonable size which results in an optimal information rate for a given access structure, improves the efficiency of the scheme.... 

Ever-encreasing progress in the field of wireless communication has paved the way for utilisation based on radio frequency authentication systems. In recent years, ITU started the scenario of "Internet of Things". RFID systems have an exclusive role in development ofthis idea, since they can make it possible for things which have an ID to be identified automatically in a large scale. The necessity of identification and tracing people and things has caused the deployment of RFID technology. Onthe other hand, these systems communicate via a wireless channel to transmit data, therefore they are vulnerable to eavesdropping and interception. Thus, one of the most important issues in using such... 

Wide use of communication systems and transmission of huge data has increased the need for cryptographic and encoding systems with high speed operations and lower energy consumptions. Code based and joint encryption encoding schemes are examples of such systems, which enjoy simultaneously high performance and security against quantum computers. So far, there has not been developed any quantum algorithm which can solve any of the hard instances of coding theory based on which these cryptographic schemes have been designed. That is why such systems are called post quantum cryptosystems. The main disadvantage of such cryptosystems is large size the key which is to be stored or shared. In this... 

Secret sharing schemes are used for storage and management of sensitive data such as cryptographic keys and bank account numbers. These schemes provide both confidentiality and reliability. Advances in quantum computers have threatened the security of currently used public-key cryptographic algorithms. In this thesis, for designing quantum resistant cryptographic schemes, we construct a lattice based threshold multi-stage secret sharing (TMSSS) scheme. Using a lattice based one-way function, even after some secrets are revealed, the computational security of the unrecovered secrets is provided against quantum computers. The scheme is multi-use and verifiable meaning that the participants can... 

Quantum computers can be used to process complex computations much more efficient than their classical counterparts. Cryptanalysis of classical ciphers is no exception to this rule. Most quantum cryptanalysis of symmetric ciphers is based on finding the periodicity, linear structure, and confidential information of the target algorithm. In this thesis, assuming that the attacker has access to the quantum computer, we propose a quantum method for deriving the appropriate differential characteristic of block ciphers, using Simon's quantum algorithm. It is worth noting that, unlike classical methods, where increasing the number of rounds of the target block cipher increases the complexity of... 

With the dramatic advancements in information technology and the industry requirements for security and privacy, proof systems play a crucial role in cryptography. Among the vast variety of proof systems, succinct non-interactive arguments (SNARG) seem to be the most appealing class of proof systems, due to their attractive properties. SNARGs are usually made up of two constructive components, namely the information theoretic part and the cryptographic part. The GKR protocol was introduced as a proof system for a tractable family of languages called “log-Space Uniform Circuits”. The log-space uniformity is a necessary condition for the protocol to be succinct. Hence, it is not possible to... 

With the advent of quantum computers, the security of cryptographic primitives, including digital signature schemes, has been compromised. Therefore, a kind of signature scheme has been introduced to resist these computers. These schemes are based on hard problems that have so far resisted against quantum computers, known as post-quantum schemes. One group of these schemes is based on the hard problems of coding theory, which are called code-based cryptographic schemes. The first code-based cryptosystem was the McEliece encryption scheme. A number of code-based signature schemes are based on the McEliece encryption scheme. Some of these schemes use three matrices including a non-singular... 

With the dramatic advancements in information technology and the industry requirements for security and privacy, proof systems play a crucial role in cryptography. Among the vast variety of proof systems, succinct non-interactive arguments (SNARG) seem to be the most appealing class of proof systems, due to their attractive properties. SNARGs are usually made up of two constructive components, namely the information theoretic part and the cryptographic part. The GKR protocol was introduced as a proof system for a tractable family of languages called “log-Space Uniform Circuits”. The log-space uniformity is a necessary condition for the protocol to be succinct. Hence, it is not possible to... 

In this study, the co-pyrolysis process of textile waste based on cellulose and polyethylene terephthalate (PET) was investigated using thermogravimetric analysis (TGA/DTG). Additionally, the pyrolysis kinetics of the thermal degradation process for each material and their combinations were studied. Kinetic parameters, including activation energy (E) and pre-exponential factor (A), were calculated for the pyrolysis of the samples using iso-conversional kinetic models such as Friedman (FR), Kissinger-Akahira-Sunose (KAS), and Flynn-Wall-Ozawa (FWO) based on the TGA and DTG curve data and compared with the corresponding reported values. Furthermore, the pyrolysis kinetics were modeled using... 

Stream ciphers are one of the cryptography primitives which have high speed in encryption process and less amount of hardware which they used. Guess and determine attacks are general attacks on stream ciphers which have often been implemented in ad hoc manner. Finding the minimal basis for internal state of a cryptosystem based on connection equations is the main challenge in stream ciphers. In recent years, some efforts for applying this attack in the systematic manner are started. First work in this area is the Simple Guess and Determine attack. The dvanced and Heuristic Guess and Determine attacks then presented, which all of them have heuristic structure. In this thesis, after reviewing... 

Many lattice-based public key encryption schemes are forced to use long keys. To deal with this problem, structured lattices such as NTRU can be used. Lattice-based searchable public key encryption schemes are no exception. Furthermore, in most existing searchable public key encryption schemes, it is assumed that the user searches documents with only one keyword. However, using a conjunctive keyword search, we can search for documents containing several keywords by executing the trapdoor generation algorithm only once. On the other hand, in most of the published searchable public key encryption schemes, storing and searching encrypted keywords are responsibility of a central entity called... 

Secret sharing is a method for sharing a secret among a number of participants in a manner that an authorized subset of participants can recover the secret using some information, called share, and some public values. One type of secret sharing schemes is Multi-stage Multisecret sharing scheme (MSSS). In such schemes, assigning one share to each participant, an authorized subset of participants is able to recover the secrets in different stages without any information leakage from the recovered secret(s) to the other one(s), using some public information.The number of public values is one of the most important parameters in secret sharing schemes. In this thesis we show that the number of... 

Blockchain systems are considered a subclass of distributed systems and have drawn a lot of attention from academia in recent years. The deployment of these systems in various contexts has been the focus of extensive studies. Because of their design, which places a strong focus on security and decentralization, blockchain systems have some challenges. The most prominent challenge is the one that involves scalability. A blockchain's scalability refers to its capacity to achieve a target throughput and solid performance in confirming transactions with an ever-increasing workload or number of nodes in the network. A solid performance suggests that the blockchain system can sustain its... 

Ciphertext-Policy Attribute-Based Encryption (CP-ABE) is a promising access control mechanism for securing cloud data. In ciphertext-policy attribute-based encryption schemes, an access structure is sent along each ciphertext to specify authorized recipients. However, this approach can inadvertently expose users' privacy and compromise the integrity of the messages. To address these challenges, hidden-policy attribute-based encryption schemes have been developed. While numerous methods have been proposed to hide the access policy, limited research has comprehensively evaluated their effectiveness, particularly in terms of security and privacy guarantees. In this thesis, we first review and... 

Nowdays, because of the promotion and developments of Information technology (IT) and the need for powerful resources for computation and storage, we are the witness of migration from existing computing schemes to the cloud computing and cloud storage environment. As a result, we can see that the client and the IT users are the beneficiaries of the services which are provided by the existing cloud providers. However, the cloud providers are not usually fully trusted and we need to protect the privacy of our sensitive documents. In this case, we should use cryptographic primitives to encrypt our data before outsourcing to the cloud. Searchable encryption and attribute based encryption are two... 

Collision attack is one of the usual attacks in cryptanalysis of hash functions. One method for applying this attack is linear approximation of equivalent compress function. Finding collisions for linearized compress function, having low Hamming weight, with a large probability could provide collisions for the main compress function. For this purpose, the problem of finding collision for used compress function is modeled by that of the linearized one, having low Hamming weight differences with a large probability. The problem of finding collisions with low Hamming weight differences is equivalent to the problem of finding codewords with low Hamming weights in linear binary code. Canteaut and... 

Code-based cryptosystems could be a suitable alternative to the cryptosystems based on number theory. It is shown that cryptosystems based on descrete logarithm and factoring is vulnerable to the Shor’s algorithm running on quantum computers, while code-based cryptosystemsare thought to be secure against this cryptanalysis. Despite its security, large key size and low transmission rate keep thesecryptosystems impractical. Reliability is one of our inevitable desires in communication systems along with security.In order to fulfill these desires, joint encryption-encoding schemes has been released.Using LDPC codes in joint encryption-encoding schemes, as an alternative to classical linear... 

gas adsorption in porous solids and adsorbent,is the important cases in gas processing industries. Including the removal of compounds such as carbon dioxide, hydrogen sulfide and mercaptans from sour gas in natural gas sweetening industry is of great importance.Also in solving the greenhouse phenomena extraction of gases such as methane and carbon dioxide of important environmental issues.To understand and predict the behavior of the absorption capacity of various adsorbents and thus the possibility of absorption systems, a mathematical model is needed.Library resources available in order to determine the kinetics and thermodynamic processes of gas adsorption on solid absorbents use... 

Economic loss due to wax precipitation in oil exploitation and transportation has reached several billion dollars a year recently. Development of amodel for better understanding of the process of wax precipitation is therefore veryimportant to reduce the loss.Currently, there are two types of soild models. One is the solid-solution model, and the other is the multisolid-phase model.In this work, a new thermodynamic model that combines the two methods mentioned above, has been presented. The PC-SAFT EOS is used to describe the vapor, liquid and wax phase.SAFT equations describe the phase behavior of large molecules accurately. Intermolecular forces in wax phase are caused by London and Van-... 

In order to provide both security and availability for a given secret, one way is to distribute it among a number of parties called participants. The distribution should be accomplished in such a way that any subset of participants, the size of which is at least equal to a given number, be able to reconstruct the secret, using their shares. More specifically, a (t, n)-threshold secret sharing scheme refers to the procedure of assigning each of the n participants a private share, such that every subset of at least t participants could recover the secret. Due to the possibility of quantum attacks in future, we need to construct secure channels for transmitting secret shares. Such channels...