Sharif Digital Repository

Regarding to the emergence of quantum computers and advances in the large scale implementation of them, exploiting hard problems of coding theory is of great importance.This family of problems, like those of lattices, has an acceptable resistance against classic and quantum attacks. Thus, they can be replaced with hard problems in number theory and algebra. Increasing dominance of cyberspace made us highly dependent to it. Consequently, preparing tools and methods to mitigate future threats is necessary. Digital signatures constitute an important family of cryptographic primitives. Code-based digital signatures are mostly suffering from two main drawbacks: large public key and long time of... 

User cooperation is an important concept in wireless networks to achieve best performance subject to low cost. Dynamic and broadcast natures of wireless networks are two indispensable characteristics of these networks. Therefore, when a user transmits his signal to the receiver, other users in his neighborhood may receive the signal as well. Hence, these neighbors can decide to cooperate with each other in transmitting their messages to the receiver. On the other hand, due to the dynamic nature of the wireless networks, one can assume that each user has some knowledge about the interference signals or the fading coefficient that are not known to the other users. Therefore by utilizing user... 

Reliable and secure communication requires low error probability and low information leakage, respectively. But there are different metrics for error probability and information leakage. Two important reliability metrics are ϵ or zero probability of error. An ϵ-error criterion requires the (average or maximal) error probability to vanish as the blocklength increases, while a zero-error criterion, demands the error to be exactly zero for every given bloklength. Three important security metrics are weak, strong, or perfect secrecy. A weak notion of secrecy requires the percentage of the message that is leaked to vanish as the code blocklength increases, while a strong notion of secrecy... 

The rapid evolution of artificial intelligence (AI) technologies has led to the widespread adoption of AI systems in diverse research and industrial fields. Deep neural networks, at the forefront of AI's power, demonstrate high performance by leveraging large volumes of training data. However, acquiring such vast amounts of data requires collaboration among individual data owners, who may have concerns about privacy. To address these concerns, various privacy-preserving methodologies have been proposed. These methodologies share a common goal of striking a balance between preserving privacy and maintaining data utility. This study aims to explore and analyze these privacy protection... 

Secure routing is a major key to service maintenance in wireless ad hoc networks. Ad hoc nature exposes the network to several types of node misbehavior or attacks. Wormhole attack is one of the most severe active attacks to mobile ad hoc networks. Noting the resource limitations in such networks, nodes may have a tendency to behave selfishly. Selfish behavior can have drastic impacts on network performance. We have proposed a Misbehavior-Tolerant Multipath Routing protocol (MTMR) which detects and punishes all types of misbehavior such as selfish behavior, wormhole, sinkhole and grey-hole attacks. The protocol utilizes a proactive approach to enforce cooperation. In addition, it uses a... 

Development of Smart Grid and deployment of smart meters in large scale has raised a lot of concerns regarding customers’ privacy. Consequently, several schemes have been proposed to overcome the above mentioned issue. These schemes mainly rely on data aggregation as a method of protecting users’ privacy from the grid operators. However, the main problem with most of these schemes is the fact that they require a large amount of processing power at the meter side. This, together with the fact that smart meters don’t usually have a powerful processor, can cause the unavailability of smart meter data at the required time for operators of the grid, and at the same time disables smart meters from... 

High-quality online services demand reliable and fast packet delivery at the network layer. However, clear evidence documents the existence of compromised routers in the ISP and enterprise networks, threatening network availability and reliability. A compromised router can stealthily drop, modify, inject, or delay packets in the forwarding path to launch Denial-of-Service, surveillance, man-in-the-middle attacks, etc. So researches tried to create intrusion detection methods to identify adversarial routers and switches. To this end, data-plane fault localization (FL) aims to identify faulty links and is an effective means of achieving high network availability. FL protocols use... 

The subject of Secret Sharing refers to the methods that a secret information is shared (by a dealer) among a number of parties, so called participants, each having one or more shares, such that the predefined subsets of the participants, called authorized subsets, are able to regenerate the secret using their shares. A more general scheme which is called Multi Stage Secret Sharing (MSSS) is to share multiple secrets among the participants such that the authorized subsets can reconstruct the secrets in the consecutive stages; in these schemes, in addition to the individual shares, the dealer should provide the participants with a number of public values regarding the secrets. A Multi-Secret... 

“Decode and Forward” is one of coding strategies used in relay channel and networks in which each relay in the network decodes the transmitted message by the sender and other relays in the network and then re-encode and transmit it to the other relays and the receiver in the network. This process results in the cooperation between the relays and the sender to remove the uncertainty of the receiver about the transmitted message. “irregular encoding-successive decoding”, “regular encoding-backward decoding” and “regular encoding-sliding window decoding” are different methods usually used in “Decode and Forward” strategy. “partial decode and forward” is a generalized form of “decode and... 

In this thesis the problem of simultaneously generating multiple keys among multiple terminals is investigated. After providing a survey on the related works and exploiting the problem of sharing a common secret key and a private key among three terminals, we introduce a new source model which we call it the Four-Terminal model. We consider the problem of generating three kinds of keys among legitimate terminals and investigate the problem under two different scenarios, Backward and Forward scenarios, which depend on the kind of restrictions imposed on the usage of the public channel. We derive an achievable region for the backward scenario, using the combination of superposition coding and... 

Due to the remarkable growth in using communication networks such as ad-hoc, wireless and sensor networks, studying the building blocks of these networks (e.g., relay channel and interference channel) from information theoretic viewpoint and investigating cooperative strategies have recently received considerable attention. Also, owing to wide range of applications, state-dependent channels have been widely studied. In this dissertation, at first, we consider a state-dependent relay channel (SD-RC). For the cases wherein channel states are available causally or non-causally at different nodes, we propose new achievable rates for this channel using compress-and-forward strategy. Also we... 

Interference management is one of the key issues in wireless networks wherein multiple source-destination pairs share the same medium and frequency band and interfere with each other. The exponentially growth in the number of wireless systems and services results in the spectrum shortage which has emerged the technologies that increase the spectral efficiency. Providing the users with cognition equipments in these networks enables them to sense the medium and use the obtained data to adapt their transmissions to cooperate with other users and improve their own rates as well as the others which results in the cognitive radio technology. In order to access the full potential of this technology,... 

In this thesis, the problem of secret key agreement in the channel model with side information is studiedx. In order to investigate the problem, we consider three models according to the different ways of side information availability which are the state-dependent broadcast channel; the state-dependent broadcast channel in presence of public channel and the state-dependent wiretap channel with degraded versions of the channel state information.For the problem of secret key agreement in broadcast channels, two models, the state-dependent wiretap channel and state-dependent three receivers broadcast channel, are considered. We obtain inner and outer bounds of the secret key capacity region for... 

In order to utilize cooperative resources of a network, we have to investigate cooperative networks and strategies. Information theory, by introducing different concepts such as coordination and common information, gives us a suitable tool to study cooperative networks and using their resources. In this thesis, we investigate coordination and secret key agreement in cooperative networks. Our study consists of two parts. In the first part, we consider coordination and its applications in cooperative networks. For this purpose, we introduce the triangular multiterminal network as a proper model for studying empirical coordination in multiterminal networks and establish an inner and outer... 

Energy Harvesting has been appeared as an approach in order to make the green communications possible. In Energy Harvesting systems, nodes extract energy from the nature to extend their lifetimes. The harvested energy can also be used for the purpose of communication and specially for the transmission process. Compared to the conventional batterypowered systems, the Energy Harvesting systems have access to an unbounded source of energy (like vibration absorption devices, water mills, wind turbines, microbial fuel cells, solar cells, thermo-electric generators, piezoelectric cells, etc). However, the diffused nature of this energy makes it difficult to be used for communication. In this work,... 

Security for wireless communication networks has become a crucial issue because of the broadcast nature of wireless channels. On every cellular network Since it is possible for passive nodes to easily overhear the confidential information transmitted between transmitter and receiver, it is necessary to provide secure communication.When a cellular user wants to communicate with a node or base station on the cellular network, it expects that information will be transmitted and received without interference, with enough quality and without eavesdropping. On the other hand In practical scenarios, we know that cellular network nodes are not equipped with ideal equipment and some percent... 

In the research area of network information theory, except in some cases, finding the exact capacity of different communication channels has always been a challenge. In the high-signal to noise ratios, the capacity of all Gaussian channels can be calculated within a low difference. Therefore, a parameter called the degree of freedom is introduced that describes the behaviour of the capacity of different networks at high signal-to-noise ratios. In other words, the DoF region is used as a tool to determine the capacity of interference channels at high $ \mathrm{SNR}$ values. In almost all interference networks, the idea of interference alignment can be used to achieve the... 

Block ciphers are one of the most necessary primitives for security protocols, and so, cryptanalysis of them is also essential as well. On the other hand, the security assessment of block ciphers is usually manual work with various specific-idea according to the target block cipher type and algorithm. Some basic known methods for cryptanalysis of block ciphers include linear, differential, meet in the middle, and integral attacks. However, cryptanalysis methods are not limited to them, and many new hybrid or innovative methods are employed to provide better results. Recently, automated cryptanalysis of block ciphers and proposing new methods for analyzes of these ciphers are in the... 

With the advent of the Internet of Things (IoT), e-health has become one of the main topics of research. Due to the sensitivity of patient information, patient privacy seems challenging. Nowadays, patient data is usually stored in the cloud in healthcare programs, making it difficult for users to have enough control over their data. The recent increment in announced cases of security and surveillance breaches compromising patients' privacy call into question the conventional model, in which third-parties gather and control immense amounts of patients' Healthcare data. In this work, we try to resolve the issues mentioned above by using blockchain technology. We propose a blockchain-based...