Sharif Digital Repository

The lightweight blockciphers and lightweight authenticated encryptions are special kind of the symmetric schemes designed to use in cases that resources such as power, area, etc. are limited. Sensor Networks and RF-Tags are two good example of lightweight schemes application. This limitations force designers to use simpler functions and operators in comparison to conventional schemes; therefore, providing the security in this case is so important. A novel approach for evaluating security of an algorithm is to apply different kinds of attack. Impossible differential cryptanalysis is one of the shortcut cryptanalysis, using the impossible differential characteristic. Impossible differential... 

Road accidents are currently one the main existing problems and a big challenge in Iran that is putting the lives of Iranian citizens in danger. Each accident is the result of a complex interplay between road users, vehicles, roads and environment. One of the main goals of accident data analysis is to identify and determine the main factors of a road accident. The dataset used here was obtained from the road traffic police and is stored in 3 different databases and corresponds to the accidents that happened between years 1390 and 1395 according to the Shamsi calendar. In this thesis, in order to deal with the inherent complexity and heterogeneity of the accident data, we will first introduce... 

On the demand of higher directivity, higher throughput and less interference, phased array systems and MIMO have become attractive to reach these goals. The advantages of phased array systems include the ability to perform electronic beam steering and arbitrary pattern synthesis. Phase shifters and variable gain amplifiers are used to reach these advantages. In this thesis, a ‘’ C‘’ band (4.8 GHz-6.2 GHz) vector-sum phase shifter using HMIC technology for realizing a phased array receiver is designed. This phase shifter can change the signal phase and amplitude. The main blocks of this phase shifter are low-insertion phase and low-noise variable gain amplifier, which consists of current... 

A voting protocol was introduced by Popveniuc in 2006; it is receipt freeness, so appeared to be resistant against the bribed voter. In 2009 Clark using the bribed contract, and attacked to the voting protocol, in the elections by two candidates. He showed that this protocol is not resistant to the bribed voter. In this thesis, we use game theory and graph to extend the bribed contract for n candidates. Thus with the help of a graph, we show that this protocol with any number of candidates, is not resistant to the bribed voter, and we show that by increasing the number of candidates, the resistance of the protocol would be high and if the number of candidates is n, the success rate of this... 

In order to protect the proxy signers’ privacy, many anonymous proxy signature schemes have been proposed. As far as we know, there is no provable secure anonymous proxy signature scheme without random oracles. Because the provable security in the random oracle model has received a lot of criticism, in this thesis, the aim is proposing the first provable secure anonymous proxy signature in the standard model based on existing standard assumptions.It is shown in the standard model that, the proposal is anonymous against full key exposure and existential unforgeable with the computational Diffie Hellman (CDH) and Subgroup Hiding (SGH) assumptions in bilinear groups.To provide anonymity for proxy... 

Electronic voting is one of the applications of electronic systems, in which collection and tallying the votes are performed electronically. In electronic voting systems, cryptography is used to provide security requirements but defects of the cryptography have made the electronic voting protocol designers to use strong assumptions which are impractical or hard to implement.In this research, some of the strong assumptions in electronic voting protocols are studied and the soloution of solving them is given. First, the assumption of trusting to the tally authority in electronic voting protocols which are based on deniable authentications is considered and a new internet voting protocol based... 

Wireless communication networks such as cell phones are rapidly expanding and every day they attract more users. Information flow in these networks is basically through free space. Thus, theses networks are inherently subjected to eavesdropping. Cryptography based on the secure key is the well known traditional way of protecting information security in wireless networks. However, these cryptosystems are just computationally secure and have their vulnerabilities. Information theoretic security is a relatively new method which doesn’t rely on any preshared key or key infrastructure and uses inherent characteristics of the communication channel to provide robust and provable security.In this... 

Fair exchange of digital items through computer network is an important research topic of modern cryptography. Generallty, a fair exchange protocol is a way which helps two parties to exchange their digital items fairly, so that at the end of the protocol execution, both parties recieve their desirable items or none of them recieves anything. In a practical applications, fair exchange protocol is widely used in different but related fields like contract signing protocols,non-repudiation protocols,e-ayment system and certified e-mails. In a fair exchange protocol, a trusted third party (TTP) acts as an arbitrator between two parties. To reduce the traffic load of the arbitrator, the concept... 

Increasing use of World Wide Web and users tend to accompany in the electronic transaction has caused a fair exchange becomes very important and pervasive electronic transaction. These transactions have a lot of usage and benefits. In this study, the optimistic fair exchange has been studied. It is a special case of the fair exchange while having more advantages than the fair exchange. A situation considered that a signer could not sign a message by himself so he delegates his right in signing to a person as a proxy signer. In this study, a protocol of optimistic fair exchange for proxy signature is presented. So, the proxy signer can accompany in the optimistic fair exchange instead of the... 

In recent years membrane bioreactors (MBR) were widely used as an advanced process for industrial wastewater treatment. But fouling issue causes some restrictions in these systems. So for fouling reduction many researches were done.In this study for fouling improvement, blending of hydrophilic polymer (PAN) with main polymer PVDF was used. Blending membrane in phase inversion process with DMAC as solvent and water as non-solvent was made. For pore-forming in membranes, PVP as hydrophilic additive with three different composition was added and then for more enhancement of antifouling property of membrane, TiO2 nanoparticles with four different composition were added to polymeric...