Sharif Digital Repository

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... 

Due to the increase of computer networks and their application, the need for secure communication has become a very important concern. We introduce Cmail, a secure messaging system that works without any central party and in addition protects privacy of users. This system is on top of a cryptocurrency system such as Bitcoin. Bitcoin is cryptocurrency that works on the Internet. The suply of bitcoins and verification of transactions is done by Bitcoin miners, which are some typical client of Bitcoin system, in a decentralizd maner. In Cmail the miners have additional task, they should transfer messages of users and if needed, they hold it for a limited time. In order to increase incentive of... 

In this study, an adaptive controller design for nonlinear fractional order systems in the strict-feedback form has been investigated. The system is subject to unknown dynamics, unmeasured state variables, quantized input and output, and input nonlinearity. A linear observer is used to solve the problem of unmeasured state variables. The fuzzy logic system is used to estimate the unknown functions, and instead of updating all the regressor weights, only the upper bound of their norms is updated, which significantly reduces the computational load. In the controller design, limitations due to the bandwidth of data transmission have been considered by applying quantizers in the input and output... 

Multiplicative weight update method, is an statistical method to produce approximation algorithms. These algorithms have applications in various fields, including: Graph Algorithms, Machine learning, Optimization, Computational geometry, Game theory, etc.Recently, with this method, fast approximation algorithms have been created for many problems, including, solving linear programming. Here, we try to study this method and it’s applications, in detail  

Kinetic roughening has attracted a lot of attention over the last few decades not only because of its practical importance for the growth of solid films, but olso as an example for a dynamical mechanism
that drives a system into a spatially and temporally scale invariant state. Most of the models of kinetic roughening studies so far can be described by the KPZ equation. In this project growth processes which
cannot be described by KPZ will be studied. The first nonequilibrium growth models including deposition and surface diffusion were intruduced independently by Wolf and villain. The Schramm-Loewner evolution (SLE_, where _ is the diffusivity) proposed recently is a very successful... 

Realistic simulation of tool-tissue interactions can help developing more effective surgical training systems and simulators. In this study we used finite element and meshless modeling approaches to simulate the grasping procedure of a large intra-abdominal organ, i.e., kidney, during laparoscopic surgery. Results indicated that the accuracy of meshless method was comparable with that of the finite element, with the RMS errors in the range of 0.8 to 3.4 mm in different directions. The computational cost of the meshless method however, was much less than that of the finite element model. In order to verify the model, a silicon sample was used with the ship kidney geometry. For determination...