To My Mother Моей Маме

1 Introduction Today, peer-to-peer technology has reached the peak of its popularity. Currently, P2P file sharing represents the dominant usage component of Internet bandwidth. Moreover, P2P networks enable sharing of other different computer resources and services, including distributed (collaborative) computing, processing cycles, instant messaging, CPU and storage resources, etc. Peer-to-peer (P2P) is a communication model in which multiple autonomous devices interact as equals. In a pure P2P network each node implements functions of both client and server, and either peer can initiate a communication session at any moment. In terms of Internet users, P2P is a sort of " transient " network that allows a group of computers with the same networking software (P2P client) to directly access files from one another's hard drives via Internet connection – simple to join, simple to use. Nevertheless, P2P network is a quite complex system that represents a synthesis of several technological components, and one of them is overlay network. Nowadays, overlay networks based on Distributed Hash Tables (DHT) are a building block of many peer-to-peer applications. DHT mechanisms provide guaranteed data retrieval, moderate lookup times, automatic load balancing and self-organizing data storage and lookup system [1]. However, DHT-based peer-to-peer networks represent a particular environment susceptible to some specific threats and attacks due to their completely distributed nature without any centralized control. Generally, these attacks are caused by malicious behaviour of some nodes of the network and aimed at routing and lookup processes. Security in DHT-based Peer-to-Peer Networks 10 This thesis is meant to find solutions for some specific security problems of DHT-based P2P environment. At the same time, it is also proposed to exploit the advantages of DHT mechanisms applying them to systems where they can be effective against some particular problems. In other words, the research was conducted in two directions: • DHT as an object to specific attacks; • DHT as a security improving tool. Regarding the first direction, we propose some solutions based on the use of trust and reputation evaluation mechanisms to cope with some types of specific attacks of DHT environment. The character of interactions between peers and the presence of " misbehaving " and " honest " peers indicate an analogy between P2P environment and human communities. So, it makes sense to apply reputation evaluation techniques to avoid further contacts with nodes that have already demonstrated malevolent behaviour in order to resolve the problem of polluting …