Peer-to-peer networks based on Distributed Hash Tables (DHTs) have received considerable attention ever since their introduction in 2001. Unfortunately, DHT-based systems have shown to be notoriously difficult to protect against security attacks. Various reports have been published that discuss or classify general security issues, but so far a comprehensive survey describing the various proposed defenses has been lacking. In this paper, we present an overview of techniques reported in the literature for making DHT-based systems resistant to the three most important attacks that can be launched by malicious nodes participating in the DHT: (1) the Sybil attack, (2) the Eclipse attack, and (3) routing and storage attacks. We review the advantages and disadvantages of the proposed solutions and in doing so, confirm how difficult it is to secure DHT-based systems in an adversarial environment.

Researchers at the University of Washington recently proposed Vanish [19], a system for creating messages that automatically “self-destruct ” after a period of time. Vanish works by encrypting each message with a random key and storing shares of the key in a large, public distributed hash table (DHT). Normally, DHTs expunge data older than a certain age. After they expire, the key is permanently lost, and the encrypted data is permanently unreadable. Vanish is an interesting approach to an important privacy problem, but, in its current form, it is insecure. In this paper, we defeat the deployed Vanish implementation, explain how the original paper’s security analysis is flawed, and draw lessons for future system designs. We present two Sybil attacks against the current Vanish implementation, which stores its encryption keys in the million-node Vuze BitTorrent DHT. These attacks work by continuously crawling the DHT and saving each stored value before it ages out. They can efficiently recover keys for more than 99 % of Vanish messages. We show that the dominant cost of these attacks is network data transfer, not memory usage as the Vanish authors expected, and that the total cost is two orders of magnitude less than they estimated. While we consider potential defenses, we conclude that public DHTs like Vuze probably cannot provide strong security for Vanish. Update – September 28, 2009 After we shared these findings with the Vanish team, they released a software update that attempts to defend against our attacks [20] and a report detailing potential countermeasures [18]. We respond to these developments in the update section at the end of this paper. 1

Abstract—A private BitTorrent site (also known as a “Bit-Torrent darknet”) is a collection of torrents that can only be accessed by members of the darknet community. The private BitTorrent sites also have incentive policies which encourage users to continue to seed files after completing downloading. Although there are at least 800 independent BitTorrent darknets in the Internet, they have received little attention in the research community to date. We examine BitTorrent darknets from macroscopic, medium-scopic and microscopic perspectives. For the macroscopic analysis, we consider 800+ private sites to obtain a broad picture of the darknet landscape, and obtain a rough estimate of the total number of files, accounts, and simultaneous peers within the entire darknet landscape. Although the size of each private site is relatively small, we find the aggregate size of the darknet landscape to be surprisingly large. For the medium-scopic analysis, we investigate content overlap between four private sites and the public BitTorrent ecosystem. For the microscopic analysis, we explore in-depth one private site and examine its user behavior. We observe that the seed-to-leecher ratios and upload-to-download ratios are much higher than in the public ecosystem. The macroscopic, medium-scopic and microscopic analyses when combined provide a vivid picture of the darknet landscape, and provide insight into how the darknet landscape differs from the public BitTorrent ecosystem. I.

Abstract — Distributed Hash Tables (DHTs) are widely studied from the fixed computer point of view, but very little research has been done on the performance of mobile clients. We have implemented a DHT client for mobile phones and carried out energy measurements to analyze the power profile of the application. By connecting to a widely deployed DHT, we were able to observe how mobile clients perform in a million user environment. We show that using a mobile phone as a full-peer is feasible only for a couple of hours due to the high power consumption. We also define the client-only mode that allows mobile nodes to use the services of the DHT with minimal energy consumption. Keywords- DHT; mobile phones; power demand; peer-to-peer I.

Abstract. BitTorrent users and consumer ISPs are often pictured as having opposite interests, with end-users aggressively trying to improve their download times, while ISPs throttle this traffic to reduce their costs. However, inefficiencies in both download time and quantity of long-distance traffic originate in BitTorrent randomly selecting peers to interact with. We show that biasing the link selection allows one to reduce both median download times by up to 32 % and long-distance traffic by up to 16%. This optimization can be deployed by modifying only the BitTorrent trackers. No external infrastructure nor specialized client-side software deployment is necessary, thereby facilitating the adoption of our technique. 1

Over the last few years, there has been a rise on the Internet of so-called social web services. In contrast with traditional web services where users only consume information, social web services enable users to interact with each other. This new approach to web services is often referred to as Web 2.0. Socialized web services have become very popular as shown by examples like YouTube, Flickr, and Wikipedia. However, these sites operate in a centralized way, and the drawback of their popularity is the increased operating costs. At the same time, Peer-to-Peer (P2P) technology has gained much popularity in the area of content distribution because of its lack of central components, which causes P2P systems to scale well. Therefore, P2P technology may be a solution for the poor scalability of current Web 2.0 services. This thesis describes the research we have conducted in providing Web 2.0 services with scalable P2P technology. We have extended Tribler, an existing P2P client, such that it enables each user to easily share his videos, photos, etc., with other users. Furthermore, we have developed a flexible system that provides access to the large collections of content items available from current popular Web 2.0 web sites. This system is easily extensible, and adding support for a new web site requires only to define structure of the site. Using these interfaces, we also decentralize current Web 2.0 web sites by distributing retrieved items in the Tribler network. ivPreface

jukka.k.nurminen at nokia.com This paper presents a novel mechanism that enables mobile devices to efficiently participate in Distributed Hash Tables (DHTs). By selectively dropping messages, we can significantly reduce traffic and thus the energy consumption of mobile nodes with only minor performance degradation. The protocol extension preserves compatibility with existing widely used DHT systems. We evaluate the solution by measuring the energy-consumption of mobile phones participating in the Kademlia-based Mainline BitTorrent DHT, which consists of over one million nodes. We further analyze the impact of our mechanism on the performance of the DHT by means of an analytical model. 1.

Abstract—The current wave of evolution that leads BitTorrent towards a fully decentralized architecture is both promising and risky. Related work demonstrates that BitTorrent’s Mainline DHT is exposed to several identified security issues. In parallel, the KAD DHT has been the core of intense research and was improved over years. In this paper, we present a study that motivates the integration of both worlds. We provide a performance comparison of both DHTs in terms of publishing efficiency. We investigate the security threats and show that the current BitTorrent Mainline DHT is much more vulnerable to attacks than KAD. On the other hand, we demonstrate that the file download service provided by BitTorrent outperforms the one of KAD. Given the strengths and weaknesses of both DHTs, we propose a design in which the two P2P networks can be merged to form a fully distributed, efficient and safe P2P eco-system.