Bøger af Roger Wattenhofer

Since its inception in late 2008, Bitcoin has enjoyed a rapid growth, both in value and in the number of transactions. Its success is mostly due to innovative use of a peer-to-peer network to implement all aspects of a currency's lifecycle - from creation to its transfer between users. Bitcoin offers cash-like transactions that are near-instantaneous and non-refundable, while at the same time allowing truly global transactions, processed at the same speed as local ones. It offers a public transaction history, enabling trustless auditability, and it introduces many new and innovative use-cases such as smart property, micropayments, contracts, and escrow transactions for dispute mediation.However, the same features that make Bitcoin attractive for its end-users are also its main limitations. Its decentralized nature limits the number of transactions and the speed at which transactions can be performed and confirmed. The problem with the slow confirmations is compounded with the semantics of the confirmations which are not final, requiring multiple confirmations and further delaying acceptance of a transaction.In the first part of the thesis we analyze whether the current Bitcoin protocol scales and what the scalability limits are. We find that Bitcoin does not scale, because its synchronization mechanism, the blockchain, limits the maximum rate of transactions the network can process. In order to address the scalability problem we propose Duplex Micropayment Channels, which increase the rate at which Bitcoin transfers can be performed by several orders of magnitude, by moving the transfers off the blockchain and using the blockchain solely for dispute mediation.Another form of scalability problem is the fact that more and more blockchain based applications are being created, eachwith their own small isolated blockchain, and vulnerable to attacks. We present PeerCensus, a subsystem that acts as a certification authority, manages peer identities in a peer-to-peer network and does not store application specific data in the blockchain. Using PeerCensus, any number of applications can share a single blockchain, decoupling confirmations from block generation rate and enhancing Bitcoin and similar systems with strong consistency.Being a relatively new technology, Bitcoin has a number of new security challenges and innovative properties. We analyze these properties and challenges in the second part of the thesis. The first novel property is that the transaction history, in the form of the blockchain, is public and accessible by anyone. Making use of the open nature of the blockchain, we were able to dispell claims by MtGox, once the world's largest Bitcoin exchange, that a bug in the Bitcoin protocol was used in a large scale attack to defraud them. We then use the blockchain to build a prototype of an audit protocol that allows a fiduciary, such as a Bitcoin exchange, to demonstrate that its assets cover its liabilities, without resorting to trusted third parties.Bitcoin also shifts the responsibility of managing and securing funds from a trusted third party to the end-user, which may not have the necessary tools to protect her funds. We show how a merchant may accept fast-payments, i.e., transactions without waiting for confirmations, with reasonable security against doublespending attacks by observing how transactions propagate in the network. Finally, we present a prototype of a secure device that stores private keys in tamper resitant storage and allows the user to independently verify a payment before authorizing it.

About the book: The Internet is a distributed system, but so are wireless communication, cloud or parallel computing, multi-core systems, mobile networks. Also an ant colony, a brain, or even the human society can be modeled as distributed systems. In this book we will be highlighting common themes and techniques. In particular, we study some of the fundamental issues underlying the design of distributed systems, for example, communication, coordination, fault-tolerance, locality, parallelism, symmetry breaking, synchronization, and uncertainty.About the author: Roger Wattenhofer is a professor at ETH Zurich. Before joining ETH Zurich, he was at Brown University and Microsoft Research. His research interests include fault-tolerant distributed systems, efficient network algorithms, and cryptocurrencies such as Bitcoin. He has published more than 300 scientific articles. In 2017, he published the book Blockchain Science.

A new edition of this book is available as of January 2019. Same price, more and better content! It is called "Blockchain Science: Distributed Ledger Technology" by Roger Wattenhofer.

people volunteer their time and energy and work in a dedicated fashion to pull everything together each year, including our very supportive Steering Comm- tee members led by Sukumar Ghosh. However, the success of ICDCN is mainly due to the hard work of all those people who submit papers and/or attend the conference. We thank you all. January 2009 Prasad Jayanti Andrew T. Campbell Message from the Technical Program Chairs Welcome to the proceedings of the 10thInternationalConferenceon Distributed Computing and Networking (ICDCN) 2009. As ICDCN celebrates its 10th - niversary,ithasbecomeanimportantforumfordisseminatingthelatestresearch results in distributed computing and networking. We received 179 submissions from all over the world, including Algeria, A- tralia, Canada, China, Egypt, France, Germany, Hong Kong, Iran, Italy, Japan, Malaysia, The Netherlands, Poland, Singapore, South Korea, Taiwan, and the USA, besides India, the host country. The submissions were read and evaluated by the Program Committee, which consisted of 25 members for the Distributed Computing Track and 28 members for the Networking Track, with the ad- tional help of external reviewers. The Program Committee selected 20 regular papers and 32 short papers for inclusion in the proceedings and presentation at the conference. We were fortunate to have several distinguished scientists as keynote speakers. Andrew Campbell (Dartmouth College, USA), Maurice Herlihy (Brown University, USA), and P. R. Kumar (University of of Illinois, Urbana-Champaign) delivered the keynote address. Krithi Ramamritham from IIT Bombay, India, delivered the A. K. Choudhury Memorial talk.

Thousands of mini computers (comparable to a stick of chewing gum in size), equipped with sensors,are deployed in some terrain or other. After activation thesensorsformaself-organizednetworkandprovidedata,forexampleabout a forthcoming earthquake. The trend towards wireless communication increasingly a?ects electronic devices in almost every sphere of life. Conventional wireless networks rely on infrastructure such as base stations; mobile devices interact with these base stations in a client/server fashion. In contrast, current research is focusing on networks that are completely unstructured, but are nevertheless able to communicate (via several hops) with each other, despite the low coverage of their antennas. Such systems are called sensor orad hoc networks, depending on the point of view and the application. Wireless ad hoc and sensor networks have gained an incredible research momentum.Computerscientistsandengineersofall?avorsareembracingthe area. Sensor networks have been adopted by researchers in many ?elds: from hardware technology to operating systems, from antenna design to databases, from information theory to networking, from graph theory to computational geometry.

This book constitutes the refereed post-proceedings of the 9th International Conference on Principles of Distributed Systems, OPODIS 2005, held in Pisa, Italy in December 2005. The volume presents 30 revised full papers and abstracts of 2 invited talks. The papers are organized in topical sections on nonblocking synchronization, fault-tolerant broadcast and consensus, self-stabilizing systems, peer-to-peer systems and collaborative environments, sensor networks and mobile computing, security and verification, real-time systems, and peer-to-peer systems.