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BIBEBUVAMaster of Science in EngineeringA Secure Distributed Bulletin BoardE-Voting / Thesis advisor: Prof. Dr. Eric DubuisExpert: Prof. Dr. Andreas SteffenMore and more data are published on the Internet everyday. How can we ensure that the displayed contenthas not been modified? In serious contexts (e.g., e-voting) it is essential to prove the correctness of the data.Using a secure bulletin board, authorized users will be able to post messages and have the assurance thatthey will never be changed, moved or deleted. Also, the messages will be available to everyone. The goalof this thesis is to describe a working solution that produces correct results even in presence of activelycorrupt parties.José Beuchatjose.beuchat@bluewin.chIn this master thesis, we show thatbulletin boards can be used in differentcontexts (e.g. e-voting, auctions,system logs, etc) to allowusers to post and read messagesthat will never be removed, modifiedor moved. Moreover, theyshould always be available, possessno single point of failure andtheir users are able to prove it ifany of those properties are notrespected.We also present a distributed solution(see Figure 2) running at nparties, of whom less than onethird can be corrupt without affectingthe correctness of the bulletinboard. The users randomlychoose a party, post their messagesand receive a receipt forthem. This solution, based on themaster thesis of R.A. Peters, usesa secure broadcast channel describedby M.K. Reiter.As you can see in Figure 1, oursolution has a relaxed layered architecturecomposed of the followingseven layers: Network, SecureGroup Membership, EchoMulticast, Reliable Multicast,Atomic Multicast, SynchronizedMulticast and Application. When amessage is posted at a party, it isbroadcast to the n parties by goingthrew all of those layers. Sixlayers have currently been implementedand tested. The applicationlayer remains to be done.However, we realized a simpleimplementation that gives us theability run our protocols and tomonitor and manage them using aJMX client. Note that this last layercan differ a lot depending on thecontext. A major difference incomparison to Peter’s solution isthat our prototype, implementedin Java, is multi-threaded. It makesus gain performance and givesmore possibilities to test the system.Unfortunately, thread-safeprotocols were hard to realize.Future work will add group-thresholdsignatures in the Echo MulticastProtocol, making it more efficient.It will be important to usea scheme not requiring a trusteddealer. Otherwise, a single point offailure will exist. Additionally, alternativemessage formats (ASN.1,XML, JSON, etc.) will be used inorder to make our bulletin boardready for interoperability with possibleother implementations.Figure 1: Relaxed Layered ArchitectureFigure 2: Distributed Solution22 ti.bfh.ch