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M. Maffei, A. Kate, G. Malavolta, P. Moreno-Sanchez, S. Ravi:
"Concurrency and Privacy with Payment-Channel Networks";
in: "CCS '17 Proceedings of the 2017 ACM SIGSAC Conference on Computer and Communications Security", ACM Digital Library, New York, 2017, ISBN: 978-1-4503-4946-8, 455 - 471.

PermissionlessblockchainsprotocolssuchasBitcoinareinherently limitedintransactionthroughputandlatency.Currenteffortsto address this key issue focus on off-chain payment channels that canbecombinedinaPayment-ChannelNetwork(PCN)toenable anunlimitednumberofpaymentswithoutrequiringtoaccessthe blockchainotherthantoregistertheinitialandfinalcapacityof eachchannel.Whilethisapproachpavesthewayforlowlatency andhighthroughputofpayments,itsdeploymentinpracticeraises severalprivacyconcernsaswellastechnicalchallengesrelatedto theinherentlyconcurrentnatureofpaymentsthathavenotbeen sufficientlystudiedsofar. In this work, we lay the foundations for privacy and concurrency in PCNs, presenting a formal definition in the Universal Composability framework as well as practical and provably securesolutions.Inparticular,wepresentFulgorandRayo.Fulgor isthefirstpaymentprotocolforPCNsthatprovidesprovableprivacyguaranteesforPCNsandisfullycompatiblewiththeBitcoin scriptingsystem.However,Fulgorisablockingprotocolandtherefore prone to deadlocks of concurrent payments as in currently available PCNs. Instead, Rayo is the first protocol for PCNs that enforcesnon-blocking progress (i.e.,atleastoneoftheconcurrent payments terminates). We show through a new impossibility result that non-blocking progress necessarily comes at the cost of weakerprivacy.AtthecoreofFulgorandRayoisMulti-HopHTLC, anewsmartcontract,compatiblewiththeBitcoinscriptingsystem, thatprovidesconditionalpaymentswhilereducingrunningtime andcommunicationoverheadwithrespecttopreviousapproaches. OurperformanceevaluationofFulgorandRayoshowsthatapaymentwith10intermediateuserstakesasfewas5seconds,thereby demonstratingtheirfeasibilitytobedeployedinpractice.

http://dx.doi.org/10.1145/3133956.3134096

http://publik.tuwien.ac.at/files/publik_268463.pdf