Eventually Consistent Distributed Ledger Relying on Degraded Atomic Broadcast The great increase of the Bitcoin cryptocurrency value these last months has brought blockchain and distributed ledger technologies back on the front stage. However, it also gives incentive to malicious users who would like to break the system or take advantage of it (steal money, hide some information stored in the ledger, isolate a particular node from the rest of the network, etc.). Thus, our research focuses on overcoming potential attacks to distributed ledgers. In this paper, we focus on attacks that damage underlying networks of distributed ledgers. Underlying networks offer useful communication primitives such as an atomic broadcast, however such attacks may degrade the property of the primitives and make distributed ledgers relying on the primitives no longer work. Hence we should design algorithms to make the distributed ledgers still work even when some attacks degrade the primitives. As the first study of this problem, we consider distributed ledgers that rely on an atomic broadcast to maintain consistency. We define different cases of attacks affecting the uniform agreement property of the atomic broadcast: We first assume that a single server can fail to deliver a particular message and then we assume that several servers can fail to deliver the message. Afterwards, we present an algorithm to recover the states of the servers that fail to deliver some messages, and consider the situation where the maximum of servers which keep a wrong sequence is known and bounded.

A Permissioned Distributed Ledger for Monitoring Web Content Online publishing of news and information enables important content to reach a much larger audience than traditional paper publishing, but there is no guarantee of longterm, reliable, and persistent access to the content. Over time, links decay because they are not reliably updated when content changes location, or even worse, content is deliberately altered from its original published form or deleted altogether. We present the design of Watchdog, a permissioned distributed ledger that securely and reliably monitors and preserves dynamic web content such as government and news sites. Unlike prior preservation systems, our system design allows nodes to be dispersed across multiple administrative domains, thus eliminating single points of trust and at the same time, is the first to monitor how content changes both over time and across geographical location. Watchdog achieves Byzantine faulttolerance via a novel interactive consistency algorithm that offers a twist on the traditional definition and use of interactive consistency; the algorithm enables a set of mutually suspicious nodes, with arbitrarysized, potentially overlapping data collections, to efficiently agree on the exact data collection in possession by each node. We demonstrate how our algorithm enables Watchdog nodes to exchange and agree upon the content each has observed individually, in a faulttolerant, tamperproof manner.