Eventually Consistent Distributed Ledger Relying on Degraded Atomic Broadcast

Abstract

The great increase of the Bitcoin crypto-currency 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.