Intrusion-Tolerant Reliable Broadcast

We consider a system with n processes where some of them can show a byzantine behavior. A byzantine process can deviate arbitrarily from the protocol, e.g., dropping messages, changing the content of a message (different recipients may receive different content of the same broadcast message), delivering messages not sent by any process or creating fake messages. This paper introduces a new broadcast abstraction, namely Intrusion-tolerant Reliable Broadcast (IT-RB). IT-RB ensures that if a message M(v) (where v is the content of M) is broadcast by a correct process, all correct processes will deliver v. Consider now {v1, . . . , vk} (with 1 ≤ k < n) be the set of contents associated with M and sent to other processes by a byzantine one, IT-RB ensures: if the sender of the broadcast M is byzantine, then a subset of correct processes deliver the same content v ∈ {v1, . . . , vk} while the rest of correct processes deliver ⊥. We first provide a protocol P implementing IT-RB and prove its correctness. P can deliver an infinite number of ⊥ in an infinite run. Secondly, we introduce an oracle detecting byzantine processes running P , namely P-LO, and provide an implementation of P-LO. Such oracle exploits the information collected by a process running P to infer bad process behaviors and to remove them from the computation. Considering an infinite run generated by P∗, i.e. a protocol that runs together P plus P-LO, we finally show that the number of ⊥ delivered by correct processes is finite.