We propose a fast and resource-efficient agreement protocol on a request set, which is used to realize Byzantine fault tolerant server replication. Although most existing randomized protocols for Byzantine agreement exploit a modular approach, that is, a combination of agreement on a bit value and a reduction of request set values to the bit values, our protocol directly solves the multi-valued agreement problem for request sets. We introduce a novel coin tossing scheme to select a candidate of an agreed request set randomly. This coin toss allows our protocol to reduce resource consumption and to attain faster response time than the existing representative protocols.

We propose a new method that accelerates asynchronous Byzantine Fault Tolerant (BFT) protocols designed on the principle of state machine replication. State machine replication protocols ensure consistency among replicas by applying operations in the same order to all of them. A naive way to determine the application order of the operations is to repeatedly execute the BFT consensus to determine the next executed operation, but this may introduce inefficiency caused by waiting for the completion of the previous execution of the consensus protocol. To reduce this inefficiency, our method allows parallel execution of the consensuses while keeping consistency of the consensus results at the replicas. In this paper, we also prove the correctness of our method and experimentally compare it with the existing method in terms of latency and throughput. The evaluation results show that our method makes a BFT protocol three or four times faster than the existing one when some machines or message transmissions are delayed.

We present a distributed protocol for achieving totally unbiased global coin flipping in the presence of an adversary. We consider a synchronous system of n processors at most t of which may be corrupted and manipulated by a malicious adversary, and assume a complete network where every two processors are connected via a private channel. Our protocol is deterministic and assumes a very powerful adversary. Although the adversary cannot eavesdrop, it is computationally unbounded, capable of rushing and dynamic. This is the same model that is adopted in Yao's global coin flipping protocol, which we use as the base of our protocol. Our protocol tolerates almost n/3 processor failures and terminates in t+4 rounds. The resilience of our protocol is greatly improved from that of Yao's protocol at the slight expense of running time, which is only added just two rounds.