On Nxt Proof of Stake Algorithm: A Simulation Study

In this paper, we present a simulation study of a Proof of Stake (PoS) consensus algorithm used in a public blockchain called Nxt. We first provide an overview of Nxt and its PoS consensus algorithm design, and introduce a concise mathematical analysis of the Nxt PoS algorithm. We then present an experimental study on the Nxt PoS behavior in a simulated environment and in a small network running Nxt private nodes. We show that the fractions of the blocks generated by forgers in the system are generally proportional to their weight in stakes provided that the weight is relatively small, which is consistent with the mathematical analysis. We consider two scenarios of double-spending attacks: (1) a single dominating forger with large stakes; and (2) a group of colluding forgers that collectively control large stakes. The simulation results show that the single-forger attack is more advantageous over using a pool of forgers in launching successful double-spending attacks. Finally, we propose a quota-based mechanism to limit the fraction of blocks that any forger could generate. We show that the mechanism is highly effective in mitigating the single-forger attack, but has limited success in preventing the double-spending attacks based on forging pools.