Analysis on the Block Reward of Fork After Withholding (FAW)

The irreversible trend in clustering of mining power raises severe concerns on stability and security of PoW based on cryptocurrency. It has been shown that, in some case, reward of a mining pool can be significantly increased by deviating honest mining strategy. As a result, many attacking strategies are proposed to maximize pools’ reward. Very recently, Kwon et. al. proposed Fork After Withholding (FAW) attack by combining selfish mining and Block Withholding (BWH) attack, which, as they stated, has a better reward than BWH. However, it is not always the case after our further investigation. In this paper, we firstly give a detailed comparation between the BWH and FAW attack, and show the implications behind them. We also consider honest mining to make the analysis of the block reward more clear. We demonstrate the imperfection of FAW in relative reward, reward after the fork and the fork state. Our main finding for FAW attack includes that the reward of victim pool increases faster compared to BWH attack, and for some cases, the attack should adopt honest mining strategy to maximize its reward, therefore, we present an improved FAW strategy, and propose a protocol for the pool’s manager to resist FAW’s attacker. Finally, we discuss the underlying flaws of FAW attack as well as countermeasures to alleviate it.