Revisiting FAW attack in an imperfect PoW blockchain system

Malicious miners in a Proof-of-Work (PoW) blockchain can apply less computing power to perform fork-after-withholding (FAW) attack than that to selfish mining and other withholding attacks. Quantitative study of FAW attack enables an in-depth understanding of the attack and then helps design countermeasures. The existing quantification studies of FAW attack only considered a perfect Bitcoin blockchain, where there is no block propagation delay. This paper aims to quantitatively investigate FAW attack in imperfect Bitcoin and Ethereum systems. We first establish an analytic model to capture the chain dynamics under FAW attack in a PoW system where the longest-chain protocol is used. Then the model is explored to derive closed-formed metric formulas for Bitcoin and Ethereum, respectively. These closed-formed formulas enable the evaluation of both the profitability of FAW adversaries and the impact of FAW attack on system throughput. Experimental results reveal that FAW adversaries can get more revenue in the network with propagation delay than without delay. FAW attack can reduce the blockchain throughput, especially in Bitcoin.