Paper 2020/019

Short Selling Attack: A Self-Destructive But Profitable 51% Attack On PoS Blockchains

Suhyeon Lee and Seungjoo Kim

Abstract

There have been several 51% attacks on Proof-of-Work (PoW) blockchains recently, including Verge and GameCredits, but the most noteworthy has been the attack that saw hackers make off with up to $18 million after a successful double spend was executed on the Bitcoin Gold network. For this reason, the Proof-of-Stake (PoS) algorithm, which already has advantages of energy efficiency and throughput, is attracting attention as an alternative to the PoW algorithm. With a PoS, the attacker needs to obtain 51% of the cryptocurrency to carry out a 51% attack. But unlike PoW, attacker in a PoS system is highly discouraged from launching 51% attack because he would have to risk losing his entire stake amount to do so. Moreover, even if a 51% attack succeeds, the value of PoS-based cryptocurrency will fall, and the attacker with the most stake will eventually lose the most. In this paper, we try to derive the results that go against these conventional myths. Despite of the significant depreciation of cryptocurrency, our method can make a profit from a 51% attack on the PoS blockchains using the traditional stock market's short selling (or shorting) concept. Our findings are an example to show that the conventional myth that "a destructive attack that destroys the blockchain ecosystem totally will not occur because it is fundamentally unprofitable to the attacker itself" may be wrong.