Paper 2019/752

Fact and Fiction: Challenging the Honest Majority Assumption of Permissionless Blockchains

Runchao Han, Zhimei Sui, Jiangshan Yu, Joseph Liu, and Shiping Chen


Honest majority is the key security assumption of Proof-of-Work (PoW) based blockchains. However, the recent 51% attacks render this assumption unrealistic in practice. In this paper, we challenge this assumption against rational miners in the PoW-based blockchains in reality. In particular, we show that the current incentive mechanism may encourage rational miners to launch 51% attacks in two cases. In the first case, we consider a miner of a stronger blockchain launches 51% attacks on a weaker blockchain, where the two blockchains share the same mining algorithm. In the second case, we consider a miner rents mining power from cloud mining services to launch 51% attacks. As 51% attacks lead to double-spending, the miner can profit from these two attacks. If such double-spending is more profitable than mining, miners are more intended to launch 51% attacks rather than mine honestly. We formally model such behaviours as a series of actions through a Markov Decision Process. Our results show that, for most mainstream PoW-based blockchains, 51% attacks are feasible and profitable, so profit-driven miners are incentivised to launch 51% attacks to gain extra profit. In addition, we leverage our model to investigate the recent 51% attack on Ethereum Classic (on 07/01/2019), which is suspected to be an incident of 51% attacks. We provide insights on the attacker strategy and expected revenue, and show that the attacker’s strategy is near-optimal.

Note: All types of feedback are welcome.

Available format(s)
Cryptographic protocols
Publication info
Published elsewhere. Minor revision. AsiaCCS'21
BlockchainProof-of-Work51% Attacks
Contact author(s)
runchao han @ monash edu
zhimei sui1 @ monash edu
jiangshan yu @ monash edu
joseph liu @ monash edu
shiping chen @ data61 csiro au
2021-02-28: last of 11 revisions
2019-06-26: received
See all versions
Short URL
Creative Commons Attribution


      author = {Runchao Han and Zhimei Sui and Jiangshan Yu and Joseph Liu and Shiping Chen},
      title = {Fact and Fiction: Challenging the Honest Majority Assumption of Permissionless Blockchains},
      howpublished = {Cryptology ePrint Archive, Paper 2019/752},
      year = {2019},
      doi = {10.1145/3433210.3453087},
      note = {\url{}},
      url = {}
Note: In order to protect the privacy of readers, does not use cookies or embedded third party content.