Fact and fiction: challenging the honest majority assumption of permissionless blockchains

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

Research output: Chapter in Book/Report/Conference proceedingConference PaperResearchpeer-review

4 Citations (Scopus)


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.

Original languageEnglish
Title of host publicationASIA CCS'21 - Proceedings of the 2021 ACM Asia Conference on Computer and Communications Security
EditorsTsz Hon Yuen
Place of PublicationNew York NY USA
PublisherAssociation for Computing Machinery (ACM)
Number of pages15
ISBN (Electronic)9781450382878
Publication statusPublished - 2021
EventACM Asia Conference on Computer and Communications Security 2021 - Online, Hong Kong
Duration: 7 Jun 202111 Jun 2021
Conference number: 16th
https://dl.acm.org/doi/proceedings/10.1145/3433210 (Proceedings)


ConferenceACM Asia Conference on Computer and Communications Security 2021
Abbreviated titleAsiaCCS 2021
Country/TerritoryHong Kong
Internet address


  • 51% attacks
  • blockchain
  • proof-of-work

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