Over the past weekend, on block 11380000, a solution for the prevention of 51% attacks was introduced to the Ethereum Classic (ETC) community. Several such attacks have recently placed ETC at a crossroads, leaving the very survival of the chain uncertain. In the weeks that followed these attacks, the community worked to evaluate numerous potential solutions.
The first one that has been implemented is called MESS, which stands for Modified Exponential Subjective Scoring. Its predecessor was first suggested by Vitalik Buterin back in 2014. It builds on the assumption that while small chain reorganizations that go back a few blocks are perfectly normal, the ones proposing reorganizations going back hundreds and even thousands of blocks are highly suspicious.
With most proof-of-work blockchains, the longest chain with the most work wins. This means that malicious attackers must mine a longer chain in isolation and then, propose it to the world. This was the case during recent attacks, which cost honest participants millions of dollars.
MESS disincentivizes shadow mining by weighing chains differently depending on the time of publication. Isaac Ardis, one of ETC Core’s developers, explained this mechanism to Cointelegraph:
“The intention with that is to weight chains which occur and are available first over chains that come later. And so in that way, there is an incentive to publish work on the chain and it disincentivizes chains that are defined in private and that would come later.”
The algorithm employs a multiplier that determines the required difficulty from a proposed chain in order to be considered canonical. The multiplier ranges from 1 to 31 and depends on the aforementioned time of publication. The more suspicious the proposed reorganization, the higher