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Last weekend, on block 11380000, a solution for the prevention of 51% of attacks was presented to the Ethereum Classic (ETC) community. Many of these attacks have recently placed ETC at a crossroads, leaving the chain’s very survival uncertain. In the weeks following these attacks, the community worked to evaluate a number of 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 in 2014. It is based on the assumption that while reorganizations of small chains that go back a few blocks are perfectly normal, those that propose reorganisations that go 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 have to extract a longer chain in isolation and then propose it to the world. This was the case during the recent attacks, which cost honest participants millions of dollars.
MESS discourages the extraction of shadows by weighing the chains differently depending on the time of publication. Isaac Ardis, one of the developers of ETC Core, explained this mechanism to Cointelegraph:
“The intention is to burden the chains that occur and are available earlier than the chains that come later. And so in this way, there is an incentive to publish the work on the chain and discourages the chains that are defined in private and that would come later.
The algorithm employs a multiplier that determines the difficulty required by a proposed chain 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 the multiplier. So a shadow chain would have to provide much more proof of work to be considered canonical.
MESS does not make 51% attacks impossible as it is rather a probabilistic and nondeterministic solution, but it makes them prohibitively expensive. This is one of the reasons the community discussed implementing it alongside a checkpoint solution. Ardis said that while it’s a possibility, there doesn’t seem to be much benefit to this duplication:
“While you can use them together, you may not have to use them together and you may not even want to use them together.”
MESS has several advantages. The code base is compact and will not require a hard fork for implementation. Nodes that choose to MESS will be compatible with those that don’t. Any discord will only come into play when and if another 51% attack occurs, Ardis said:
“If a large attack occurs, the miners, operators and nodes that have activated MESS, we certainly hope, will be able to dissuade the attacker while those nodes that have not been updated will move into the attacker’s chain.”
The assumption is that most of these attacks tend to be short-term and opportunistic. Once the attacker leaves, the remaining honest miners will rejoin the canon fork.
MESS appears to be a short and medium term solution. Although Ardis believes that no chain is immune to a 51% attack, he agreed that the only viable protection is network growth. One of the bets is that Ethereum (ETH) miners will join Ethereum Classic after the former has passed the proof-of-stake consensus. Another is to take advantage of the compatibility between the two networks, which allows for a painless migration from a congested Ethereum to Ethereum Classic. Ardis said that now that they are done with this mess, the team can focus on developing new tools for ETC.
