In August 2020, Ethereum Classic experienced multiple 51% attacks that resulted in deep chain reorganizations and double-spend attempts. The community responded with MESS (Modified Exponential Subjective Scoring), a novel defense mechanism.
The 2020 Attacks
Three separate 51% attacks occurred in August 2020:
- August 1: 3,693 block reorganization (~12 hours of chain history rewritten)
- August 6: 4,000+ block reorganization
- August 29: 7,000+ block reorganization (the deepest)
The attacker used rented hashrate to build alternative chains in secret, then broadcast them to reorganize the canonical chain. This enabled double-spend attacks against exchanges.
MESS: The Response
ECBP-1100, known as MESS (Modified Exponential Subjective Scoring), was implemented in October 2020. The mechanism works by penalizing chain reorganizations that go back many blocks:
How MESS Worked
- Each node tracks the "gravity" of the chain it's following
- A competing chain that reorganizes N blocks back must demonstrate exponentially more work than the current chain tip
- Short reorgs (1-2 blocks, normal PoW behavior) are unaffected
- Deep reorgs (100+ blocks) become effectively impossible even with majority hashrate
Key Properties
- No consensus change: MESS is a node-level policy, not a consensus rule
- Gradual penalty: The penalty increases with reorganization depth
- Configurable: Node operators could adjust or disable the penalty
The Merge Changes Everything
When Ethereum transitioned to proof-of-stake in September 2022, GPU miners migrated to ETC in massive numbers. Network hashrate increased from ~25 TH/s to over 200 TH/s: an 8x increase.
This dramatically changed the economics of 51% attacks. The cost to rent sufficient hashrate to attack ETC became prohibitively expensive.
Default off under ECBP-1110
With the post-Merge hashrate providing strong natural protection, ECBP-1110 recommended shipping MESS off. The community determined that the economic security from high hashrate made the additional subjective penalty unnecessary by default. MESS was not removed: it stays in the client, and each node operator can turn it on or off according to their own security needs.
Lessons Learned
The 2020 attacks and MESS response demonstrated ETC's ability to:
- Respond quickly to security incidents
- Implement novel defense mechanisms
- Remove temporary measures when they're no longer needed
- Rely on fundamental PoW economics for long-term security
