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.
Deprecation
With the post-Merge hashrate providing strong natural protection, MESS was deprecated. The community determined that the economic security from high hashrate made the additional subjective penalty unnecessary — and removing it simplified the client codebase.
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