On proof-of-work blockchains like Ethereum Classic, transaction security is measured in block confirmations. Each confirmation makes it exponentially more expensive to reverse a transaction.
What Is a Confirmation?
When your transaction is included in block N, it has one confirmation. When block N+1 is mined on top of block N, your transaction has two confirmations. Each new block adds another confirmation.
Why Confirmations Matter
In PoW, miners compete to extend the longest chain. Occasionally, two miners find valid blocks at nearly the same time, creating a temporary fork. The network resolves this by following the chain with more cumulative work — the "heaviest chain."
Transactions in the shorter fork are returned to the mempool and may be re-included in a future block. This is a chain reorganization (reorg).
Confirmation Guidelines
The appropriate number of confirmations depends on the value at stake:
| Transaction Value | Recommended Confirmations | Wait Time (~13s/block) |
|---|---|---|
| Small (< 100 ETC) | 6 | ~78 seconds |
| Medium (100–10K ETC) | 20 | ~4.3 minutes |
| Large (> 10K ETC) | 40 | ~8.7 minutes |
| Exchange deposits | 40–100 | 8.7–21.7 minutes |
The Economics of Reorgs
To reverse a transaction with N confirmations, an attacker must:
- Mine an alternative chain starting from the block before the transaction
- This chain must be longer than the current chain (requiring > 50% of network hashrate sustained for N blocks)
- The cost scales linearly with N: more confirmations = more mining power × more time needed
After Ethereum's Merge, ETC's hashrate increased dramatically, making 51% attacks substantially more expensive than pre-Merge levels.
MESS and Additional Protections
In 2020, following 51% attacks, ETC implemented MESS (Modified Exponential Subjective Scoring, ECBP-1100) to penalize deep reorganizations. MESS was later deprecated after the post-Merge hashrate increase made attacks economically infeasible.
Compared to PoS Finality
Proof-of-stake chains like Ethereum offer deterministic finality — after a certain point, transactions cannot be reversed at all. PoW chains offer probabilistic finality — transactions become increasingly secure but are never mathematically irreversible. The tradeoff is that PoW finality requires no trusted validator set.