In theory, the records on a blockchain cannot be altered because enough disparate parties are incentivized to maintain an accurate accounting of who owns what. But a 51 percent attack can change that calculus, allowing someone to falsify transactions and spend the same holding more than once, fundamentally damaging the usefulness of a digital payment system — and people’s trust in it.
Proponents of blockchain technology often talk up the security of the network, but a 51 percent attack is one of the flaws. They are currently quite rare but have occurred in some instances, particularly with smaller cryptocurrencies. Last year, litecoin cash and zencash both suffered 51 percent attacks. The size of a cryptocurrency’s network is thought to therefore be a defense: it becomes prohibitively expensive to take over more than half of a gigantic distributed system.
As of Tuesday morning, ethereum classic had a market capitalization of more than $500 million. That pales in comparison to bitcoin’s nearly $70 billion market cap, but still means the token ranks in the top 20 cryptocurrencies measured by CoinMarketCap.