Is it a reliable option to launch a 51% attack to prevent evil?

Recently, two Bitcoin Cash (BCH) mining pools launched a 51% attack on the blockchain, apparently in an attempt to reverse a miner’s transactions.

The move is related to the BCH network hard fork on May 15. The two mining pools BTC.com and BTC.top took this action to prevent unknown miners from taking coins that did not belong to them after the code change. On that day, the attacker exploited a bug unrelated to the upgrade (which has since been fixed), which caused the network to split and miners to mine empty blocks for a short period of time.

In the context of cryptocurrencies like BCH, a 51% attack requires one entity or organization to control a majority of the computing power, allowing them to do something they normally can’t and wouldn’t be allowed to do, like attempting to rewrite the network’s transaction history.

This has always been a controversial topic, and some currencies have suffered similar attacks due to the decline in computing power in the network.

BTC.top’s hashrate did exceed 50% for a short period of time, but BTC.com and BTC.top were able to work together to reverse the transactions in the block.

What’s interesting about this particular attack on BCH, though, is that the purpose of carrying it out was arguably to do something ostensibly good for the community, rather than to reward the attacker or take the money for themselves.

But not all BCH community members agree. A BCH developer named moniker Kiarahpromises wrote in a post on May 17:

“This is a 51% attack by reorganizing to revert unknown transactions. Probably the worst attack. It’s mentioned in the whitepaper. What about (miners and developers) decentralized and censorship-resistant cash? Whatever is convenient, right?”

The internal details of this attack by the mining pool (and the attack that triggered it) are complex.

Bitcoin Podcast host Guy Swann explained on Twitter:

“Since the launch of BCH in 2017, a large number of coins have been accidentally sent to “anyone can spend” addresses (due to signed transaction compatibility, but BCH does not support SegWit), or they may have been resent from the Bitcoin network to the BCH network.”

But he added that those coins could be spent once a code change was removed in BCH’s May 15 hard fork, “essentially giving those bitcoins to miners.”

Unidentified miner attackers decided to take these coins. That’s when BTC.top and BTC.com suddenly stepped in and reversed the transactions.

“When the unidentified miner tried to take the coins for himself, BTC.top and BTC.com noticed and immediately decided to reorganize and delete these (transactions) in favor of their own (transactions), using the same P2SH coins.”

There are also some BCH users who think this approach is correct.

BCH supporter Jonathan Silverblood said:

“This is a very unfortunate situation, but it’s also a reflection of proof of work. The miners did choose to abandon the high hashrate this time, but from what I understand it was because they believed one of the transactions was invalid.”

However, some people believe that this is not a good thing for BCH. They believe that this incident shows that the cryptocurrency is too centralized.

51% attacks have become a common concern for PoW currencies (as mentioned above, some blockchains have already had to deal with this issue due to declining hashrate). Half of Bitcoin’s hashrate is currently in the hands of three mining pools.