ETC is frequently attacked by 51. How do attackers succeed?

After suffering a 51 attack on August 1, media reported on August 6 that ETC suffered another 51 attack. These frequent 51 attacks were a heavy blow to ETC.

Among the current blockchain projects, ETC is the one that has suffered the most 51 attacks and still survived. For ETH, which has the same origin, such a thing has not happened, and 51 attacks are basically rare. In fact, as early as 2019, a team warned that ETC was at risk of 51 attacks, and then ETC encountered a 51 attack a few days later. This time, it has encountered one attack after another, which is rare.

Many old crypto players should be familiar with the 51 attack. It is mainly to control a large amount of computing power of the entire network, and then fork out another chain, so that this chain becomes the longest chain. If the chain changes, some large transactions will be invalid. Therefore, the attacker can obtain huge profits in this way. The simplest way is to deposit your own coins to the exchange, then sell the withdrawn coins, and then start mining in the blocks before the deposit, making the original chain invalid. The new chain removes the coins that you deposited to the exchange, so that your coins return to your wallet.

The 51 attack is actually similar to the fork of digital currency, but the biggest difference is that after the fork, both the original chain and the new chain are running normally, and at least one chain has taken anti-replay attack measures, so that the two chains can be separated to prevent miners from mixing up blocks after mining. For example, if there is no anti-replay attack measure right after the fork, the blocks mined by BCH miners may take effect on the BTC chain, which causes some inconvenience.

The 51 attack occurs when a fork occurs. The original chain and the new chain will inevitably have one chain longer and one chain shorter within a certain period of time. This will cause miners to only recognize the longest chain and not mine the shorter chain. This means that the short chain is invalid and dead, and the long chain is the only valid chain.

Of course, this is what we generally understand as 51 attacks and forks. This is also based on the theory on the Bitcoin chain and is more suitable for BTC, BCH and other chains. For ETH and ETC, our understanding of 51 attacks may be a little more complicated.

The block time of ETH/ETC is generally shorter, unlike Bitcoin which requires a lot of calculations. Therefore, in this case, ETH/ETC is prone to forks. Therefore, the GHOST protocol was created. Simply put, it allows the existence of forked blocks, which we call uncle blocks.

The generation of uncle blocks ensures that even if a miner accidentally mines a block, subsequent miners can integrate the forked blocks together, thus avoiding miners fighting over the length of the chain.

The next block is usually packaged before the uncle block is significantly extended, which greatly reduces the possibility of block forks and thus prevents 51 attacks to a certain extent.

Therefore, as long as the miners of ETH/ETC compete fairly, it is basically unlikely that a 51 attack will occur. The 51 attack that ETC suffered this time was actually mainly because the attackers were secretly mining and kept it a secret. They waited until they significantly exceeded the main chain and then made the blocks public, thereby consolidating their competitive advantage.

This behavior of secretly mining and then announcing it at a certain time is also called a block hiding attack or a concealed block attack by some people (hiding one's own blocks and announcing them at a certain time. There is also a type of mining that is done by a mining pool, which hides the mined blocks and does not announce them. In simple terms, it is a behavior that harms others and does not benefit oneself).

One condition that needs to be noted here is that the profit gained by the attacker generally needs to be greater than the cost of renting computing power for secret mining.

At this point, we should think about why attackers like to carry out 51 attacks on ETC. The main reason is that when ETC carries out such an attack, the computing power of the entire network can actually be hidden without being discovered.

Simply put, we all know that ETC and ETH use the same mining algorithm. According to the current situation, ETH's total network computing power is much higher than ETC. When attackers need to attack ETC, they must rent or purchase a large amount of computing power in advance. This part of the computing power needs to be subtracted from the total network computing power in advance. If it is other currencies, a large amount of computing power disappears, which is a clue that is easy to be discovered, but ETC will not, because it can be directly subtracted from the ETH computing power, and almost no one finds out. This is very similar to the strategy of the early BCH attack.

After the attacker rents the computing power, he hides and mines. Compared with ETH, it is not easy to be discovered, so it is easy to implement the strategy of secret mining. In addition, miners on ETC actually have to compete. In fact, in theory, as long as the computing power has an overwhelming advantage, 51% of the computing power is not required.

Of course, similar to the 51 attack method like ETC, BCH finally intervened to make the attacker's chain invalid. Now BCH also has a strategy to prevent 51 attacks and has implemented a function such as 0 confirmation, so it can avoid this type of attack. If ETC cannot effectively take certain measures to avoid irreversible confirmation of blocks, then even if the currency rushes into the exchange and confirms hundreds or even nearly a thousand blocks, it is still possible for the attacker to succeed.