The Philosophy of 51% Attack
51% attacks are a terror for any network. A 51% attack happens when a person or a group tries to gain control of the network by acquiring more than 50% of the network computing power. The percentage may vary from blockchain to blockchain, but in the Bitcoin example, the 51% attack is 50%.
Blockchain initial data implies that there is no central authority, miners can track the transactions validity. Miners always follow the longest chain rule, and according to the consensus mechanism, the consensus can be reached by the approval of more than 51% of the participants.51% percent attacks may happen only in permissionless blockchains. For permissioned blockchains there is no concern of 51% attacks since the participants identify themselves in a network; the governance system is protecting the network. Such control would also allow to reverse already completed transactions which may lead to double-spending. That means an ordering transaction can be modified, and all mining activities would be stuck. That’s precisely how a 51% attacker reverses a transaction that causes double-spending within the blockchain.
One of the reasons 51% attacks can succeed is that these blockchains are not large in terms of computing power. The capacity to resist 51% attacks grows with the number of miners. For example, in Bitcoin blockchain, billions of dollars should be spent to succeed in a 51% attack. In smaller blockchains, you can go to one of the mining platforms and purchase computing power ( hashing power) to get 51% of a smaller blockchain.
Blockchains are improving to avoid such types of attacks in the future by adjusting the block size which making the attack more difficult. Mechanisms such as Digi Shield reduce successful 51% attacks by reducing and increasing the block size according to the needs.
Example of blockchains that suffered from 51% attacks
The original Ethereum blockchain suffered a malicious 51% attack at the beginning of 2019. Analysts found a shady private mining pool that could grow its hashing power up to 3,263 GH/s. In a little while, it retrieved to the 300 GH/s. The attack iterated ten hours later. According to the CoinNess report, the secret mining pool controlled approximately 63% of the ETC network hash rate.
An additional example of 51% is the attack on Bitcoin Cash. In November 2018 Craig Write and a group of BCH supporters wanted to create a hard fork through a 51% attack by increasing the block size from 32MB to 128 MB. Although both Blockchains could cleanly split in a hard fork, Bitcoin Cash supporters wanted Bitcoin cash to come out as the only survivor. Additional rewards were given to join the network. The fork was not successful. Bitcoin Cash tried to gather the resources for the takeover by giving miners additional rewards.Bitmain , mining devices producer allocated for a period of time 90,000 Antminer S9 devices to the Bitcoin Cash network.This was extremely hated by the users of Bitcoin Cash.
Verge employed an algorithm named “Dark Gravity Wave” to modify the difficulty level on the network, utilizing the average block-confirmation time over a 30-minute window. The Verge attack was quite smart, in that the attacker changed time stamps on the chain to decrease the difficulty level of Verge and successfully attacked it with far less than 51% of the network hash power. Before the attack, Verge’s difficulty was around 139093, and it dropped down to a low of .00024414 during the attack.
The Verge team updated the protocol as a response but accidentally initiated a hard fork on the network that then was restored.
This is a collection of coins and the theoretical cost of a 51% attack on each network.
The risk of a 51% attack poses a threat on every decentralized network. Nevertheless, to execute such an infringement, hackers must surpass themselves in order to gain control over a blockchain. The protocols are being upgraded, the mining difficulty for most blockchains is growing all the time. The war goes on!
