What is a 51% attack?

 

A 51% attack is a type of attack against a cryptocurrency network based on blockchain technology, where the attacker gains control of more than 51% of the network's computational power, thereby acquiring control over the entire network. To understand a 51% attack, it is essential first to grasp the basic principles of mining and blockchain.

 

Mining is a process within the blockchain network used to validate and package transactions, subsequently adding them to the blockchain. This process involves solving complex mathematical problems, often referred to as "Proof of Work" (PoW). Solving these problems requires a significant amount of computational power, which is the work performed by miners. Miners, participants in the network, compete to compute hash values to create new blocks. The miner who first finds the correct answer is granted the right to add a new block and receives a certain amount of cryptocurrency as a reward, a process known as "mining" new coins. Miners not only validate transactions but also ensure the security and decentralization of the blockchain.

 

Blockchain is a decentralized distributed ledger composed of multiple nodes on the network, eliminating central authority and relying on consensus among nodes. Each node possesses a complete copy of the ledger, meaning there is no single server or entity controlling the entire system. This decentralized nature imparts high censorship resistance and fault tolerance to the blockchain. Each node maintains a replicated ledger synchronized with others. When new transactions are added to the blockchain, they are broadcasted to the entire network, with each node verifying the validity of these transactions. Only validated transactions are included in the next block. This distributed replication and verification mechanism ensures data consistency and security.

 

The distributed nature of blockchain makes it challenging to attack. To alter data on the blockchain, an attacker would need to control a majority of the nodes on the network, which is a costly and difficult task. This resistance to attacks is a crucial feature of blockchain, especially in public blockchain networks like Bitcoin and Ethereum.

What is a 51% attack?

A 51% attack occurs when an attacker controls more than 51% of the total computational power in a cryptocurrency network. This means the attacker has enough power to control the process of creating new blocks and can manipulate transaction verification on the network. Attackers can exploit this advantage for malicious activities, including double-spending and denial-of-service attacks.

 

Assuming the total computational power of the Bitcoin network is 100 EH/s (exahashes per second), an attacker with at least 51 EH/s of computational power poses the possibility of a 51% attack. Once the attacker controls more than 51% of the computational power, they can initiate the attack. For example, they can create a private branch of the blockchain, continuously adding new blocks to the private chain without broadcasting them to the public network. Subsequently, they can engage in transactions on the public chain while sending the same bitcoins to the private chain, executing a double-spending attack. Since the attacker controls more than 51% of the computational power, their private chain will surpass the public chain, ultimately replacing it as the main chain, making double-spending a reality.

Likelihood of a 51% attack

The likelihood of a 51% attack is typically low because the security of a cryptocurrency network relies on the computational power distributed among a large number of miners worldwide. Gaining control of more than 51% of this computational power is both expensive and challenging. Additionally, once a 51% attack is detected, the cryptocurrency community usually takes measures to address it, such as updating algorithms or implementing other defensive measures.

 

In conclusion, a 51% attack poses a serious threat, but in most cryptocurrency networks, the likelihood of an attack is low due to the substantial computational power and resources required. Nevertheless, the cryptocurrency community continues to work on improving security to reduce the risk of such threats.