## Introduction
A 51% attack, also known as a majority attack, is one of the most significant threats to blockchain systems based on Proof of Work (PoW) and Proof of Stake (PoS) protocols. This article discusses the mechanisms of the 51% attack, its consequences for blockchains, and also offers recommendations for protection against such attacks.
## 51% attack mechanism in PoW systems
In PoW systems, an entity that controls more than 50% of the hashing power can gain full control of the blockchain. This allows an attacker to:
1. Fork the main chain: An attacker can create an alternative branch of the blockchain by starting mining on his branch.
2. Get ahead of the main chain: Gradually, the attacker will be able to get ahead of the main chain as his hashing power exceeds 50%.
3. Monitor transactions: An attacker can reverse or modify transactions, leading to double spending and other types of fraud.
Since block generation in PoW is probabilistic, there is a possibility of conflicting branches. However, as the 51% barrier is overcome, the likelihood of a successful attack increases significantly.
## 51% attack mechanism in PoS systems
In PoS protocols, a majority attack is also possible, but of a slightly different nature. In this case, one validator or a coordinated group of validators owning more than 34% (for BFT PoS) of the blockchain share can:
1. Return finality: An already completed block can be challenged by finalizing another competing block.
2. Cause a denial of viability: This results in a denial of the blockchain’s viability or censorship of it.
## Consequences of the 51% attack
A 51% attack can have catastrophic consequences for blockchain systems:
1. Double Spending: An attacker can spend the same coins several times.
2. Breach of Trust: Users lose trust in the system, which can lead to a drop in the value of the cryptocurrency.
3. Transaction censorship: An attacker can block or change transactions, which violates the principle of decentralization.
## Security recommendations
To protect against 51% attacks, it is recommended:
1. Increasing mining difficulty: This will make it more difficult for an attacker to gain control of the network.
2. Decentralization of hashing power: Distributing the power among a large number of participants reduces the risk of attack.
3. Confirmation of transactions: The exchange or recipient must complete the payment after the transaction has been confirmed by a sufficient number of blocks.
## Conclusion
A 51% attack poses a serious threat to blockchain systems, both PoW and PoS based. Understanding the mechanism of this attack and its consequences allows developers and users to take measures to protect their systems. Implementing recommendations to increase mining difficulty, decentralize hashing power, and confirm transactions will help reduce the risk of a successful 51% attack.
## Links
Research on Long Range Attacks for Proof of Stake Protocols: IEEE Xplore