A blockchain reorganization attack, also known as a 51% attack, is a security scenario that occurs when a single entity or group of entities manages to control the majority of the hash power in a cryptocurrency network. Hash power is the computational power that miners use to validate and add new transactions to the blockchain. If a malicious actor manages to control more than 50% of the hashing power, he can potentially disrupt the network and manipulate the blockchain in many ways.
To understand how a blockchain reorganization attack works, we first need to understand the cryptocurrency mining process. Mining is the process by which new transactions are verified and added to the blockchain. This is done by miners, who use computational power to solve complex mathematical problems. The first miner to solve the problem gets the right to add the next block of transactions to the blockchain and is rewarded with an amount of cryptocurrency.
The blockchain is designed to be decentralized and secure, which means that no one individual or group should be able to control the majority of the hashing power. However, if a malicious actor manages to control the majority of the hashing power, he can potentially manipulate the blockchain. This can be done in a number of ways, including blockchain reorganization, which means the malicious actor can change the order of blocks on the blockchain.
A blockchain reorganization attack occurs when the malicious actor starts mining blocks in secret. While the rest of the network is working on the next block, the malicious actor is working on his own version of the blockchain. If the malicious actor can mine blocks faster than the rest of the network, he can eventually create the longest chain of blocks. According to blockchain rules, the longest chain is considered the correct blockchain version.
Once the malicious actor has the longest chain, he can then broadcast his version of the blockchain to the rest of the network. Since your chain is longer, the rest of the network will recognize it as the correct version and start working on it. This is known as a blockchain reorganization and can have a number of negative consequences.
Firstly, the malicious actor can double-spend the same cryptocurrency, also known as a double-spending attack. This is because the malicious actor can include a transaction in their version of the blockchain and then not include it when they reorganize the blockchain. This allows them to spend the same cryptocurrency twice.
Second, the malicious actor can deny transactions. This is because they can choose not to include certain transactions in their version of the blockchain. This can be used to censor transactions or to harm certain users or businesses.
Third, the malicious actor can rollback transactions. This is because they can add a transaction to their version of the blockchain and then remove it when they reorganize the blockchain. This can be used to reverse transactions they don't want to happen.
In summary, a blockchain reorganization attack is a serious security scenario that can have a number of negative consequences. Although difficult to execute, due to the need to control the majority of hashing power, it is still a potential threat that must be taken into account when designing and using cryptocurrency networks.
