What is a 51% attack and why is it dangerous for bitcoin


A 51% attack is an attack on a blockchain network, where an attacker or a group of attackers control more than 50% of the network’s mining (or hashing) power. This type of attack is dangerous for bitcoin as it allows the attacker to reverse transactions on the blockchain, double-spend coins, prevent certain transactions from being confirmed, and stop other miners from mining new blocks. Additionally, the attacker can monopolize new coins generated by the network, effectively controlling the supply of the cryptocurrency.

How does a double spend of cryptocurrency (double spend)?

Imagine that the attacker has significant computing power. He pays for the goods or service with the supplier, he accepts a large amount in cryptocurrency and the transaction is almost completed. The transaction is sent to the general blockchain and after three confirmations, the participants in the transaction say goodbye.

When the villain is convinced that the victim will not find him, he “returns” the coins to himself. To do this, the attacker, after sending the money, rolls back the blockchain to an earlier state.

Another, more stealthy option: the attacker mines a parallel chain of blocks, in the manner of selfish mining. There, instead of a fair transaction, a double-spend transaction was included. Such a transaction sends the same coins to another address owned by the scammer. It remains to “feed” an alternative portion of blocks (with the correct PoW) to the valid chain, in the expectation that the network will accept them.

What is a 51% attack?

Thus, the network will “exclude” the correct transaction from the history. The supplier looks into the wallet and sees that he has lost his coins and there is no evidence of the transaction. He did not even take screenshots of the wallet, did not copy the transaction ID when he received the coins.

In theory, if a transaction has one or more confirmations, double spending is excluded. Many do not know what to do when a transaction “disappears” from a bitcoin wallet.

Thanks to such “schemes”, the coins keep returning to the attacker’s wallet, and you can spend them twice, thrice, and so on. Frequent double spending leads to the threat of withdrawal from trading on exchanges affected by double spending. In addition, attacked cryptocurrencies lose market capitalization after the attack. For example, the Verge cryptocurrency was attacked in May 2018 and has since lost over 95% of its value.


A 51% attack is a malicious attack on a cryptocurrency network in which an attacker controls more than 50% of the network’s computing power. This gives the attacker the ability to prevent new transactions from being confirmed, and even reverse previous transactions. The attacker can also double-spend coins, meaning they can spend the same coins multiple times. This kind of attack is particularly dangerous for Bitcoin because of its decentralized nature, as it means the entire network can be compromised. If a 51% attack is successful, it can cause significant damage to the credibility of Bitcoin, as well as lead to financial losses for investors in the cryptocurrency.


What is a 51% attack and why is it dangerous for bitcoin?

What is a 51% attack and why is it dangerous for bitcoin?

A 51% attack is a situation where a single entity or group of entities control more than 50% of the hash rate (computing power) of a cryptocurrency network. This allows the entity or group of entities to control the network and allow for double-spend transactions, blocking transactions or reversing transactions. This could be highly damaging for the Bitcoin network, as it would have a negative impact on its trustworthiness and reputation, leading to a decrease in its value.

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