What is an attack 51%?

The attack 51% is the vulnerability of the Pow blockchains, with which the attacker captures control over transaction confirmation and block generation.

Which gives possession 51% of the network capacity?

Having at its disposal 51% of the capacity, attacking:

• do not allow the rest of the miners (validators) to find blocks (Selfish mining);
• conduct a double waste of coins to steal from suppliers of services, exchanges or exchangers (Double Spend);
• Fork the main blockchain, dividing the network into two competing chains;
• do not allow transactions or blocks to be confirmed;
• During the attack, all rewards for the block and commissions are collected from transactions.

The attack wears more serious character, if the attackers control much more than 51% of the network. Then they:

• They steal from any contracts such as deposit-mhallenge-verify and state channels/Lightning Network, if the attackers were their participants;
• reduce and manipulate the complexity of the network;
• They steal coins that are not dated by the genesis block (by pumping out old blocks and recruiting awards for these blocks);
• Remove contracts or the history of transactions (due to the rollback of old blocks and editing the list of transactions on).

Attack 51% in itself It does not allow attacking:

• get your private key or fake a signature;
• get coins obtained as a result of incorrect operation of the contract;
• send, freeze in stakeing or burn your coins instead of you (with the exception of techniques mentioned above);
• Manage decisions of holders of full nodes (validators).

How many resources will be required to execute an attack of 51%?

A malicious mining pool can hire additional resources and launch an attack on the selected cryptocurrency. Based on the 51crypto service data, the authors of the study “Studying the types of blockchain attacks” compiled a table of six cryptocurrencies and the price of attack per hour is indicated.

Data received in April 2019. As you can see to attack bitcoin, you need to spend $ 486,000 per hour. Attack on Dash (with a market capitalization of $ 2.3 billion), costs only $ 15,000 per hour.

What is selfish mining (Selfish mining)?

Strategy that allows miners to increase their profits by concealing blocks from a public network. Miners do not send a block to the network every time after generation, but continue to minute new blocks on top of any independent blocks found. While competitors are mining over older blocks, a selfish miner gets an advantage.

There is a secret race between the public chain of “honest miners” and the private chain of “selfish miners”. Attackers should have enough computing power to make a secret chain longer than a public.

As soon as the private blockchain becomes longer than the public, attackers release it to the network to get awards for finding blocks and user commissioners. If the private network capacity is from 25% of the capacity of the entire network, selfish miners will continue to win the chain race until they are displaced by another selfish miner or an infringed minority.

In the Proof-OF-WORK (POW) blockchain, not the longest chain is important, but the most “reinforced”.

The longest chain is the majority of computing power only if there is no monopolist on the network (the owner of 51% or more power). If it is, then the longest chain may not represent the will of most miners.

How is a double cryptocurrency waste (Double Spend)?

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

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

Another, more secretive option: attacking minute parallel chain of blocks, in the manner of selfish mining. Instead of an honest transaction, a double waste transaction was enabled. Such a transaction sends the same coins to another address belonging to a fraudster. It remains to “feed” the valid chain of an alternative portion of blocks (with the right POW), in anticipation that the network will accept them.

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

In theory, if the transaction has one and more confirmation, double expenses are excluded. Many do not know what to do when the transaction “disappears” from the wallet for bitcoins.

Thanks to such “schemes”, the coins are now and then returned to the attacker’s wallet, and you can spend them twice, three times, and so on. Frequent double expenses lead to a threat of removing cryptocurrency from trading on the exchanges affected by double expenses. In addition, attacked cryptocurrencies lose market capitalization after the attack. For example, Verge cryptocurrency was attacked in May 2018, and has since lost more than 95% of its cost.

Harder after an attack of 51% as a method of creating a new asset

With an attack of 51%, you can create a new cryptocurrency. The Pow consensus algorithm was developed to prove the integrity of the chain, not to prevent the appearance of branches.

Suppose attacking secretly mines several blocks, and then “drop” them to the main network. If there is no community support for the attackers, an honest minority of the remaining 49% will reject such a chain. But several secretly found blocks allow the attacking one to separate from the network and continue to minute his own chain, while the rest of the miners will continue the old. So two assets appear, one is known to everyone, and the other is new.

While miners are enough for the blockchain to work, even the new blockchains formed as a result of the hardforn.

Is it possible to attack 51% without having 51% of the capacity?

The famous Bitcoin-Guru Andreas Antonopoulos believes that the Bitcoin network is no longer at risk of an attack of 51% due to the resources that miners spend the network maintenance. Andreas states that in 2019 it makes no sense to attack bitcoin, it will be too expensive even for governments. But you can attack less powerful altcoins, said Andreas.

In order to attack the blockchain, an attacker does not always need to have 51% or more computing power. The probability of success is calculated based on the time of attack and the number of computing power.

Even if the attacker controls 40% of the network, he will be able to attack for two blocks with a 40% probability of success.

But miners are only a small part of the blockchain security model. After 10 years, we saw real examples of attacks and it turned out that the threat was greatly exaggerated.

Examples of attacks 51% of the history of cryptocurrencies

Attack 51% is not not feasible. Ghash mining pool.IO in a short period of time in July 2014 had more than 50% of Bitcoin computing capacity. This led to the fact that the pool voluntarily tried to reduce his share in the network. The statement said that in the future it will not reach 40% of the total production.

In August 2016, a group of hackers from “51 Crew” hacked Blockchain projects Krypton and Shift. Using a series of double expenses, they managed to take approximately 20,000 Krypton tokens.

In May 2018, a group of malicious hackers gained control over 51% of the Altcoin Gold altcoin network, which allowed them to steal $ 18 million in cryptocurrency in Bittrex, Binance, Bitinka, BitHumb and Bitfinex. Bittrex accused the developers of negligence and demanded compensation from them, otherwise the exchange promised to close the auction in BTG. The developers replied that this is a well -known type of threat. The Bittrex exchange did not take care of precautions and is to blame for what happened.

In June 2018, Monacoin, Zencash, Verge, and Litecoin Cash were attacked by 51%. This led to multimillion -dollar losses. As a result of the attack, some exchanges lost approximately $ 90,000 in Monacoin, $ 500,000 in Zencash and $ 1.7 million in Verge.

In November 2018, as a result of an attack of 51% on Aurum Coin, more than $ 500,000 was stolen from the Cryptopia exchange.

In May 2019, two large mining pools attacked 51% on the Bitcoin Cash network. According to them, they prevented the theft of unprotected segwit coins, which were stored on addresses, from where anyone could take them. These coins remained on the Web after the department from Bitcoin in 2017, but were blocked by developers. Until they were accidentally unlocked by the hard fork in May 2019.