The news about 51% attack on bitcoin with US$760,000 was widely spread on the network, and some well-known currencies such as ETH, BCH and BSV had all become the targets. It seems that the cost of launching a 51% attack is far less than our imagination. Many “smart guys” quickly came up with the idea of launching a 51% attack on bitcoin to get profits in the futures market. Unfortunately, they are just daydreaming.

Why 51% attack is impossible for some major currencies?

First, we’d better introduce the source of news about 51% attack with US$760,000: crypto51.

Crypto51 is a long-standing website whose data provided are also quite interesting, all about the 51% attack against all the cryptocurrencies. Here we can read that the cost of launching a one-hour 51% attack against BTC is about US$860,721.

However, can such amount of money really buy one-hour of 51% hashrate on BTC? Definitely not.

The source of hash price comes from the data of Nicehash, a well-known hashrate market. The page of hashrate rent and sale at Nicehash is as follows:

It’s very clear that there’s a market for hash rent, but the total hashrate available for rent (350PH) is less than 1% of the total hashrate on network (67,000 PH). Moreover, during a large amount of renting, the hashrate price will be naturally inflated so that the cost of launching attack will far exceed the quoted price now. Secondly, to successfully launch a double-spending attack, it is not enough to control only 51% hash within just one hour. The whole process may take several days with huge spending.

So, according to some conspiracy theory, how about joining hands with some large mining pools? Is it feasible?

If they can really convince the top pools in the market to launch attacks, it is feasible. But I believe nobody is willing to destroy the entire industry for such a tiny profit.

Some people may ask, since a few 51% attacks really occurred last year, will this hidden danger still exist now?

During the period from May to June last year, the 51% attacks against small currencies had occurred. However, it was in special time that the 51% attacks were easier due to low cost.

Some currencies such as BTG, XVG and ZEN were under attack then. They all have the same features:

• Adopt the same Equihash algorithm as Zcash, but smaller overall market values and hashrate;
• Transactions on the Binance platform with certain scale;
• The double-spending by attack is profitable.

Therefore, the hackers can make use of small number of new miners to mine ZEC to launch attacks at the small currencies with low costs and high profits. As for relevant details, please refer to the article: 51% Attack, the Doom of POW Cryptocurrency and ASIC Miners?

After repeated 51% attacks, the developers of all POW currencies have gradually focused on the solutions against the 51% attacks. For example, ZEN completed the upgrade in the next few months, which basically realized defense against 51% attacks.

Since then, the same story only appeared on the ETC, which suffered from 51% attack in January this year. Fortunately, although the hackers successfully launched attacks to realize double-spending, the resulting losses were not so serious, just about million dollars. However, once it was attacked, the subsequent plunge would be inevitable.

It’s not difficult to see that the currencies which had been or would be attacked will have following features:

• Adopt the same hash algorithm with the high-value currency in the market. The attackers can make use of the powerful miners for big currencies to launch attacks on small currencies;
• The currency has not yet been able to resist 51% attacks at the bottom layer while its market value is not low with certain scale of transaction, so the attackers can make profits from the attacks.

As the market has been improved this year, some small currencies are also advocated and mushroomed. But all the investors are reminded to more understand the algorithm and technical level of the currencies so as not to become the cash machine of hackers.