Proof-of-Work (PoW) is supposed to be very hard to attack, requiring a lot of hardware and electricity to power a double-spending attack. That might be the case for some cryptocurrencies, but it's not the case for all of them.

A new website called [Crypto51](https://www.crypto51.app/) has done some calculations to show the theoretical cost for pulling of a 51% double-spending attack. The site creator goes by /u/xur17 on Reddit, and showed that the attack is more feasible if you take into account renting the hardware from NiceHash (a Slovenian cryptocurrency cloud mining marketplace) for example, rather than buying it.

## PoW 51% Attack Cost

<center><img src="https://i.imgur.com/Oovm9Nr.jpg" /><br/><em><sub><a href="https://www.crypto51.app/">Source</a></sub></em></center>

> "There was an attack against Bitcoin Gold a few days ago, which made me curious what the cost of an attack would be with rented hashing power. I did the math and was honestly kind of shocked – someone could attack a cryptocurrencies worth close to a billion dollar for < $10k an hour and even less than that if you include the block rewards.<br/>
This got me to calculate the same figures for other coins, and again it was cheaper than I thought it should be. The goal of the site is to drive more attention to what I see as a pretty glaring problem with these smaller coins. Hashing power is easily retargetable, so not only can people rent hashing power, the larger mining pools can redirect hashing power at smaller coins for a few hours to attack them."

To attack the Bitcoin network, it would require $1 billion worth of hardware to pull off, and $500,000 for electricity per hour. The juice NiceHash rents out only covers 2% of the required computing power to attack bitcoin. Bitcoin is pretty safe at this point, but other coins aren't so safe according to these calculations.

Bitcoin Gold was recently attacked and double-spending was made to the sum of $18,000,000. NiceHash alone can pull this off and it would cost less than $4,000 per hour. Bytecoin has nearly a $1 billion market cap, and they could be attacked with NiceHash for the little cost of $600/hour. It's possible double-spending has been happening elsewhere and people don't know about it, as NiceHash cuts the cost and makes it easy for those in the know to pull it off with smaller Proof-of-Work networks.

# Double Spending 51% Attacks

<div class="pull-right"><center><img src="https://i.imgur.com/t0QN13p.jpg" /><br/><em><sub><a href="https://pixabay.com/en/security-cryptocurrency-bitcoin-2972105/">Source</a></sub></em></center></div>

If you don't know what a 51% attack or double spending is, it's being able to control the hash rate of a currency and spend the same funds twice. Double-spending has been an issue and preventing digital currencies from becoming a reality, until Bitcoin came around with the PoW system and cryptographic calculations to "mine" coins. If you are fast enough to guess numbers and solve algorithms to verify transaction, then you get to be the one to add a new block to the immutable ledger. The hash rate is the number of guesses per second.

Bitcoin didn't used to be such a large network,but now it is. The amount of nodes in the network make it expensive to attack because you need a lot of processing power to beat everyone else trying to win and be the next block producer. But smaller networks have fewer nodes and require less power to win. If you can control 51% of the hash rate in the network, then you can send funds to an exchange, trade it for another currency, then erase your transaction. This means you spent the coins when you traded them, but then erased that record so the coins are still received in your wallet, and you successfully double spent them.

# PoS Attacks Cost More

Proof-of-Stake (PoS) systems like Steem are also vulnerable. In PoS, you can pull off the attack if you buy half the coin supply. This would cost you a lot though, usually more than it would cost for hardware in a PoW system, but it's still possible. Also, as one would start to buy coins, this would push up the price and cost more and more to acquire more coins to reach 51% of the supply.

In light of these calculations done with the NiceHash service to cut costs for PoW attacks, there are security implications for many of the cryptocurrencies out there that are vulnerable to be attacked successfully. If someone combined NiceHash with other services or their own top-tier hardware rigs, they could attack larger cryptos like Ethereum Classic that already has 89% of an attack satisfied by NiceHash. Dash has 27% covered by NiceHash. It would be harder to pull off, but not impossible.

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References:
- [How Much Does it Cost to Attack Bitcoin, Ethereum and Other Coins](https://cryptonews.com/news/how-much-does-it-cost-to-attack-bitcoin-ethereum-and-other-c-1885.htm)
- [Website Outlines The Cost of 51% Attack on Altcoins: It’s Lower Than You Think](https://www.ccn.com/website-outlines-the-cost-of-51-attack-on-altcoins-its-lower-than-you-think/)

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