In modern times, pretty much everyone is aware of Bitcoin, Blockchain, Decentralization and all the fancy words that come with the Distributed Ledger Technology scene.

Whether we invest, trade, or utilize the technology behind the famous cryptocurrencies or not, it has now become an everyday-life standard for mass-media outlets, politicians and regulators all over the globe, and even people we know from work, school, personal relationships.

Bitcoin and the general concept behind most cryptocurrencies is the solid fact, that it relies on transactions' authenticity and integrity being monitored and processed by an almost autonomous network and not a third party individual or group of people.

What happens tho, when you, or I, or anyone has control over a big chunk of the network's computing and/or processing resources? Could that person somehow manipulate the outcome or the result of an ongoing transaction, and how would he do that?

As a matter of fact, not only that could happen, but it already happened several times in the past, hence the term, as well as the malicious implications of the so-called '51% attack' make the round of the internet, alongside top-shelf alternative monetary systems, such as Bitcoin itself.

What exactly is a "51% attack"?

As mentioned before, most cryptocurrency projects were created in order to avoid the third-party authoritarian glove, giving freedom to internet users to perform peer-2-peer transactions regardless of their social status, tax-history, government-relations, etc.

That is possible thanks to the protocols underlying most Distributed Ledger Technology (DTL) projects. Nevertheless, that protocol can be trusted to be working accurately as long as the hash-rate / processing power of the network is distributed in a decentralized fashion, meaning that there is no single entity or group of users in the network that could control a majority of the network's operations in any case.

Sometimes, that happens, and although it's extremely costly in most cases, and a time-consuming performance, as it basically requires you to control a little more than half of the respective network's resources, some users or groups of users could turn a decentralized network in a centralized 'digital servant' of their own needs.

While malicious actors cannot generate transactions for a third wallet, unless they have access to that specific wallet's private keys, they could reverse their own transactions, which results to the prevention of confirmations for new transactions.

In a nutshell, a 51% attacker, would not be able to steal someone else's crypto, however, he could spend his own crypto, then reverse the transaction, ending up with his initial crypto plus the physical and/or digital goods he just paid for. We usually use the term "double-spend attack" instead of '51% attack' in such cases.

The official definition for 51% attacks according to Bitcoin.org developer's technical glossary would be:

The ability of someone controlling a majority of the network hash rate to revise transaction history and prevent new transactions from confirming.

How often does a "51% attack" occur?

There is a similar portal called "crypto51.app" where you can find all the details regarding the necessities or 'minimum requirements' of a possible 51% attack targeting a specific network or coin.

You'll notice that it's almost impossible to 'hack' major projects such as and , while it's more than just possible to influence 'irrelevant' altcoins that are either amateur business-wise or have a weak cybersecurity protocol.

The last famous major attempt of a successful '51% attack' occurred earlier this year on Ethereum Classic's network, where an individual user managed to "hack" his way into possessing $100,000k worth of extra coins.

Cryptocurrency exchange and broker Coinbase, and Gate.io (where the initial attack took place) announced on Jan 7, 2019, that they are freezing all transactions for ETC holders, in order to secure user's funds.

Verdict

Concluding, "51% attacks" are not very often in general, but when they appear, they seem to penetrate the network's defense mechanisms and create temporal chaos during the period of the influencer's operations.

While minor altcoins are easier to influence, a "51% attack" targeting a major coin such as Bitcoin, would definitely require a heavy pocket on the attacker's side, as it would consume over several hundred thousand dollar's per hour to carry out the attempt.

Last but not least, ambitious hackers could rent hash-rate / processing power from large mining companies and use third-party computer systems in order to perform their attack(s), which essentially makes it harder for authorities to track the initial source of the attack.