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You can think of a public key as a gym locker and a private key as the locker combination. Trainers & members can insert letters and notes through the opening in your locker. However, the only person who can retrieve the mailbox’s contents is the one that has the unique key. However, it should be noted that while gym locker combinations are kept in the gym manager’s office, there is no central database that keeps track of a blockchain network’s private keys. If users misplace their private key, they will lose access to their bitcoin wallet.

The Single Public Chain

In the Bitcoin network, the blockchain is not only shared and maintained by a public network of users – but it is also agreed upon. When users join the network, their connected computer receives a copy of the blockchain updated whenever a new block of transactions is added. But what if, through human error or the efforts of a hacker, one user’s copy of the blockchain is manipulated to be different from every other copy of the blockchain?

The blockchain protocol discourages the existence of multiple blockchains through a process called “consensus.” In the presence of multiple, differing blockchain copies, the consensus protocol will adopt the longest chain available. More users on a blockchain mean that blocks can be added to the end of the chain quicker. By that logic, the blockchain of record will always be the one most user’s trust. The consensus protocol is one of blockchain technology’s greatest strengths and allows for one of its most significant weaknesses.

Hacker-Proofing The Block

A hacker can take advantage of the majority rule in what is referred to as a 51% attack. Here’s how it would happen. Let’s say that there are five million computers on the Bitcoin network, a gross understatement for sure but an easy enough number to divide. To achieve a majority on the network, a hacker would need to control at least 2.5 million of those computers. In doing so, an attacker or group of attackers could interfere with the process of recording new transactions. They could send a trade – and then reverse it, making it appear as though they still had the coin they just spent. This vulnerability, known as double-spending, is the digital equivalent of a perfect counterfeit and would enable users to spend their bitcoins twice.

Such an attack is extremely difficult to execute for a blockchain of Bitcoin’s scale, as it would require an attacker to gain control of millions of computers. When Bitcoin was first founded in 2009 and its users numbered in the dozens, it would have been easier for an attacker to control most computational power in the network. This defining characteristic of blockchain has been flagged as one weakness for fledgling cryptocurrencies.

User fear of 51% attacks can actually limit monopolies from forming on the blockchain. In “Digital Gold: Bitcoin and the Inside Story of the Misfits and Millionaires Trying to Reinvent Money,” New York Times journalist Nathaniel Popper writes of how a group of users, called “Bitfury,” pooled thousands of high-powered computers together to gain a competitive edge on the blockchain. Their goal was to mine as many blocks as possible and earn bitcoin, which were valued at approximately $700 each.

Harnessing Bitfury

By March 2014, however, a company called Bitfury was positioned to exceed 50% of the blockchain network’s total computational power. Instead of continuing to increase its hold over the network, the group elected to self-regulate itself and vowed never to go above 40%. Bitfury knew that if they chose to continue increasing their control over the network, bitcoin’s value would fall as users sold off their coins in preparation for the possibility of a 51% attack. In other words, if users lose their faith in the blockchain network, the information on that network risks becoming completely worthless. Blockchain users, then, can only increase their computational power to a point before they begin to lose money.

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