Newer cryptocurrencies and blockchain networks are susceptible to 51% attacks. These attacks are extremely difficult to execute due to the computational power required to gain majority control of a blockchain network, but NYU computer science researcher Joseph Bonneau said that might change. Bonneau released a report last year estimating that 51% attacks were likely to increase, as hackers can now simply rent computational power, rather than buying all of the equipment.


For example, Ethereum (CCY: ETH-USD), which has a nearly $116 billion market cap and is the second-largest cryptocurrency behind bitcoin, currently has 200 organizations testing a version of its blockchain technology. Yes, traditional banks are testing out Ethereum's blockchain, but so are companies in the technology and energy industries. Integrated oil and gas giant BP (NYSE:BP) envisions using a version of Ethereum's blockchain to aid it with energy futures trading. If these transactions were to settle faster, BP could presumably improve its margin. 
The potential for added efficiency in share settlement makes a strong use case for blockchains in stock trading. When executed peer-to-peer, trade confirmations become almost instantaneous (as opposed to taking three days for clearance). Potentially, this means intermediaries — such as the clearing house, auditors and custodians — get removed from the process.
In 2014, researchers at the University of Kentucky found "robust evidence that computer programming enthusiasts and illegal activity drive interest in bitcoin, and find limited or no support for political and investment motives".[128] Australian researchers have estimated that 25% of all bitcoin users and 44% of all bitcoin transactions are associated with illegal activity as of April 2017. There were an estimated 24 million bitcoin users primarily using bitcoin for illegal activity. They held $8 billion worth of bitcoin, and made 36 million transactions valued at $72 billion.[224][225] A group of researches analyzed bitcoin transactions in 2016 and came to a conclusion that "some recent concerns regarding the use of bitcoin for illegal transactions at the present time might be overstated".[226]
The blockchain protocol discourages the existence of multiple blockchains through a process called “consensus.” In the presence of multiple, differing copies of the blockchain, the consensus protocol will adopt the longest chain available. More users on a blockchain means that blocks can be added to the end of the chain quicker. By that logic, the blockchain of record will always be the one that the most users trust. The consensus protocol is one of blockchain technology’s greatest strengths, but also allows for one of its greatest weaknesses.
Several news outlets have asserted that the popularity of bitcoins hinges on the ability to use them to purchase illegal goods.[129][221] Nobel-prize winning economist Joseph Stiglitz says that bitcoin's anonymity encourages money laundering and other crimes, "If you open up a hole like bitcoin, then all the nefarious activity will go through that hole, and no government can allow that." He's also said that if "you regulate it so you couldn't engage in money laundering and all these other [crimes], there will be no demand for Bitcoin. By regulating the abuses, you are going to regulate it out of existence. It exists because of the abuses."[222][223]
Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain. Bitcoin was invented by an unknown person or group of people using the name Satoshi Nakamoto[9] and released as open-source software in 2009.[10] Bitcoins are created as a reward for a process known as mining. They can be exchanged for other currencies,[11] products, and services. Research produced by the University of Cambridge estimates that in 2017, there were 2.9 to 5.8 million unique users using a cryptocurrency wallet, most of them using bitcoin.[12]
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