Cryptocurrency Glossary / dictionary

A 51% attack refers to the ability of one or more miners combine their computing power so that they have 51% or more of a cryptocurrency's hashing power. With this power, they would have the ability to create a new branch of the blockchain. In this new branch then can insert false transactions, such as reversing old transactions so that they can spend the same coins more than once. This is called "double spending". They can also prevent other legitimate transactions from being approved. This may also be called a "Majority Attack". These attackers would make money by double spending, and also by "earning" the rewards for adding new blocks to the blockchain. This attack would not allow them to alter " really old" transactions - they can only modify more recent transactions. These old transactions are hardwired into the blockchain and are unchangeable. They also would not be able to change the maximum limit of total coins. Cryptocurrencies which don't have many miners are more easily attacked than more common cryptocurrencies. For example, bitcoin would be very hard to attack since there are so many independent miners working on the blockchain. But it would still not be impossible to launch a successful 51% attack on bitcoin. Several different cryptocurrencies have been attacked in this way over the years. The attacks usually only last a short period of time before the attack is discovered and/or thwarted. The attackers may also choose to stop the attack once they have achieved what they wanted. Sometimes, after discovery, a cryptocurrency's developers will create an emergency fork in the software to stop the attack.

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