• Blockchains are distributed ledgers that record every transaction made on the cryptocurrency network.
  • A 51% attack is an attack on the blockchain by a group of miners who control more than 50% of the mining hashrate on the network.
  • Attackers with the most control over the network can interrupt the writing of new blocks, preventing other miners from completing blocks.
  • Modifying historical blocks is difficult due to hard coding of past transactions in the Bitcoin software.
  • Although a successful attack on Bitcoin or Ethereum is unlikely, smaller networks are often the targets of 51% attacks.