Page 3 of Satoshi Nakamoto’s whitepaper, Bitcoin: A Peer-to-Peer Electronic Cash System, states the following:
“If a majority of CPU power is controlled by honest nodes, the honest chain will grow the fastest and outpace any competing chains.”
The “honest[y]” of more than half of miners is a core requirement for the security of Bitcoin and any proof-of-work cryptocurrencies based on Bitcoin. Honest action, in this context, means following the behavior described in the Bitcoin white paper. This is sometimes described as a “security risk” or “attack vector,” but is more accurately described as a known limitation to the proof-of-work model.
Failure to meet this requirement breaks several core guarantees of the Bitcoin protocol, including the irreversibility of transactions. Many other cryptocurrencies, such as Ethereum Classic, have also adopted proof-of-work mining.
The function of mining is to add transactions to the universal, shared transaction history, known as the blockchain. This is done by producing blocks, which are bundles of transactions, and defining the canonical history of transactions as the longest chain of blocks*. If a single miner has more resources than the entirety of the rest of the network, this miner could pick an arbitrary previous block from which to extend ...