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A double spend is generally what people mean by "rewriting history". A 51% attack only lets people steal money by taking back recent transactions and sending t
by AgentME 6y ago
A double spend is generally what people mean by "rewriting history".
A 51% attack only lets people steal money by taking back recent transactions and sending them elsewhere. Transactions that have been received a long time ago are generally safe. (An attacker can reach further back in time if they run the 51% attack longer or with even more hashpower.)
- brian_cloutier 6y agoThis is not quite right. 51% lets you rewrite arbitrarily-deep history, limited only by how much time you're willing to spend on the attack. You have more hash strength than the rest of the network combined! You pick some ancient block and start building blocks off of it, and the very definition of a 51% attack is that your new fork will eventually outgrow the "real" fork. Double spends are possible well before 51%. Even a 10% attacker has the ability to (sometimes) double-spend. A 10% attacker can try to double-spend transactions and has a certain chance of being able to. The chance of success decreases exponentially, the deeper in the chain the target transaction is. However, for a high-enough value transaction, attempting to double spend it can have a higher EV than the mining revenue you lose out on by mining honestly.