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Thursday Dec 14th 2017
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Double Spend

Definition of "Double Spend"

Regarding virtual currency: What occurs when someone makes a transaction using a cryptocurrency and then makes a second purchase from someone else, using that same cryptocurrency, and convinces the rest of the network to confirm only one of the transactions by hashing it in a block. Double spending is not easy to do, but is a risk run by those accepting zero-confirmation transactions. Regarding Ethereum: A deliberate fork, where a user with a large amount of mining power sends a transaction to purchase some product, then after receiving the product creates another transaction, sending the same coins to themselves. The attacker then creates a block at the same level as the block containing the original transaction but that contains the second transaction instead, and starts mining on the fork. If the attacker has more than 50 percent of all mining power, the double spend is guaranteed to succeed eventually at any block depth. Below 50 percent there is some probability of success, but it is usually only substantial at a depth up to about 2-5. For this reason, most cryptocurrency exchanges, gambling sites, and financial services wait until six blocks have been produced (“six confirmations”) before accepting a payment.

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