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==== Race attack ====
If Eve offers to pay Alice a bitcoin in exchange for goods and signs a corresponding transaction, it is still possible that she also creates a different transaction at the same time sending the same bitcoin to Bob. By the rules, the network accepts only one of the transactions. This is called a [[Race condition|race attack]], since there is a race between the recipients to accept the transaction first. Alice can reduce the risk of race attack stipulating that she will not deliver the goods until Eve's payment to Alice appears in the blockchain.<ref name="cornell4220">{{cite web |url=
A variant race attack (which has been called a Finney attack by reference to Hal Finney) requires the participation of a miner. Instead of sending both payment requests (to pay Bob and Alice with the same coins) to the network, Eve issues only Alice's payment request to the network, while the accomplice tries to mine a block that includes the payment to Bob instead of Alice. There is a positive probability that the rogue miner will succeed before the network, in which case the payment to Alice will be rejected. As with the plain race attack, Alice can reduce the risk of a Finney attack by waiting for the payment to be included in the blockchain.<ref>{{cite journal |title=Two Bitcoins at the Price of One? Double-Spending Attacks on Fast Payments in Bitcoin |url=http://eprint.iacr.org/2012/248.pdf |last1=Karame |first1=Ghassan O. |last2=Androulaki |first2=Elli |last3=Capkun |first3=Srdjan |publisher=International Association for Cryptologic Research |website=iacr.org |year=2012 |access-date=22 October 2014}}</ref>
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