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Some bitcoin ATMs allow the exchange from bitcoins to paper money. Since waiting in front of an ATM for a confirmation is not feasible, they must defend themselves in another way against double-spends. How do they do that?

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I can't talk about other ATMs but in BTCPoint (The Bticoin ATM of Barcelona) works the following way:

  1. The user want to SELL bitcoin
  2. The bitcoin machine provides a fresh new public address for the transfer
  3. The user sends the bitcoins to that address
  4. Bitcoin network detects the transfer sent and issues a ticket with a confirmation code
  5. The user waits for some minutes while the transaction is confirmed (the ATM can be used by other people)
  6. After some confirmations the user scans the "ticket with the confirmation code" in the QR reader.
  7. The ATM validates the confirmation and gives money out if everything is valid.

Check out the operations in: http://www.btcpoint.org

For some extra security (camera recording the confirmation code in step 4.) the machine can be configured so that a PIN code is inserted. This PIN code is then required in step 6.

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