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I have a question concerning the transaction of bitcoins. If, for example, I send some BTC to a specific address, does the receiver have any chance in verifying that I have already sent the BTC, before they are being registered in his wallet? I know there is http://blockexplorer.com/, where I can look the transactions up. But is there any functionality for it in the Bitcoin-Client, which I could use in RPC-Calls?

Use case for this: Assume there is an application that uses Bitcoin via RPC-Calls to the Client. I want to buy something via the application and the seller should be able to see that I paid the correct amount of BTC without having to wait for the transaction to be finally redeemed in his client.

Thanks in advance!

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The client will often know about received transactions seconds after they have been sent. Transactions that have not gotten into a block yet are said to have 0 confirmations. You can see them by adjusting the parameters of listreceivedbyaddress, etc. so that they show 0-confirmation transactions.

  • Seems to do what I want. Of course they are not confirmed and therefore unreliable in a way, but at least I can see the transaction before confirmation. – Fenriswolf Jan 1 '12 at 15:34
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There's a Bitcoin Charts page that shows pending transactions. There are also patches available for the official client (they may be out of date though) to show pending transactions.

To do it right, you need to run more than one Bitcoin client and connect them as close to the major mining pools as possible. You need to check pending transactions at all of your nodes and if you see a conflicting transaction at any of those nodes, wait for confirmations.

Be warned, this method, even if done right, is vulnerable to the Finney attack. To launch a Finney attack, you must risk losing a block reward, so if a transaction is worth more than 50 Bitcoins (the current block reward), I'd recommend waiting for at least one confirmation.

The Finney attack is a form of double-spending attack where the attacker pre-mines a block containing a conflicting transaction and then publishes a transaction. A merchant relies on the published transaction to do something irreversible. The attacker then succeeds if he can publish the pre-mined block after the merchant has committed and before the network can find a different block. (The merchant must act irrevocably on a published, but unconfirmed, transaction.) -- Question about the Finney attack.

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You can see transactions, whether confirmed or not yet confirmed, using the search field on Blockchain.info.

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