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Author Topic: Can the same bitcoin be spent twice (or more) in one block?  (Read 931 times)
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August 23, 2011, 10:37:29 PM
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Lets say my friend calls me and needs some money fast. I send him some money from my smart phone, he recieves it and says thanks. Then he wants to spend it on whatever he needed it for. Assume that he spends it before he gets any confirmations, is it possible for both of our transactions end up in the same block?
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jackjack
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August 23, 2011, 10:58:29 PM
 #2

Yes

Own address: 19QkqAza7BHFTuoz9N8UQkryP4E9jHo4N3 - Pywallet support: 1AQDfx22pKGgXnUZFL1e4UKos3QqvRzNh5 - Bitcointalk++ script support: 1Pxeccscj1ygseTdSV1qUqQCanp2B2NMM2
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August 23, 2011, 11:05:26 PM
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Awesome!

Thanks :-)
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August 23, 2011, 11:13:21 PM
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However... I don't think the standard software will let you spend it until it has at least 1 confirmation.

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August 23, 2011, 11:22:27 PM
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However... I don't think the standard software will let you spend it until it has at least 1 confirmation.

Are you sure? I think that with enough fee it's ok
Just a thought though

Own address: 19QkqAza7BHFTuoz9N8UQkryP4E9jHo4N3 - Pywallet support: 1AQDfx22pKGgXnUZFL1e4UKos3QqvRzNh5 - Bitcointalk++ script support: 1Pxeccscj1ygseTdSV1qUqQCanp2B2NMM2
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August 23, 2011, 11:33:08 PM
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However... I don't think the standard software will let you spend it until it has at least 1 confirmation.

Are you sure? I think that with enough fee it's ok
Just a thought though

Well, I'm not sure about all versions, but I think when it's on 0/unconfirmed it's still grey, not black and won't update your balance with the new amount until it gets into at least 1 block.

Also, using command line bitcoind or something might get around this.

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August 23, 2011, 11:45:03 PM
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It is technically okay to have unconfirmed inputs, old versions allowed this from the GUI. The newer ones don't. I can't remember the exact justification.

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August 23, 2011, 11:56:58 PM
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However... I don't think the standard software will let you spend it until it has at least 1 confirmation.

Sure, I'm not really that interested in client limitations though, since there will be plenty of clients with different features. I'm interested in the theory of bitcoin and the protocol, and this was just a random question that popped up in my head.
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August 24, 2011, 12:19:29 AM
 #9

what about this?


Scenario: You can't wait around for one or several confirmation blocks. Instead, you receive the transaction, broadcast it to nodes you know and wait for a couple of seconds to see if you notice any double-spend attempts. If not, you accept the payment right away.

In the current Bitcoin scheme, one can't accept transactions until it has been incorporated into a block. Suppose two transactions "spending the same coins" enter the network at different points. On average, half the network will have one transaction and half the other. The only way out of this deadlock is which happens to make it into a block first. So you can see that the race across the network is unimportant but the race to get into a block is the deciding factor.

Hal's attack above would yield a reliable income.

ByteCoin


and what about mybitcoin, who claims it was a 1 confirmation problem

sounds to me like they would be in separate blocks

mooo for rent
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August 24, 2011, 12:42:21 AM
 #10

Clients don't spend 0/unconfirmed coins they get from somewhere else.
They will spend their own 0/unconfirmed change if left no other option.
Original reason for that, "viral bitdust":
A transaction that split 0.01 btc into 100s of tiny outputs and had no fee, so it staid at 0/unconf "forever".
Clients that received one of the outputs sooner or later used them as inputs in their own transactions... and as a tx can only go into a block if all its inputs are in the same or older blocks...
Yeah.

bitcoin: 1Fb77Xq5ePFER8GtKRn2KDbDTVpJKfKmpz
i0coin: jNdvyvd6v6gV3kVJLD7HsB5ZwHyHwAkfdw
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