Topic: Double spending while spending bitcoin on stores (Read 1777 times)

OP's scenario seems to describe a race attack:

Quote from: Mitchell on March 13, 2015, 10:15:19 AM

Would double spending cause the network to undo both spend attempts? That is how I thought I works, would someone correct me on this if I am wrong.

The network will see both transaction attempts and invalidate one of them. The other transaction attempt should proceed normally.

Is it possible to know which one it will invalidate or is random?

Miners can choose which transaction to include in the next block. Most pools will choose the one that came first although some (e.g. BitUndo) will choose the one with the higher fee. So in most situations, the second transaction would be invalidated and never included in the blockchain.

If you want to read a good explanation of how it works, see the first answer below:

http://bitcoin.stackexchange.com/questions/26926/could-any-standard-fee-transaction-be-double-spent-by-using-a-higher-fee

The bitundo actually sounds like a good idea but wouldn't they have to find the block for it to work?

Yes, they would need to find a block. Fortunately, they are a tiny pool with a tiny hashrate so their success rate is very low.

From their FAQ:

Quote

Q: How likely is it to undo my transaction

A: The likelihood is approximately the same as the percentage of mines using bitundo. Right now, this is pretty low.

I guess if you make a large mistake then it is probably better than nothing!

picolo

hero member

Activity: 1022

Merit: 500

Double spends are possible, but really hard to do because you would have to send both of them quickly and to two different parts of the network and hope that the seller picks up the one you are going to drop and the miners the one you want to be confirmed. Once a miner has seen a transaction it will reject any address that contains the same inputs.

I'm sure DannyHamilton or DeathAndTaxes can explain this a lot better. See this post by DeathAndTaxes for more information about sellers that accept 0 confirmation transactions.

Hard to do, not successfull open and it would be considered a fraud when done successfully so the shop would investigate and find a picture of you from the surveillance camera and get you next time you shop with them.

Lorenzo

sr. member

Activity: 406

Merit: 250

Quote from: cramved on March 13, 2015, 11:35:23 AM

Quote from: Lorenzo on March 13, 2015, 11:25:14 AM

OP's scenario seems to describe a race attack:

Quote from: Lorenzo on March 13, 2015, 11:25:14 AM

Would double spending cause the network to undo both spend attempts? That is how I thought I works, would someone correct me on this if I am wrong.

The network will see both transaction attempts and invalidate one of them. The other transaction attempt should proceed normally.

Is it possible to know which one it will invalidate or is random?

Miners can choose which transaction to include in the next block. Most pools will choose the one that came first although some (e.g. BitUndo) will choose the one with the higher fee. So in most situations, the second transaction would be invalidated and never included in the blockchain.

If you want to read a good explanation of how it works, see the first answer below:

http://bitcoin.stackexchange.com/questions/26926/could-any-standard-fee-transaction-be-double-spent-by-using-a-higher-fee

The bitundo actually sounds like a good idea but wouldn't they have to find the block for it to work?

Yes, they would need to find a block. Fortunately, they are a tiny pool with a tiny hashrate so their success rate is very low.

From their FAQ:

Quote

Q: How likely is it to undo my transaction

A: The likelihood is approximately the same as the percentage of mines using bitundo. Right now, this is pretty low.

cramved

newbie

Activity: 56

Merit: 0

OP's scenario seems to describe a race attack:

Would double spending cause the network to undo both spend attempts? That is how I thought I works, would someone correct me on this if I am wrong.

The network will see both transaction attempts and invalidate one of them. The other transaction attempt should proceed normally.

Is it possible to know which one it will invalidate or is random?

Miners can choose which transaction to include in the next block. Most pools will choose the one that came first although some (e.g. BitUndo) will choose the one with the higher fee. So in most situations, the second transaction would be invalidated and never included in the blockchain.

If you want to read a good explanation of how it works, see the first answer below:

http://bitcoin.stackexchange.com/questions/26926/could-any-standard-fee-transaction-be-double-spent-by-using-a-higher-fee

The bitundo actually sounds like a good idea but wouldn't they have to find the block for it to work?

Lorenzo

sr. member

Activity: 406

Merit: 250

OP's scenario seems to describe a race attack:

Quote from: CIYAM on March 13, 2015, 11:09:22 AM

Would double spending cause the network to undo both spend attempts? That is how I thought I works, would someone correct me on this if I am wrong.

The network will see both transaction attempts and invalidate one of them. The other transaction attempt should proceed normally.

Is it possible to know which one it will invalidate or is random?

Miners can choose which transaction to include in the next block. Most pools will choose the one that came first although some (e.g. BitUndo) will choose the one with the higher fee. So in most situations, the second transaction would be invalidated and never included in the blockchain.

If you want to read a good explanation of how it works, see the first answer below:

http://bitcoin.stackexchange.com/questions/26926/could-any-standard-fee-transaction-be-double-spent-by-using-a-higher-fee

EDIT: Actually it seems that the above is only true if the transactions were sent very far apart since it assumes that the first transaction has been fully propagated by the network. If they were sent almost simultaneously (which is what would happen in most double spend attempts), whether or not the first or second transaction goes through would be essentially random. This is because while the reference client is designed to accept the first transaction received, there would be no way for all nodes in the network to agree on which transaction was first sent. Sending two double spend transactions to different nodes a few seconds apart would result in some nodes seeing A first and others B. If one of these happen to be the one that mines the next block, then the first transaction that it receives will be added to the next block (exceptions like BitUndo aside).

cramved

newbie

Activity: 56

Merit: 0

Is it possible to know which one it will invalidate or is random?

It isn't really possible to know (although fees could make one tx more likely than another to confirm).

But assuming that the fees are the same then it would effectively be random (as it will depend on which tx got included in the best PoW chain).

So that makes it hard to use it as a way to change your mind about a transaction, which is a good thing.

CIYAM

legendary

Activity: 1890

Merit: 1086

Ian Knowles - CIYAM Lead Developer

Is it possible to know which one it will invalidate or is random?

It isn't really possible to know (although fees could make one tx more likely than another to confirm).

But assuming that the fees are the same then it would effectively be random (as it will depend on which tx got included in the best PoW chain).

My guess is that services like BitPay already detect such attempts and would warn their clients ASAP.

cramved

newbie

Activity: 56

Merit: 0

OP's scenario seems to describe a race attack:

Would double spending cause the network to undo both spend attempts? That is how I thought I works, would someone correct me on this if I am wrong.

The network will see both transaction attempts and invalidate one of them. The other transaction attempt should proceed normally.

Is it possible to know which one it will invalidate or is random?

Lorenzo

sr. member

Activity: 406

Merit: 250

OP's scenario seems to describe a race attack:

Would double spending cause the network to undo both spend attempts? That is how I thought I works, would someone correct me on this if I am wrong.

The network will see both transaction attempts and invalidate one of them. The other transaction attempt should proceed normally.

Josef27

hero member

Activity: 784

Merit: 1000