How do you execute a double spend?

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How can you do a double spend? When I click on send, a certain amount is deducted from my balance. I cannot send the same amount again. So how to race attackers do double spends?

Posted on August 11, 2014 at 4:04 AM

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IvanB

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Location: New York

You would need some scripting knowledge as any of the default bitcoin core/qt or other wallets won't allow you to easily execute a double spend.

Any particular reason you want to try this? Just to see how resilient the protocol is? Short of having a ton of mining resources at your disposal, it's going to a bit hard to pull off even a 0 confirmation double spend since most miners will reject the transaction by comparing what's in mempool. For a 1-3 confirmation doubles spend, the difficulty becomes exponentially higher.

In short, there's no GUI 'test a double spend' client that I'm aware of.

Posted on August 11, 2014 at 6:02 AM
Last Edited: August 11, 2014 at 6:02 AM
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Rob

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I think Higas was asking how one would TRY to execute a double spend. And my more layman's answer (as I'm less technical than Ivan) is that you'd want to somehow transmit one spend to one part of the network, and one spend to another, and hope you can fool the two people you are paying long enough to get what you paid for, before the network sees both transaction attempts, and invalidates one.

Your wallet is set up to follow rules, so that's why it doesn't allow a double spend. Feel free to correct me if I'm not quite dead on (and I'll learn something too).

Posted on August 11, 2014 at 6:28 AM
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highas_akite

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I know how to basics work, I am just interested in how it would actually work since no ' normal' wallet lets you double spend your coins.

Posted on August 11, 2014 at 8:08 AM
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meadefreling

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Double spending isn't really permitted by major wallets so try to avoid it.

Posted on August 11, 2014 at 8:58 AM
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phobosbtc

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Location: Vancouver B.C.

The basics of it are as follows, although you need technical knowledge to actually execute this. My understanding is that double spending is extremely trivial if you know what your doing.

1 send bitcoin to someone, do not include a fee

2 receive what you were going to get for your bitcoins

3 before the transaction confirms send the same bitcoin to another address you control and include a fee

4 the miner should pick up the transaction with the fee and the one without a fee with disappear

you now have whatever you bought and you still have your bitcoins.

Posted on August 11, 2014 at 12:09 PM
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Rob

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@highasakite I know how to basics work, I am just interested in how it would actually work since no ' normal' wallet lets you double spend your coins.

I think a wallet protecting the user by not allowing double-spends is just the very tiny tip of the iceberg in the difficulty of a double spend. There are lots of wiki posts and BitcoinTalk.org discussions of the complexities, and I think the very basic assumption is that the entity trying the double-spend has their own code they use to attempt the attack, rather than a wallet that's coded to follow the rules.

Posted on August 11, 2014 at 3:10 PM
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dpc

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Location: Central California

@rob

I think a wallet protecting the user by not allowing double-spends is just the very tiny tip of the iceberg in the difficulty of a double spend. There are lots of wiki posts and BitcoinTalk.org discussions of the complexities, and I think the very basic assumption is that the entity trying the double-spend has their own code they use to attempt the attack, rather than a wallet that's coded to follow the rules.

Exactly.
Plus for a double spend to be successful the vendor would need to accept payment and release whatever product before the transaction was fully propagated through the network.

The bitcoin is only "spent" once. Meaning it is only going from one address to another. The second "double" transaction would never actually be delivered to the second address. In a double spend you are trying to make the second vendor believe that they are getting bitcoin long enough so that they will release product before they find out the transaction will fail.

If you really want to do a double spend try a 51% attack. You will need a few extra computers to mine bitcoin :)

Posted on August 11, 2014 at 3:51 PM
Last Edited: August 11, 2014 at 3:52 PM
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highas_akite

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@dpc

If you really want to do a double spend try a 51% attack. You will need a few extra computers to mine bitcoin :)

Nice one. Let me save up a few million $ first. :)

Posted on August 12, 2014 at 11:05 AM
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dpc

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Location: Central California

@highasakite

@dpc

If you really want to do a double spend try a 51% attack. You will need a few extra computers to mine bitcoin :)

Nice one. Let me save up a few million $ first. :)

I have some old pentium II's you can borrow :)

Posted on August 12, 2014 at 11:06 AM
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stevegreen88

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You can easily double spend without any scripting knowledge on Glass Hunt: http://www.glasshunt.co/exploits/bitcoin-double-spend. Video demo: https://www.youtube.com/watch?v=RtGzV_-agcI&feature=youtu.be

Posted on September 8, 2016 at 10:15 PM
Last Edited: September 8, 2016 at 10:16 PM
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