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You can do this right now, with Armory.   If you switch Armory to Expert usermode, you can combine coin-control with unsigned transactions to do exactly this.  It's because Armory doesn't "lock" coins used in previous unsigned transactions, until they're actually broadcast and confirmed to be "out in the wild".  This was done for simplicity to avoid people getting arbitrarily-locked coins, even though it means you end up accidentally double-spending if you try to create two different unsigned transactions from the same wallet without sign&broadcasting the first one.

So here's what you do:
(1) Switch to "Expert" usermode in Armory
(2) Open any wallet (you don't need a watch-only wallet, full wallet is fine)
(3) In the "Send Bitcoins" window, click coin-control
(4) Create a transaction using one sufficiently large input
(5) Click "Create Unsigned Transaction" and save it
(6) Repeat 3-5 with the same coin, but sending to yourself, specify a larger fee
(7) Go into "Offline Transactions" and "Sign and Broadcast Transactions"
(8) Load tx1, sign & broadcast
(9) Load tx2, sign & broadcast

This only works if your Bitcoin-Qt/bitcoind client has the replace-by-fee patch, since Armory uses Bitcoin-Qt/bitcoind as a gateway to the network. Otherwise, the second tx will be DOA.  But you don't have to mess with Armory other than switching it to Expert mode to get to the coin-control feature.

- -Alan

P.S. -- If you try this, Armory is likely to not show the second tx as having ever happened (Bitcoin-Qt will send it back to us and we ignore it because we already have a tx).  But if your Bitcoin node has the modification, it /will/ reach the network


On 05/15/2013 08:19 AM, Peter Todd wrote:
> On Wed, May 15, 2013 at 07:38:27AM -0400, Peter Todd wrote:
>> So I'm offering 2BTC for anyone who comes up with a nice and easy to use
>> command line tool that lets you automagically create one version of the
>> transaction sending the coins to the desired recipient, and another
>> version sending all the coins back to you, both with the same
>> transaction inputs. In addition to creating the two versions, you need
>> to find a way to broadcast them both simultaneously to different nodes
>> on the network. One clever approach might be to use blockchain.info's
>> raw transaction POST API, and your local Bitcoin node.
>
> Oh, and while we're at it, a good starting point for your work would be
> Gavin's spendfrom utility in the contrib/spendfrom directory in the
> Bitcoin-QT respository.
>
> Also please do keep in mind that it's much better for the community if
> an attack is demonstrated first, followed by releasing the code some
> time later.
>
>
>
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>
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