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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.<br>
<br>
So here's what you do:<br>
(1) Switch to "Expert" usermode in Armory<br>
(2) Open any wallet (you don't need a watch-only wallet, full wallet
is fine)<br>
(3) In the "Send Bitcoins" window, click coin-control<br>
(4) Create a transaction using one sufficiently large input<br>
(5) Click "Create Unsigned Transaction" and save it<br>
(6) Repeat 3-5 with the same coin, but sending to yourself, specify
a larger fee<br>
(7) Go into "Offline Transactions" and "Sign and Broadcast
Transactions"<br>
(8) Load tx1, sign & broadcast<br>
(9) Load tx2, sign & broadcast<br>
<br>
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.<br>
<br>
- -Alan<br>
<br>
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<br>
<br>
<br>
On 05/15/2013 08:19 AM, Peter Todd wrote:<br>
<span style="white-space: pre;">> On Wed, May 15, 2013 at
07:38:27AM -0400, Peter Todd wrote:<br>
>> So I'm offering 2BTC for anyone who comes up with a nice
and easy to use<br>
>> command line tool that lets you automagically create one
version of the<br>
>> transaction sending the coins to the desired recipient,
and another<br>
>> version sending all the coins back to you, both with the
same<br>
>> transaction inputs. In addition to creating the two
versions, you need<br>
>> to find a way to broadcast them both simultaneously to
different nodes<br>
>> on the network. One clever approach might be to use
blockchain.info's<br>
>> raw transaction POST API, and your local Bitcoin node.<br>
><br>
> Oh, and while we're at it, a good starting point for your
work would be<br>
> Gavin's spendfrom utility in the contrib/spendfrom directory
in the<br>
> Bitcoin-QT respository.<br>
><br>
> Also please do keep in mind that it's much better for the
community if<br>
> an attack is demonstrated first, followed by releasing the
code some<br>
> time later.<br>
><br>
><br>
><br>
>
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