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Date: Mon, 24 Oct 2011 10:29:57 +0200 (CEST)
From: "Jan Vornberger" <jan@uos.de>
To: "Bitcoin Dev" <bitcoin-development@lists.sourceforge.net>
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Subject: [Bitcoin-development] Determine input addresses of a transaction
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Hi there!

As part of my green address endeavor, I'm currently trying to extend the
'gettransaction' call to include an extra field "inputaddresses" which
should return a list of the Bitcoin addresses associated with the inputs
of the transaction.

I understand that this is not generally possible, because of the different
possible structures enabled through the scripting language. But it would
be fine, if this only worked for 'regular' transactions.

So my first shot at this is to go through the inputs of a transaction and
see if the scriptSig field has only two opcodes. If that is the case, I
assume that it is of the structure <sig> <pubKey> and calculate the
Bitcoin address from <pubKey>. The patch for this is here:

https://github.com/javgh/bitcoin/compare/vps_wheezy...showinputaddresses

But then I started to wonder if this is safe. Can this be tricked somehow?
Would it be possible to create a valid transaction which has an input that
has only two opcodes but with an arbitrary pubKey at the second position?
Could someone who has a better grasp on the scripting capabilities comment
on this?

Or alternatively: should I determine the input addresses of a transaction
in a different way? if so, how?

Regards!
Jan