Bitcoin Forum

Hi all, just a basic question really.... is it possible to prove that A sent X BTC to B?
Thanks :)

Blockexplorer.com

http://blockexplorer.com/

You can prove you sent bitcoins to an address by spending the change. You cannot prove who received the bitcoins without their cooperation.

Hi, Can you explain more please? Thanks! :)

Not really. Unless:
-all your money is in one address in one wallet
-you announce you will send another amount to another address of yours and then do (has to be a pretty unique amount)
-but even then, someone else could see the announcement and send it again
-all that would prove is the from address is the same and was sent again after you announced

Which part do you want to have explained?

The "change" part... and how I find it and spend it. :)

You could come pretty close, but the software support is rather lacking.  With a patched client, you can sign a message with the key pair used in the transaction.  The message could say "Jack owns this coin".  That would be pretty good evidence, though there is always the chance the real owners gave you the key pair after they spent the coin, or you paid them to sign a false message.

If you haven't yet sent it, you could in theory create a transaction and send its hash to the recipient before you execute it.  Again, the current software doesn't help much here, but with the command line you can at least find the transaction hash after the fact.  The recipient could check the hash in a database like Block Explorer.

Send somebody some money. View your transaction in Block Explorer. You will see that your money went to your recipient, AND some more money went to another address. That is the change, and it returns to your wallet at a newly generated address. You merely need to spend that particular coin.

True... if you have not already spent it and its descendants... and if the transaction did not happen to come out even (without change).

I think in the future transaction creation will be separate from execution, and sending the transaction hash first will be the straightforward solution in cases where you know beforehand that you will need proof.

Related:
  - http://forum.bitcoin.org/index.php?topic=4411.0

Thanks! Just out of interest, how long does it take for a transaction to appear in BlockExplorer? :)

it appears when it confirmed, and included in a block.

How do I spend that particular coin?

how do you spend your change?  the ability to spend the change never left your wallet, so you can just spend using the bitcoin client as normal.
oh, i hadn't read back to see what you are trying to do.

the bitcoin client chooses for you which coins to use in spending.  there is a patch that you can use to help though: http://forum.bitcoin.org/index.php?topic=23354.0

Thanks for that, although I sent the payment using a web-based client (mybitcoin.com) and would this therefore not work in my case?

That's correct. You would have to have access to mybitcoin's logs as well.

Someone being able to spend the change is not sure proof of sending the original tx. What you read as change could be another payment made simultaneously using SendMany. You need to see the person use the IN address again.

None of that is needed just to show that the tx happened though, just look in block explorer.

Another really n00b question, apologies... can someone explain to me how to read BlockExplorer.com... like which addresses mean what? For example, on http://www.bitlotto.com it says you can send a payment (the winnings) back to the person who sent the winning ticket bitcoin... so I assume another address is created which is associated with the sender of bitcoins? so confusing :(

I tell the person they HAVE TO send using Bitcoin software on their computer. Since Bitcoin sends payment from only addresses they own, I can send some back to the identical address because they own it. I don't need to know who it is! Only the owner of the address! With ewallets such as mybitcoin those addresses/keys are all shared.