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Author Topic: Can you attach a fee payment to a transaction you haven't signed?  (Read 539 times)
ByteCoin
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March 31, 2012, 11:44:05 PM
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1) Casino accepts BTC for customer deposit to customer's unique address.
2) Customer isn't warned about not depositing from online ewallet / exchange provider
3) Customer sends money from big bitcoin exchange service to Casino
4) Casino only sends withdrawals to depositing address.
5) All withdrawals from Casino initially received from the exchange provider's green address are       unrecoverable by the Customer

There are perfectly good reasons for returning unwanted payments. With Bitcoin's lack of fungibility, the only convincing way to untaint your coins is to publish a transaction which the putative owner could use to return the coins whence they came.
In the event that all transactions must have fees to be included in the block chain, there should be a way for the casino to publish some signed message to reverse the unwanted payment without paying a fee.
The signed message should be publicly available from the casino as soon as possible to show that the casino rejects the payment.

The putative owner would then use this signed message to create a transaction to retrieve the coins and would have to pay the fee to get the transaction in the block. They would also be responsible for contacting a bitcoin node to tender the transaction for inclusion in the block chain.

Is it possible for the casino to sign some message to which someone could attach a fee payment which couldn't be stripped off and redirected to another transaction? Is it possible to attach a fee payment to someone else's transaction using the existing Bitcoin System.

There are two interesting cases:
1) Someone has published a transaction without a fee or with an insufficient fee. They're uncontactable or have lost the private keys so they can't issue a new transaction with a higher fee. Is it possible to take their transaction and "wrap" it with a fee payment so that it gets in a block?

2) The case in the quoted article, the casino wants to publish some sort of signed pseudotransaction which can be seen to facilitate returning some bitcoins. Someone else then takes this pseudotransaction and creates a valid fee paying transaction returning the bitcoins where a relay node can't take off the fee payment and use it to pay the fees on some other transaction.

The first case is likely to be more difficult as the cooperation of the sender in constructing some special signed message is not possible.

Can this be done?
 
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DeathAndTaxes
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March 31, 2012, 11:49:05 PM
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No.

You could have used the unconfirmed output as an input for a new tx involving a fee.

Input:
A 10 BTC
Output:
B 10 BTC (no fee)

Input:
B 10 BTC
C 0.01 BTC
Output:
D 10 BTC (w/ 0.01 fee)

It would however require miners to have "smart" enough mining daemon to realize that tx (B+C)->D which has a fee depends on A->B which doesn't so the miner includes both in the next block.
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April 01, 2012, 12:18:33 AM
 #3

Inputs are not included in the transaction base; so once the transaction base is made; it is possible for 3rd parties to add fees. (However they cannot collect any change)

It is possible to make a transaction with any outputs, then pass it around your friends, than they each add an input.

Satoshi himself outlined one way for how you could do this:
https://bitcointalk.org/index.php?topic=2181.msg28729#msg28729

One off NP-Hard.
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