BTC re-order outputs of a signed transaction (detect change output reliably)

[dimitris-c]

Is there currently any supported way to reliably identify the change output index in WalletCore’s Bitcoin v2 signing output (as it stands today)?

We have a case where, for certain transactions, we want to re-order outputs so that some “extra outputs” are always at the very end of the transaction.

Today we set changeOutput on BitcoinSigningInput.signingV2.builder. In many cases the builder appends change at the end, but change is not guaranteed to exist (e.g. below dust / omitted), and the v2 signing output does not clearly label which output is change. As a result, clients can’t safely do outputs.last = change.
Is there an existing API/field/guarantee we can use to determine which output is change?
If not, would you consider adding explicit metadata such as change_output_index (or change_output_indices) in the v2 signing output so clients can reorder outputs safely without heuristics?

Thanks!

[satoshiotomakan]

Hi @dimitris-c, you can set the change manually in Proto.TransactionBuilder.outputs. So the flow may look like:

1. Use AnySigner.plan with Proto.TransactionBuilder.outputs and Proto.TransactionBuilder.changeOutput to get exact change amount
2. Remove Proto.TransactionBuilder.changeOutput from the signing input, and push it back to Proto.TransactionBuilder.outputs explicitly

[dimitris-c]

@satoshiotomakan Thanks for this, I missed that
I'm doing the transaction, in Swift using the BitcoinSigningInput with AnySigner.sign the input which outputs only have the scriptPubkey / value.

[dimitris-c]

Just out of curiosity do you have idea how a tx could result in having such a high fee?
Tx examples https://www.blockchain.com/explorer/transactions/btc/873a0a79c979653d0d27ccdec527308bf6d95c74fa75d09174a702784dae0457
https://www.blockchain.com/explorer/transactions/btc/3dac3c6c834c7bacc203563801789f2dfb8061719d3283ed25f202c76e7d050d

Fee is way too high, could be a result of misidentifying the change output but wouldn't WalletCore returned notEnoughUtxos (which we do use on our app)