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BIP-78: fix reference sender implementation pseudocode #2096

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BIP-78: fix reference sender implementation pseudocode #2096

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29 changes: 17 additions & 12 deletions bip-0078.mediawiki
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Original file line number Diff line number Diff line change
Expand Up @@ -422,7 +422,12 @@ public async Task<PSBT> RequestPayjoin(
if (optionalParameters.AdditionalFeeOutputIndex != null && optionalParameters.MaxAdditionalFeeContribution != null)
feePSBTOutput = signedPSBT.Outputs[optionalParameters.AdditionalFeeOutputIndex];
Script paymentScriptPubKey = bip21.Address == null ? null : bip21.Address.ScriptPubKey;

FeeRate originalFeeRate;
if (!signedPSBT.TryGetEstimatedFeeRate(out originalFeeRate))
throw new PayjoinSenderException("signedPSBT should have utxo information");
decimal originalFee = signedPSBT.GetFee();

PSBT originalPSBT = CreateOriginalPSBT(signedPSBT);
Transaction originalGlobalTx = signedPSBT.GetGlobalTransaction();
TxOut feeOutput = feePSBTOutput == null ? null : originalGlobalTx.Outputs[feePSBTOutput.Index];
Expand Down Expand Up @@ -466,29 +471,27 @@ public async Task<PSBT> RequestPayjoin(
throw new PayjoinSenderException("The receiver added keypaths to an input");
if (proposedPSBTInput.PartialSigs.Count != 0)
throw new PayjoinSenderException("The receiver added partial signatures to an input");
PSBTInput proposedTxIn = proposalGlobalTx.Inputs.FindIndexedInput(proposedPSBTInput.PrevOut).TxIn;
TxIn proposedTxIn = proposalGlobalTx.Inputs.FindIndexedInput(proposedPSBTInput.PrevOut).TxIn;
bool isOurInput = originalInputs.Count > 0 && originalInputs.Peek().OriginalTxIn.PrevOut == proposedPSBTInput.PrevOut;
// If it is one of our input
if (isOurInput)
{
OutPoint inputPrevout = ourPrevouts.Dequeue();
TxIn originalTxin = originalGlobalTx.Inputs.FromOutpoint(inputPrevout);
PSBTInput originalPSBTInput = originalPSBT.Inputs.FromOutpoint(inputPrevout);
var input = originalInputs.Dequeue();
// Verify that sequence is unchanged.
if (input.OriginalTxIn.Sequence != proposedTxIn.Sequence)
throw new PayjoinSenderException("The proposedTxIn modified the sequence of one of our inputs")
throw new PayjoinSenderException("The proposedTxIn modified the sequence of one of our inputs");
// Verify the PSBT input is not finalized
if (proposedPSBTInput.IsFinalized())
throw new PayjoinSenderException("The receiver finalized one of our inputs");

sequences.Add(proposedTxIn.Sequence);

// Fill up the info from the original PSBT input so we can sign and get fees.
proposedPSBTInput.NonWitnessUtxo = input.SignedPSBTInput.NonWitnessUtxo;
proposedPSBTInput.WitnessUtxo = input.SignedPSBTInput.WitnessUtxo;
// We fill up information we had on the signed PSBT, so we can sign it.
foreach (var hdKey in input.SignedPSBTInput.HDKeyPaths)
proposedPSBTInput.HDKeyPaths.Add(hdKey.Key, hdKey.Value);
proposedPSBTInput.RedeemScript = signedPSBTInput.RedeemScript;
proposedPSBTInput.HDKeyPaths.Add(hdKey.Key, hdKey.Value);
proposedPSBTInput.RedeemScript = input.SignedPSBTInput.RedeemScript;
}
else
Expand All @@ -511,7 +514,9 @@ public async Task<PSBT> RequestPayjoin(
if (sequences.Count != 1)
throw new PayjoinSenderException("Mixed sequence detected in the proposal");

decimal newFee = proposal.GetFee();
decimal newFee;
if (!proposal.TryGetFee(out newFee))
throw new PayjoinSenderException("The payjoin receiver did not included UTXO information to calculate fee correctly");
decimal additionalFee = newFee - originalFee;
if (additionalFee < 0)
throw new PayjoinSenderException("The receiver decreased absolute fee");
Expand Down Expand Up @@ -543,10 +548,10 @@ public async Task<PSBT> RequestPayjoin(
// Make sure the actual contribution is only paying for fee incurred by additional inputs
// This assumes an additional input can be up to 110 bytes.
int additionalInputsCount = proposalGlobalTx.Inputs.Count - originalGlobalTx.Inputs.Count;
if (actualContribution > originalFeeRate * 110 * additionalInputsCount)
if (actualContribution > originalFeeRate.GetFee(110) * additionalInputsCount)
throw new PayjoinSenderException("The actual contribution is not only paying for additional inputs");
}
else if (allowOutputSubstitution && output.OriginalTxOut.ScriptPubKey == paymentScriptPubKey)
else if (allowOutputSubstitution && originalOutput.OriginalTxOut.ScriptPubKey == paymentScriptPubKey)
{
// That's the payment output, the receiver may have changed it.
}
Expand All @@ -556,9 +561,9 @@ public async Task<PSBT> RequestPayjoin(
throw new PayjoinSenderException("The receiver decreased the value of one of the outputs");
}
// We fill up information we had on the signed PSBT, so we can sign it.
foreach (var hdKey in output.SignedPSBTOutput.HDKeyPaths)
foreach (var hdKey in originalOutput.SignedPSBTOutput.HDKeyPaths)
proposedPSBTOutput.HDKeyPaths.Add(hdKey.Key, hdKey.Value);
proposedPSBTOutput.RedeemScript = output.SignedPSBTOutput.RedeemScript;
proposedPSBTOutput.RedeemScript = originalOutput.SignedPSBTOutput.RedeemScript;
}
}
// Verify that all of sender's outputs from the original PSBT are in the proposal.
Expand Down