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Make stronger check on fee, let the merchant make bigger transaction if he pays the fee

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This commit is contained in:
nicolas.dorier 2020-04-06 23:21:02 +09:00
parent 5503132ffc
commit 9fc451c9ca
No known key found for this signature in database
GPG Key ID: 6618763EF09186FE
1 changed files with 32 additions and 14 deletions
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46
BTCPayServer/Services/PayjoinClient.cs
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@ -37,9 +37,10 @@ namespace BTCPayServer.Services
var sentBefore = -originalTx.GetBalance(derivationSchemeSettings.AccountDerivation,
signingAccount.AccountKey,
signingAccount.GetRootedKeyPath());
if (!originalTx.TryGetEstimatedFeeRate(out var oldFeeRate))
var oldGlobalTx = originalTx.GetGlobalTransaction();
if (!originalTx.TryGetEstimatedFeeRate(out var originalFeeRate) || !originalTx.TryGetVirtualSize(out var oldVirtualSize))
throw new ArgumentException("originalTx should have utxo information", nameof(originalTx));
var originalFee = originalTx.GetFee();
var cloned = originalTx.Clone();
if (!cloned.IsAllFinalized() && !cloned.TryFinalize(out var errors))
{
@ -114,7 +115,12 @@ namespace BTCPayServer.Services
}
// Making sure that our inputs are finalized, and that some of our inputs have not been added
var newGlobalTx = newPSBT.GetGlobalTransaction();
int ourInputCount = 0;
if (newGlobalTx.Version != oldGlobalTx.Version)
throw new PayjoinSenderException("The version field of the transaction has been modified");
if (newGlobalTx.LockTime != oldGlobalTx.LockTime)
throw new PayjoinSenderException("The LockTime field of the transaction has been modified");
foreach (var input in newPSBT.Inputs.CoinsFor(derivationSchemeSettings.AccountDerivation,
signingAccount.AccountKey, signingAccount.GetRootedKeyPath()))
{
@ -123,6 +129,8 @@ namespace BTCPayServer.Services
ourInputCount++;
if (input.IsFinalized())
throw new PayjoinSenderException("A PSBT input from us should not be finalized");
if (newGlobalTx.Inputs[input.Index].Sequence != newGlobalTx.Inputs[ourInput.Index].Sequence)
throw new PayjoinSenderException("The sequence of one of our input has been modified");
}
else
{
@ -131,11 +139,16 @@ namespace BTCPayServer.Services
}
}
// Making sure that the receiver's inputs are finalized
// Making sure that the receiver's inputs are finalized and P2PWKH
foreach (var input in newPSBT.Inputs)
{
if (originalTx.Inputs.FindIndexedInput(input.PrevOut) is null && !input.IsFinalized())
throw new PayjoinSenderException("The payjoin receiver included a non finalized input");
if (originalTx.Inputs.FindIndexedInput(input.PrevOut) is null)
{
if (!input.IsFinalized())
throw new PayjoinSenderException("The payjoin receiver included a non finalized input");
if (!(input.FinalScriptWitness.GetSigner() is WitKeyId))
throw new PayjoinSenderException("The payjoin receiver included an input that is not P2PWKH");
}
}
if (ourInputCount < originalTx.Inputs.Count)
@ -143,29 +156,34 @@ namespace BTCPayServer.Services
// We limit the number of inputs the receiver can add
var addedInputs = newPSBT.Inputs.Count - originalTx.Inputs.Count;
if (originalTx.Inputs.Count < addedInputs)
throw new PayjoinSenderException("The payjoin receiver added too much inputs");
if (addedInputs == 0)
throw new PayjoinSenderException("The payjoin receiver did not added any input");
var sentAfter = -newPSBT.GetBalance(derivationSchemeSettings.AccountDerivation,
signingAccount.AccountKey,
signingAccount.GetRootedKeyPath());
if (sentAfter > sentBefore)
{
var overPaying = sentAfter - sentBefore;
if (!newPSBT.TryGetEstimatedFeeRate(out var newFeeRate) || !newPSBT.TryGetVirtualSize(out var newVirtualSize))
throw new PayjoinSenderException("The payjoin receiver did not included UTXO information to calculate fee correctly");
var additionalFee = newPSBT.GetFee() - originalFee;
if (overPaying > additionalFee)
throw new PayjoinSenderException("The payjoin receiver is sending more money to himself");
if (overPaying > 2 * originalFee)
throw new PayjoinSenderException("The payjoin receiver is making us pay more than twice the original fee");
// Let's check the difference is only for the fee and that feerate
// did not changed that much
var expectedFee = oldFeeRate.GetFee(newVirtualSize);
var expectedFee = originalFeeRate.GetFee(newVirtualSize);
// Signing precisely is hard science, give some breathing room for error.
expectedFee += newPSBT.Inputs.Count * Money.Satoshis(2);
expectedFee += originalFeeRate.GetFee(newPSBT.Inputs.Count * 2);
// If the payjoin is removing some dust, we may pay a bit more as a whole output has been removed
var removedOutputs = Math.Max(0, originalTx.Outputs.Count - newPSBT.Outputs.Count);
expectedFee += removedOutputs * oldFeeRate.GetFee(294);
var actualFee = newFeeRate.GetFee(newVirtualSize);
if (actualFee > expectedFee && actualFee - expectedFee > Money.Satoshis(546))
throw new PayjoinSenderException("The payjoin receiver is paying too much fee");
expectedFee += removedOutputs * Money.Satoshis(294);
if (overPaying > expectedFee)
throw new PayjoinSenderException("The payjoin receiver increased the fee rate we are paying too much");
}
return newPSBT;