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Do not allow decrease in absolute fee

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nicolas.dorier 2020-06-19 16:06:19 +09:00
parent 549107933c
commit c449c019f6
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1 changed files with 6 additions and 2 deletions
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@ -117,6 +117,7 @@ The payjoin proposal MAY:
The payjoin proposal MUST NOT:
* Shuffle the order of inputs or outputs, the additional outputs or additional inputs must be inserted at a random index.
* Decrease the absolute fee of the original transaction.
===BIP21 payjoin parameters===
@ -249,6 +250,7 @@ The receiver needs to do some check on the original PSBT before proceeding:
The sender should check the payjoin proposal before signing it to prevent a malicious receiver from stealing money.
* Verify that the absolute fee of the payjoin proposal is equals or higher than the original PSBT.
* If the receiver's BIP21 signalled <code>pjos=0</code>, disable payment output substitution.
* Verify that the transaction version, and the nLockTime are unchanged.
* Check that the sender's inputs' sequence numbers are unchanged.
@ -529,6 +531,10 @@ public async Task<PSBT> RequestPayjoin(
if (sequences.Count != 1)
throw new PayjoinSenderException("Mixed sequence detected in the proposal");
decimal newFee = proposal.GetFee();
decimal additionalFee = newFee - originalFee;
if (additionalFee < 0)
throw new PayjoinSenderException("The receiver decreased absolute fee");
// For each outputs in the proposal:
foreach (PSBTOutput proposedPSBTOutput in proposal.Outputs)
{
@ -545,8 +551,6 @@ public async Task<PSBT> RequestPayjoin(
// The amount that was substracted from the output's value is less or equal to maxadditionalfeecontribution
if (actualContribution > optionalParameters.MaxAdditionalFeeContribution)
throw new PayjoinSenderException("The actual contribution is more than maxadditionalfeecontribution");
decimal newFee = proposal.GetFee();
decimal additionalFee = newFee - originalFee;
// Make sure the actual contribution is only paying fee
if (actualContribution > additionalFee)
throw new PayjoinSenderException("The actual contribution is not only paying fee");