Rename payjoin optional parameters, implement sender minFeeRate

BTCPayServer.Tests/PayJoinTests.cs

@@ -402,8 +402,8 @@ namespace BTCPayServer.Tests
                 var requesting = pjClient.RequestPayjoin(fakeServer.ServerUri, derivationSchemeSettings, psbt, default);
                 var request = await fakeServer.GetNextRequest();
                 Assert.Equal("1", request.Request.Query["v"][0]);
-                Assert.Equal(changeIndex.ToString(), request.Request.Query["feebumpindex"][0]);
+                Assert.Equal(changeIndex.ToString(), request.Request.Query["additionalfeeoutputindex"][0]);
-                Assert.Equal("3000", request.Request.Query["maxfeebumpcontribution"][0]);
+                Assert.Equal("3000", request.Request.Query["maxadditionalfeecontribution"][0]);
 
 
                 Logs.Tester.LogInformation("The payjoin receiver tries to make us pay lots of fee");
@@ -438,6 +438,19 @@ namespace BTCPayServer.Tests
                 ex = await Assert.ThrowsAsync<PayjoinSenderException>(async () => await requesting);
                 Assert.Contains("increased the fee rate", ex.Message);
 
+                Logs.Tester.LogInformation("The payjoin receiver can't decrease the fee rate too much");
+                pjClient.MinimumFeeRate = new FeeRate(50m);
+                requesting = pjClient.RequestPayjoin(fakeServer.ServerUri, derivationSchemeSettings, psbt, default);
+                request = await fakeServer.GetNextRequest();
+                originalPSBT = await ParsePSBT(request);
+                proposalTx = originalPSBT.GetGlobalTransaction();
+                proposalTx.Outputs[changeIndex].Value -= Money.Satoshis(3000);
+                await request.Response.WriteAsync(PSBT.FromTransaction(proposalTx, Network.RegTest).ToBase64(), Encoding.UTF8);
+                fakeServer.Done();
+                ex = await Assert.ThrowsAsync<PayjoinSenderException>(async () => await requesting);
+                Assert.Contains("a too low fee rate", ex.Message);
+                pjClient.MinimumFeeRate = null;
+
                 Logs.Tester.LogInformation("Make sure the receiver implementation do not take more fee than allowed");
                 var bob = tester.NewAccount();
                 await bob.GrantAccessAsync();
@@ -472,6 +485,36 @@ namespace BTCPayServer.Tests
                 var proposal = await pjClient.RequestPayjoin(endpoint, derivationSchemeSettings, psbt, default);
                 Assert.True(proposal.TryGetFee(out var newFee));
                 Assert.Equal(Money.Satoshis(3001 + 50), newFee);
+                proposal = proposal.SignAll(derivationSchemeSettings.AccountDerivation, alice.GenerateWalletResponseV.AccountHDKey, signingAccount.GetRootedKeyPath());
+                proposal.Finalize();
+                await tester.ExplorerNode.SendRawTransactionAsync(proposal.ExtractTransaction());
+                await notifications.NextEventAsync();
+
+                Logs.Tester.LogInformation("Abusing minFeeRate should give not enough money error");
+                invoice = bob.BitPay.CreateInvoice(
+                    new Invoice() { Price = 0.1m, Currency = "BTC", FullNotifications = true });
+                invoiceBIP21 = new BitcoinUrlBuilder(invoice.CryptoInfo.First().PaymentUrls.BIP21,
+                    tester.ExplorerClient.Network.NBitcoinNetwork);
+                psbt = (await nbx.CreatePSBTAsync(alice.DerivationScheme, new CreatePSBTRequest()
+                {
+                    Destinations =
+                    {
+                        new CreatePSBTDestination()
+                        {
+                            Amount = invoiceBIP21.Amount,
+                            Destination = invoiceBIP21.Address
+                        }
+                    },
+                    FeePreference = new FeePreference()
+                    {
+                        ExplicitFee = Money.Satoshis(3001)
+                    }
+                })).PSBT;
+                psbt.SignAll(derivationSchemeSettings.AccountDerivation, alice.GenerateWalletResponseV.AccountHDKey, signingAccount.GetRootedKeyPath());
+                endpoint = TestAccount.GetPayjoinEndpoint(invoice, Network.RegTest);
+                pjClient.MinimumFeeRate = new FeeRate(100_000_000.2m);
+                var ex2 = await Assert.ThrowsAsync<PayjoinReceiverException>(async () => await pjClient.RequestPayjoin(endpoint, derivationSchemeSettings, psbt, default));
+                Assert.Equal(PayjoinReceiverWellknownErrors.NotEnoughMoney, ex2.WellknownError);
             }
         }
 

BTCPayServer/Payments/PayJoin/PayJoinEndpointController.cs

@@ -117,8 +117,9 @@ namespace BTCPayServer.Payments.PayJoin
         [MediaTypeConstraint("text/plain")]
         [RateLimitsFilter(ZoneLimits.PayJoin, Scope = RateLimitsScope.RemoteAddress)]
         public async Task<IActionResult> Submit(string cryptoCode,
-            long maxfeebumpcontribution = -1,
+            long maxadditionalfeecontribution = -1,
-            int feebumpindex = -1,
+            int additionalfeeoutputindex = -1,
+            decimal minfeerate = -1.0m,
             int v = 1)
         {
             if (v != 1)
@@ -130,7 +131,8 @@ namespace BTCPayServer.Payments.PayJoin
                     new JProperty("message", "This version of payjoin is not supported.")
                 });
             }
-            Money allowedFeeBumpContribution = Money.Satoshis(maxfeebumpcontribution >= 0 ? maxfeebumpcontribution : long.MaxValue);
+            FeeRate senderMinFeeRate = minfeerate < 0.0m ? null : new FeeRate(minfeerate);
+            Money allowedSenderFeeContribution = Money.Satoshis(maxadditionalfeecontribution >= 0 ? maxadditionalfeecontribution : long.MaxValue);
             var network = _btcPayNetworkProvider.GetNetwork<BTCPayNetwork>(cryptoCode);
             if (network == null)
             {
@@ -345,10 +347,10 @@ namespace BTCPayServer.Payments.PayJoin
             var ourNewOutput = newTx.Outputs[originalPaymentOutput.Index];
             HashSet<TxOut> isOurOutput = new HashSet<TxOut>();
             isOurOutput.Add(ourNewOutput);
-            TxOut preferredFeeBumpOutput = feebumpindex >= 0
+            TxOut preferredFeeBumpOutput = additionalfeeoutputindex >= 0
-                                            && feebumpindex < newTx.Outputs.Count
+                                            && additionalfeeoutputindex < newTx.Outputs.Count
-                                            && !isOurOutput.Contains(newTx.Outputs[feebumpindex])
+                                            && !isOurOutput.Contains(newTx.Outputs[additionalfeeoutputindex])
-                                            ? newTx.Outputs[feebumpindex] : null;
+                                            ? newTx.Outputs[additionalfeeoutputindex] : null;
             var rand = new Random();
             int senderInputCount = newTx.Inputs.Count;
             foreach (var selectedUTXO in selectedUTXOs.Select(o => o.Value))
@@ -434,7 +436,7 @@ namespace BTCPayServer.Payments.PayJoin
                 }
 
                 // The rest, we take from user's change
-                for (int i = 0; i < newTx.Outputs.Count && additionalFee > Money.Zero && allowedFeeBumpContribution > Money.Zero; i++)
+                for (int i = 0; i < newTx.Outputs.Count && additionalFee > Money.Zero && allowedSenderFeeContribution > Money.Zero; i++)
                 {
                     if (preferredFeeBumpOutput is TxOut &&
                         preferredFeeBumpOutput != newTx.Outputs[i])
@@ -444,10 +446,10 @@ namespace BTCPayServer.Payments.PayJoin
                         var outputContribution = Money.Min(additionalFee, newTx.Outputs[i].Value);
                         outputContribution = Money.Min(outputContribution,
                             newTx.Outputs[i].Value - newTx.Outputs[i].GetDustThreshold(minRelayTxFee));
-                        outputContribution = Money.Min(outputContribution, allowedFeeBumpContribution);
+                        outputContribution = Money.Min(outputContribution, allowedSenderFeeContribution);
                         newTx.Outputs[i].Value -= outputContribution;
                         additionalFee -= outputContribution;
-                        allowedFeeBumpContribution -= outputContribution;
+                        allowedSenderFeeContribution -= outputContribution;
                     }
                 }
 
@@ -457,7 +459,7 @@ namespace BTCPayServer.Payments.PayJoin
                     // we are not under the relay fee, it should be OK.
                     var newVSize = txBuilder.EstimateSize(newTx, true);
                     var newFeePaid = newTx.GetFee(txBuilder.FindSpentCoins(newTx));
-                    if (new FeeRate(newFeePaid, newVSize) < minRelayTxFee)
+                    if (new FeeRate(newFeePaid, newVSize) < (senderMinFeeRate ?? minRelayTxFee))
                     {
                         await UnlockUTXOs();
                         await BroadcastNow();

BTCPayServer/Services/PayjoinClient.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Configuration;
+using System.Globalization;
 using System.Linq;
 using System.Net.Http;
 using System.Text;
@@ -43,8 +44,9 @@ namespace BTCPayServer.Services
 
     public class PayjoinClientParameters
     {
-        public Money MaxFeeBumpContribution { get; set; }
+        public Money MaxAdditionalFeeContribution { get; set; }
-        public int? FeeBumpIndex { get; set; }
+        public FeeRate MinFeeRate { get; set; }
+        public int? AdditionalFeeOutputIndex { get; set; }
         public int Version { get; set; } = 1;
     }
 
@@ -72,6 +74,7 @@ namespace BTCPayServer.Services
         }
 
         public Money MaxFeeBumpContribution { get; set; }
+        public FeeRate MinimumFeeRate { get; set; }
 
         public async Task<PSBT> RequestPayjoin(Uri endpoint, DerivationSchemeSettings derivationSchemeSettings,
             PSBT originalTx, CancellationToken cancellationToken)
@@ -94,7 +97,7 @@ namespace BTCPayServer.Services
             var changeOutput = originalTx.Outputs.CoinsFor(derivationSchemeSettings.AccountDerivation, signingAccount.AccountKey, signingAccount.GetRootedKeyPath())
                     .FirstOrDefault();
             if (changeOutput is PSBTOutput o)
-                clientParameters.FeeBumpIndex = (int)o.Index;
+                clientParameters.AdditionalFeeOutputIndex = (int)o.Index;
             var sentBefore = -originalTx.GetBalance(derivationSchemeSettings.AccountDerivation,
                 signingAccount.AccountKey,
                 signingAccount.GetRootedKeyPath());
@@ -102,8 +105,9 @@ namespace BTCPayServer.Services
             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();
-
+            clientParameters.MaxAdditionalFeeContribution = MaxFeeBumpContribution is null ? originalFee : MaxFeeBumpContribution;
-            clientParameters.MaxFeeBumpContribution = MaxFeeBumpContribution is null ? originalFee : MaxFeeBumpContribution;
+            if (MinimumFeeRate is FeeRate v)
+                clientParameters.MinFeeRate = v;
             var cloned = originalTx.Clone();
             cloned.Finalize();
 
@@ -220,20 +224,25 @@ namespace BTCPayServer.Services
             if (ourInputCount < originalTx.Inputs.Count)
                 throw new PayjoinSenderException("The payjoin receiver removed some of our inputs");
 
+            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");
+
+            if (clientParameters.MinFeeRate is FeeRate minFeeRate)
+            {
+                if (newFeeRate < minFeeRate)
+                    throw new PayjoinSenderException("The payjoin receiver created a payjoin with a too low fee rate");
+            }
+
             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 > clientParameters.MaxFeeBumpContribution)
+                if (overPaying > clientParameters.MaxAdditionalFeeContribution)
                     throw new PayjoinSenderException("The payjoin receiver is making us pay too much fee");
 
                 // Let's check the difference is only for the fee and that feerate
@@ -255,10 +264,12 @@ namespace BTCPayServer.Services
                 requestUri = requestUri.Substring(0, i);
             List<string> parameters = new List<string>(3);
             parameters.Add($"v={clientParameters.Version}");
-            if (clientParameters.FeeBumpIndex is int feeBumpIndex)
+            if (clientParameters.AdditionalFeeOutputIndex is int additionalFeeOutputIndex)
-                parameters.Add($"feebumpindex={feeBumpIndex}");
+                parameters.Add($"additionalfeeoutputindex={additionalFeeOutputIndex.ToString(CultureInfo.InvariantCulture)}");
-            if (clientParameters.MaxFeeBumpContribution is Money maxFeeBumpContribution)
+            if (clientParameters.MaxAdditionalFeeContribution is Money maxAdditionalFeeContribution)
-                parameters.Add($"maxfeebumpcontribution={maxFeeBumpContribution.Satoshi}");
+                parameters.Add($"maxadditionalfeecontribution={maxAdditionalFeeContribution.Satoshi.ToString(CultureInfo.InvariantCulture)}");
+            if (clientParameters.MinFeeRate is FeeRate minFeeRate)
+                parameters.Add($"minfeerate={minFeeRate.SatoshiPerByte.ToString(CultureInfo.InvariantCulture)}");
             endpoint = new Uri($"{requestUri}?{string.Join('&', parameters)}");
             return endpoint;
         }