Adapt the simple_close protocol to simple taproot channels

`partial_signature_with_nonce` TLVs are added to `closing_complete` and `closing_sig` with the same format as in `commitment_signed`

The closing workflow is similar to the standard "simple close" workflow:
- Alice and Bob exchange `shutdown`, which includes a "closing nonce" (no changes here compared to the "simple taproot channels" spec).
- Alice selects possible closing transaction (closer_output_only, closee_output_only, closer_and_closee_output) and for each of them creates
a partial_signature_with_nonce using a new random local nonce and Bob's closing nonce (which she received in Bob's `shutdown` message).
- Alice send a `closing_complete` message to Bob that include these partial_signature_with_nonce.
- Bob receive Alice's `closing_complete` message, selects one of Alice's partial_signature_with_nonce, creates partial_signature_with_nonce using.
his closing nonce and the nonce attached to the partial_signature_with_nonce and sends it to Alice in a `closing_sig` message.
- Alice receives Bob's `closing_sig` and creates a partial signature for her closing tx using her closing nonce and the nonce attached Bob's partial_signature_with_nonce.
- Alice combines this signature with Bob's and can broadcat her closing tx.

eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/Channel.scala

@@ -773,7 +773,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
             if (d.commitments.hasNoPendingHtlcsOrFeeUpdate) {
               // there are no pending signed changes, let's directly negotiate a closing transaction
               if (Features.canUseFeature(d.commitments.params.localParams.initFeatures, d.commitments.params.remoteParams.initFeatures, Features.SimpleClose)) {
-                startSimpleClose(d.commitments, localShutdown, remoteShutdown, d.closingFeerates, sendList)
+                startSimpleClose(d.commitments, localShutdown, remoteShutdown, d.closingFeerates, sendList, closingNonce, remoteShutdown.shutdownNonce_opt)
               } else if (d.commitments.params.localParams.paysClosingFees) {
                 // we pay the closing fees, so we initiate the negotiation by sending the first closing_signed
                 val (closingTx, closingSigned) = MutualClose.makeFirstClosingTx(keyManager, d.commitments.latest, localShutdown.scriptPubKey, remoteShutdownScript, nodeParams.currentBitcoinCoreFeerates, nodeParams.onChainFeeConf, d.closingFeerates, closingNonce, remoteShutdown.shutdownNonce_opt)
@@ -1592,7 +1592,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
               context.system.eventStream.publish(ChannelSignatureReceived(self, commitments1))
               if (commitments1.hasNoPendingHtlcsOrFeeUpdate) {
                 if (Features.canUseFeature(d.commitments.params.localParams.initFeatures, d.commitments.params.remoteParams.initFeatures, Features.SimpleClose)) {
-                  startSimpleClose(d.commitments, localShutdown, remoteShutdown, d.closingFeerates, revocation :: Nil)
+                  startSimpleClose(d.commitments, localShutdown, remoteShutdown, d.closingFeerates, revocation :: Nil, closingNonce, remoteShutdown.shutdownNonce_opt)
                 } else if (d.commitments.params.localParams.paysClosingFees) {
                   // we pay the closing fees, so we initiate the negotiation by sending the first closing_signed
                   val (closingTx, closingSigned) = MutualClose.makeFirstClosingTx(keyManager, commitments1.latest, localShutdown.scriptPubKey, remoteShutdown.scriptPubKey, nodeParams.currentBitcoinCoreFeerates, nodeParams.onChainFeeConf, closingFeerates, closingNonce, remoteShutdown.shutdownNonce_opt)
@@ -1636,7 +1636,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
           if (commitments1.hasNoPendingHtlcsOrFeeUpdate) {
             log.debug("switching to NEGOTIATING spec:\n{}", commitments1.latest.specs2String)
             if (Features.canUseFeature(d.commitments.params.localParams.initFeatures, d.commitments.params.remoteParams.initFeatures, Features.SimpleClose)) {
-              startSimpleClose(d.commitments, localShutdown, remoteShutdown, d.closingFeerates, Nil)
+              startSimpleClose(d.commitments, localShutdown, remoteShutdown, d.closingFeerates, Nil, closingNonce, remoteShutdown.shutdownNonce_opt)
             } else if (d.commitments.params.localParams.paysClosingFees) {
               // we pay the closing fees, so we initiate the negotiation by sending the first closing_signed
               val (closingTx, closingSigned) = MutualClose.makeFirstClosingTx(keyManager, commitments1.latest, localShutdown.scriptPubKey, remoteShutdown.scriptPubKey, nodeParams.currentBitcoinCoreFeerates, nodeParams.onChainFeeConf, closingFeerates, closingNonce, remoteShutdown.shutdownNonce_opt)
@@ -1871,7 +1871,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
         case status: ClosingNegotiation.SigningTransactions =>
           val localScript = status.localShutdown.scriptPubKey
           val remoteScript = status.remoteShutdown.scriptPubKey
-          MutualClose.signSimpleClosingTx(keyManager, d.commitments.latest, localScript, remoteScript, closingComplete) match {
+          MutualClose.signSimpleClosingTx(keyManager, d.commitments.latest, localScript, remoteScript, closingComplete, closingNonce) match {
             case Left(f) =>
               // This may happen if scripts were updated concurrently, so we simply ignore failures.
               log.warning("invalid closing_complete: {}", f.getMessage)

eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/CommonHandlers.scala

@@ -17,6 +17,7 @@
 package fr.acinq.eclair.channel.fsm
 
 import akka.actor.FSM
+import fr.acinq.bitcoin.crypto.musig2.{IndividualNonce, SecretNonce}
 import fr.acinq.bitcoin.scalacompat.{ByteVector32, Script}
 import fr.acinq.eclair.Features
 import fr.acinq.eclair.channel.Helpers.Closing.MutualClose
@@ -132,11 +133,11 @@ trait CommonHandlers {
     finalScriptPubKey
   }
 
-  def startSimpleClose(commitments: Commitments, localShutdown: Shutdown, remoteShutdown: Shutdown, closingFeerates: Option[ClosingFeerates], toSend: List[LightningMessage]) = {
+  def startSimpleClose(commitments: Commitments, localShutdown: Shutdown, remoteShutdown: Shutdown, closingFeerates: Option[ClosingFeerates], toSend: List[LightningMessage], localClosingNonce_opt: Option[(SecretNonce, IndividualNonce)] = None, remoteClosingNonce_opt: Option[IndividualNonce] = None) = {
     val localScript = localShutdown.scriptPubKey
     val remoteScript = remoteShutdown.scriptPubKey
     val closingFeerate = closingFeerates.map(_.preferred).getOrElse(nodeParams.onChainFeeConf.getClosingFeerate(nodeParams.currentBitcoinCoreFeerates))
-    MutualClose.makeSimpleClosingTx(nodeParams.currentBlockHeight, keyManager, commitments.latest, localScript, remoteScript, closingFeerate) match {
+    MutualClose.makeSimpleClosingTx(nodeParams.currentBlockHeight, keyManager, commitments.latest, localScript, remoteScript, closingFeerate, localClosingNonce_opt, remoteClosingNonce_opt) match {
       case Left(f) =>
         log.warning("cannot create local closing txs, waiting for remote closing_complete: {}", f.getMessage)
         val status = ClosingNegotiation.SigningTransactions(localShutdown, remoteShutdown, None, None, None)

eclair-core/src/main/scala/fr/acinq/eclair/transactions/Transactions.scala

@@ -374,7 +374,11 @@ super.sign(privateKey, txOwner, commitmentFormat)
 }
   }
 
-  case class ClosingTx(input: InputInfo, tx: Transaction, toLocalOutput: Option[OutputInfo]) extends TransactionWithInputInfo { override def desc: String = "closing" }
+  case class ClosingTx(input: InputInfo, tx: Transaction, toLocalOutput: Option[OutputInfo]) extends TransactionWithInputInfo {
+    // these nonces are generated on the fly at during a "simple" closing session and can be forgotten once the session ends
+    @volatile var localNonce_opt: Option[(SecretNonce, IndividualNonce)] = None
+    override def desc: String = "closing"
+  }
 
   sealed trait TxGenerationSkipped
   case object OutputNotFound extends TxGenerationSkipped { override def toString = "output not found (probably trimmed)" }

eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/ChannelTlv.scala

@@ -321,10 +321,22 @@ object ClosingTlv {
   /** Signature for a closing transaction containing the closer and closee's outputs. */
   case class CloserAndClosee(sig: ByteVector64) extends ClosingTlv
 
+  /** Signature for a closing transaction containing only the closer's output. */
+  case class CloserNoCloseePartialSignature(partialSigWithNonce: PartialSignatureWithNonce) extends ClosingTlv
+
+  /** Signature for a closing transaction containing only the closee's output. */
+  case class NoCloserCloseePartialSignature(partialSigWithNonce: PartialSignatureWithNonce) extends ClosingTlv
+
+  /** Signature for a closing transaction containing the closer and closee's outputs. */
+  case class CloserAndCloseePartialSignature(partialSigWithNonce: PartialSignatureWithNonce) extends ClosingTlv
+
   val closingTlvCodec: Codec[TlvStream[ClosingTlv]] = tlvStream(discriminated[ClosingTlv].by(varint)
     .typecase(UInt64(1), tlvField(bytes64.as[CloserNoClosee]))
     .typecase(UInt64(2), tlvField(bytes64.as[NoCloserClosee]))
     .typecase(UInt64(3), tlvField(bytes64.as[CloserAndClosee]))
+    .typecase(UInt64(4), tlvField(partialSignatureWithNonce.as[CloserNoCloseePartialSignature]))
+    .typecase(UInt64(5), tlvField(partialSignatureWithNonce.as[NoCloserCloseePartialSignature]))
+    .typecase(UInt64(6), tlvField(partialSignatureWithNonce.as[CloserAndCloseePartialSignature]))
   )
 
 }

eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/LightningMessageTypes.scala

@@ -409,12 +409,18 @@ case class ClosingComplete(channelId: ByteVector32, fees: Satoshi, lockTime: Lon
   val closerNoCloseeSig_opt: Option[ByteVector64] = tlvStream.get[ClosingTlv.CloserNoClosee].map(_.sig)
   val noCloserCloseeSig_opt: Option[ByteVector64] = tlvStream.get[ClosingTlv.NoCloserClosee].map(_.sig)
   val closerAndCloseeSig_opt: Option[ByteVector64] = tlvStream.get[ClosingTlv.CloserAndClosee].map(_.sig)
+  val closerNoCloseePartialSig_opt: Option[PartialSignatureWithNonce] = tlvStream.get[ClosingTlv.CloserNoCloseePartialSignature].map(_.partialSigWithNonce)
+  val noCloserCloseePartialSig_opt: Option[PartialSignatureWithNonce] = tlvStream.get[ClosingTlv.NoCloserCloseePartialSignature].map(_.partialSigWithNonce)
+  val closerAndCloseePartialSig_opt: Option[PartialSignatureWithNonce] = tlvStream.get[ClosingTlv.CloserAndCloseePartialSignature].map(_.partialSigWithNonce)
 }
 
 case class ClosingSig(channelId: ByteVector32, tlvStream: TlvStream[ClosingTlv] = TlvStream.empty) extends ChannelMessage with HasChannelId {
   val closerNoCloseeSig_opt: Option[ByteVector64] = tlvStream.get[ClosingTlv.CloserNoClosee].map(_.sig)
   val noCloserCloseeSig_opt: Option[ByteVector64] = tlvStream.get[ClosingTlv.NoCloserClosee].map(_.sig)
   val closerAndCloseeSig_opt: Option[ByteVector64] = tlvStream.get[ClosingTlv.CloserAndClosee].map(_.sig)
+  val closerNoCloseePartialSig_opt: Option[PartialSignatureWithNonce] = tlvStream.get[ClosingTlv.CloserNoCloseePartialSignature].map(_.partialSigWithNonce)
+  val noCloserCloseePartialSig_opt: Option[PartialSignatureWithNonce] = tlvStream.get[ClosingTlv.NoCloserCloseePartialSignature].map(_.partialSigWithNonce)
+  val closerAndCloseePartialSig_opt: Option[PartialSignatureWithNonce] = tlvStream.get[ClosingTlv.CloserAndCloseePartialSignature].map(_.partialSigWithNonce)
 }
 
 case class UpdateAddHtlc(channelId: ByteVector32,

eclair-core/src/test/scala/fr/acinq/eclair/channel/states/g/NegotiatingStateSpec.scala

@@ -518,6 +518,42 @@ class NegotiatingStateSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike
     assert(bob.stateName == NEGOTIATING_SIMPLE)
   }
 
+  test("recv ClosingComplete (both outputs, simple taproot channels)", Tag(ChannelStateTestsTags.SimpleClose), Tag(ChannelStateTestsTags.OptionSimpleTaprootStaging), Tag(ChannelStateTestsTags.AnchorOutputsZeroFeeHtlcTxs)) { f =>
+    import f._
+    aliceClose(f)
+    val aliceClosingComplete = alice2bob.expectMsgType[ClosingComplete]
+    assert(aliceClosingComplete.fees > 0.sat)
+    assert(aliceClosingComplete.closerAndCloseeSig_opt.nonEmpty || aliceClosingComplete.closerAndCloseePartialSig_opt.nonEmpty)
+    assert(aliceClosingComplete.closerNoCloseeSig_opt.nonEmpty || aliceClosingComplete.closerNoCloseePartialSig_opt.nonEmpty)
+    assert(aliceClosingComplete.noCloserCloseeSig_opt.isEmpty && aliceClosingComplete.noCloserCloseePartialSig_opt.isEmpty)
+    val bobClosingComplete = bob2alice.expectMsgType[ClosingComplete]
+    assert(bobClosingComplete.fees > 0.sat)
+    assert(bobClosingComplete.closerAndCloseeSig_opt.nonEmpty || bobClosingComplete.closerAndCloseePartialSig_opt.nonEmpty)
+    assert(bobClosingComplete.closerNoCloseeSig_opt.nonEmpty || bobClosingComplete.closerNoCloseePartialSig_opt.nonEmpty)
+    assert(bobClosingComplete.noCloserCloseeSig_opt.isEmpty && bobClosingComplete.noCloserCloseePartialSig_opt.isEmpty)
+
+    alice2bob.forward(bob, aliceClosingComplete)
+    val bobClosingSig = bob2alice.expectMsgType[ClosingSig]
+    bob2alice.forward(alice, bobClosingSig)
+    val aliceTx = alice2blockchain.expectMsgType[PublishFinalTx]
+    assert(bob2blockchain.expectMsgType[PublishFinalTx].tx.txid == aliceTx.tx.txid)
+    assert(aliceTx.desc == "closing")
+    alice2blockchain.expectWatchTxConfirmed(aliceTx.tx.txid)
+    bob2blockchain.expectWatchTxConfirmed(aliceTx.tx.txid)
+    assert(alice.stateName == NEGOTIATING_SIMPLE)
+
+    bob2alice.forward(alice, bobClosingComplete)
+    val aliceClosingSig = alice2bob.expectMsgType[ClosingSig]
+    alice2bob.forward(bob, aliceClosingSig)
+    val bobTx = bob2blockchain.expectMsgType[PublishFinalTx]
+    assert(alice2blockchain.expectMsgType[PublishFinalTx].tx.txid == bobTx.tx.txid)
+    assert(aliceTx.tx.txid != bobTx.tx.txid)
+    assert(bobTx.desc == "closing")
+    bob2blockchain.expectWatchTxConfirmed(bobTx.tx.txid)
+    alice2blockchain.expectWatchTxConfirmed(bobTx.tx.txid)
+    assert(bob.stateName == NEGOTIATING_SIMPLE)
+  }
+
   test("recv ClosingComplete (single output)", Tag(ChannelStateTestsTags.SimpleClose), Tag(ChannelStateTestsTags.NoPushAmount)) { f =>
     import f._
     aliceClose(f)
@@ -539,6 +575,27 @@ class NegotiatingStateSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike
     assert(bob.stateName == NEGOTIATING_SIMPLE)
   }
 
+  test("recv ClosingComplete (single output, simple taproot channels)", Tag(ChannelStateTestsTags.SimpleClose), Tag(ChannelStateTestsTags.NoPushAmount), Tag(ChannelStateTestsTags.OptionSimpleTaprootStaging), Tag(ChannelStateTestsTags.AnchorOutputsZeroFeeHtlcTxs)) { f =>
+    import f._
+    aliceClose(f)
+    val closingComplete = alice2bob.expectMsgType[ClosingComplete]
+    assert(closingComplete.closerAndCloseeSig_opt.isEmpty && closingComplete.closerAndCloseePartialSig_opt.isEmpty)
+    assert(closingComplete.closerNoCloseeSig_opt.nonEmpty || closingComplete.closerNoCloseePartialSig_opt.nonEmpty)
+    assert(closingComplete.noCloserCloseeSig_opt.isEmpty && closingComplete.noCloserCloseePartialSig_opt.isEmpty)
+    // Bob has nothing at stake.
+    bob2alice.expectNoMessage(100 millis)
+
+    alice2bob.forward(bob, closingComplete)
+    bob2alice.expectMsgType[ClosingSig]
+    bob2alice.forward(alice)
+    val closingTx = alice2blockchain.expectMsgType[PublishFinalTx]
+    assert(bob2blockchain.expectMsgType[PublishFinalTx].tx.txid == closingTx.tx.txid)
+    alice2blockchain.expectWatchTxConfirmed(closingTx.tx.txid)
+    bob2blockchain.expectWatchTxConfirmed(closingTx.tx.txid)
+    assert(alice.stateName == NEGOTIATING_SIMPLE)
+    assert(bob.stateName == NEGOTIATING_SIMPLE)
+  }
+
   test("recv ClosingComplete (single output, trimmed)", Tag(ChannelStateTestsTags.SimpleClose), Tag(ChannelStateTestsTags.NoPushAmount)) { f =>
     import f._
     val (r, htlc) = addHtlc(250_000 msat, alice, bob, alice2bob, bob2alice)