Add support for taproot outputs to our "input info" class (#2895)

* Refactor tx signing (no functional changes)

* Upgrade input info class to allow spending from taproot transactions

Our InputInfo class contains a tx output and the matching redeem script, which is enough to spend segwit v0 transactions.
For taproot transactions, instead of a redeem script, we need a script tree instead, and the appropriate internal pubkey.

* Use specific segwit and taproot input info types

We now use specific subtypes for segwit inputs (which include a redeem script) and taproot inputs (which include a script tree and an internal key).
Older codecs have been modified to always return a SegwitInput.
v4 codec is modified and uses an empty redeem script as a marker to specify that a script tree is being used, which makes it compatible with the current v4 codec.

Current (v4) codecs only handle segwit inputs. Support for taproot inputs will be added to v5 codecs.

---------

Co-authored-by: Pierre-Marie Padiou <pm47@users.noreply.github.com>

eclair-core/src/main/scala/fr/acinq/eclair/channel/Commitments.scala

@@ -225,7 +225,7 @@ object LocalCommit {
                     fundingTxIndex: Long, remoteFundingPubKey: PublicKey, commitInput: InputInfo,
                     commit: CommitSig, localCommitIndex: Long, spec: CommitmentSpec, localPerCommitmentPoint: PublicKey): Either[ChannelException, LocalCommit] = {
     val (localCommitTx, htlcTxs) = Commitment.makeLocalTxs(keyManager, params.channelConfig, params.channelFeatures, localCommitIndex, params.localParams, params.remoteParams, fundingTxIndex, remoteFundingPubKey, commitInput, localPerCommitmentPoint, spec)
-    if (!checkSig(localCommitTx, commit.signature, remoteFundingPubKey, TxOwner.Remote, params.commitmentFormat)) {
+    if (!localCommitTx.checkSig(commit.signature, remoteFundingPubKey, TxOwner.Remote, params.commitmentFormat)) {
       return Left(InvalidCommitmentSignature(params.channelId, fundingTxId, fundingTxIndex, localCommitTx.tx))
     }
     val sortedHtlcTxs = htlcTxs.sortBy(_.input.outPoint.index)
@@ -235,7 +235,7 @@ object LocalCommit {
     val remoteHtlcPubkey = Generators.derivePubKey(params.remoteParams.htlcBasepoint, localPerCommitmentPoint)
     val htlcTxsAndRemoteSigs = sortedHtlcTxs.zip(commit.htlcSignatures).toList.map {
       case (htlcTx: HtlcTx, remoteSig) =>
-        if (!checkSig(htlcTx, remoteSig, remoteHtlcPubkey, TxOwner.Remote, params.commitmentFormat)) {
+        if (!htlcTx.checkSig(remoteSig, remoteHtlcPubkey, TxOwner.Remote, params.commitmentFormat)) {
           return Left(InvalidHtlcSignature(params.channelId, htlcTx.tx.txid))
         }
         HtlcTxAndRemoteSig(htlcTx, remoteSig)
@@ -1142,7 +1142,10 @@ case class Commitments(params: ChannelParams,
       val localFundingKey = keyManager.fundingPublicKey(params.localParams.fundingKeyPath, commitment.fundingTxIndex).publicKey
       val remoteFundingKey = commitment.remoteFundingPubKey
       val fundingScript = Script.write(Scripts.multiSig2of2(localFundingKey, remoteFundingKey))
-      commitment.commitInput.redeemScript == fundingScript
+      commitment.commitInput match {
+        case InputInfo.SegwitInput(_, _, redeemScript) => redeemScript == fundingScript
+        case _: InputInfo.TaprootInput => false
+      }
     }
   }
 

eclair-core/src/main/scala/fr/acinq/eclair/channel/Helpers.scala

@@ -376,10 +376,10 @@ object Helpers {
 
     def makeFundingPubKeyScript(localFundingKey: PublicKey, remoteFundingKey: PublicKey): ByteVector = write(pay2wsh(multiSig2of2(localFundingKey, remoteFundingKey)))
 
-    def makeFundingInputInfo(fundingTxId: TxId, fundingTxOutputIndex: Int, fundingSatoshis: Satoshi, fundingPubkey1: PublicKey, fundingPubkey2: PublicKey): InputInfo = {
+    def makeFundingInputInfo(fundingTxId: TxId, fundingTxOutputIndex: Int, fundingSatoshis: Satoshi, fundingPubkey1: PublicKey, fundingPubkey2: PublicKey): InputInfo.SegwitInput = {
       val fundingScript = multiSig2of2(fundingPubkey1, fundingPubkey2)
       val fundingTxOut = TxOut(fundingSatoshis, pay2wsh(fundingScript))
-      InputInfo(OutPoint(fundingTxId, fundingTxOutputIndex), fundingTxOut, write(fundingScript))
+      InputInfo.SegwitInput(OutPoint(fundingTxId, fundingTxOutputIndex), fundingTxOut, write(fundingScript))
     }
 
     /**

eclair-core/src/main/scala/fr/acinq/eclair/channel/publish/ReplaceableTxFunder.scala

@@ -358,12 +358,16 @@ private class ReplaceableTxFunder(nodeParams: NodeParams,
     import fr.acinq.bitcoin.scalacompat.KotlinUtils._
 
     // We create a PSBT with the non-wallet input already signed:
+    val witnessScript = locallySignedTx.txInfo.input match {
+      case InputInfo.SegwitInput(_, _, redeemScript) => fr.acinq.bitcoin.Script.parse(redeemScript)
+      case _: InputInfo.TaprootInput => null
+    }
     val psbt = new Psbt(locallySignedTx.txInfo.tx)
       .updateWitnessInput(
         locallySignedTx.txInfo.input.outPoint,
         locallySignedTx.txInfo.input.txOut,
         null,
-        fr.acinq.bitcoin.Script.parse(locallySignedTx.txInfo.input.redeemScript),
+        witnessScript,
         fr.acinq.bitcoin.SigHash.SIGHASH_ALL,
         java.util.Map.of(),
         null,

eclair-core/src/main/scala/fr/acinq/eclair/crypto/keymanager/LocalChannelKeyManager.scala

@@ -23,7 +23,6 @@ import fr.acinq.bitcoin.scalacompat.{Block, BlockHash, ByteVector32, ByteVector6
 import fr.acinq.eclair.crypto.Generators
 import fr.acinq.eclair.crypto.Monitoring.{Metrics, Tags}
 import fr.acinq.eclair.router.Announcements
-import fr.acinq.eclair.transactions.Transactions
 import fr.acinq.eclair.transactions.Transactions.{CommitmentFormat, TransactionWithInputInfo, TxOwner}
 import fr.acinq.eclair.{KamonExt, randomLong}
 import grizzled.slf4j.Logging
@@ -113,7 +112,7 @@ class LocalChannelKeyManager(seed: ByteVector, chainHash: BlockHash) extends Cha
     Metrics.SignTxCount.withTags(tags).increment()
     KamonExt.time(Metrics.SignTxDuration.withTags(tags)) {
       val privateKey = privateKeys.get(publicKey.path)
-      Transactions.sign(tx, privateKey.privateKey, txOwner, commitmentFormat)
+      tx.sign(privateKey.privateKey, txOwner, commitmentFormat)
     }
   }
 
@@ -134,7 +133,7 @@ class LocalChannelKeyManager(seed: ByteVector, chainHash: BlockHash) extends Cha
     KamonExt.time(Metrics.SignTxDuration.withTags(tags)) {
       val privateKey = privateKeys.get(publicKey.path)
       val currentKey = Generators.derivePrivKey(privateKey.privateKey, remotePoint)
-      Transactions.sign(tx, currentKey, txOwner, commitmentFormat)
+      tx.sign(currentKey, txOwner, commitmentFormat)
     }
   }
 
@@ -154,7 +153,7 @@ class LocalChannelKeyManager(seed: ByteVector, chainHash: BlockHash) extends Cha
     KamonExt.time(Metrics.SignTxDuration.withTags(tags)) {
       val privateKey = privateKeys.get(publicKey.path)
       val currentKey = Generators.revocationPrivKey(privateKey.privateKey, remoteSecret)
-      Transactions.sign(tx, currentKey, txOwner, commitmentFormat)
+      tx.sign(currentKey, txOwner, commitmentFormat)
     }
   }