feat: basic MWEB transaction read and write

Support reading and writing transactions carrying MWEB extension data
as used by Litecoin. The 0x08 flag bit alongside the witness marker
signals MWEB data, which is carried as opaque bytes between the
witness data and the locktime. mwebBytes is preserved through
input/output modification copies and legacy sighash computation.

Ported from cypherstack/coinlib commit 7723ba3, reworked to keep the
existing expectWitness two-pass parsing API and adapted to the
Locktime abstraction.

Author: sneurlax

coinlib/lib/src/tx/sighash/legacy_signature_hasher.dart

@@ -71,6 +71,7 @@ final class LegacySignatureHasher extends SignatureHasher {
       inputs: modifiedInputs,
       outputs: modifiedOutputs,
       locktime: tx.locktime,
+      mwebBytes: tx.mwebBytes,
     );
 
     // Add sighash type onto the end

coinlib/lib/src/tx/transaction.dart

@@ -47,6 +47,9 @@ class Transaction with Writable {
   final List<Input> inputs;
   final List<Output> outputs;
   final Locktime locktime;
+  /// Raw MWEB extension data carried between the witness data and locktime,
+  /// as used by Litecoin. This is kept as opaque bytes.
+  final Uint8List? mwebBytes;
 
   /// Constructs a transaction with the given [inputs] and [outputs].
   /// [TransactionTooLarge] will be thrown if the resulting transction exceeds
@@ -59,6 +62,7 @@ class Transaction with Writable {
     required Iterable<Input> inputs,
     required Iterable<Output> outputs,
     this.locktime = Locktime.zero,
+    this.mwebBytes,
   })
   : inputs = List.unmodifiable(inputs),
   outputs = List.unmodifiable(outputs)
@@ -77,11 +81,17 @@ class Transaction with Writable {
 
     final version = reader.readInt32();
 
+    bool hasWitness = false;
+    bool hasMweb = false;
     if (witness) {
-      // Check for witness data
+      // Check for witness data. The 0x01 flag bit signals witness data and
+      // the 0x08 flag bit signals MWEB data as used by Litecoin.
       final marker = reader.readUInt8();
       final flag = reader.readUInt8();
-      if (marker != 0 || flag != 1) return null;
+      if (marker != 0) return null;
+      hasWitness = (flag & 1) != 0;
+      hasMweb = (flag & 8) != 0;
+      if (!hasWitness && !hasMweb) return null;
     }
 
     final rawInputs = List.generate(
@@ -96,17 +106,27 @@ class Transaction with Writable {
 
     // Match the raw inputs with witness data if this is a witness transaction
     final inputs = rawInputs.map(
-      (raw) => Input.match(raw, witness ? reader.readVector() : []),
+      (raw) => Input.match(raw, hasWitness ? reader.readVector() : []),
     // Create list now to ensure we read the witness data before the locktime
     ).toList();
 
+    // Any MWEB data is carried between the witness data and the locktime
+    Uint8List? mwebBytes;
+    if (hasMweb) {
+      final remaining = reader.bytes.lengthInBytes - reader.offset;
+      if (remaining > 4) {
+        mwebBytes = reader.readSlice(remaining - 4);
+      }
+    }
+
     final locktime = reader.readUInt32();
 
     return Transaction(
       version: version,
       inputs: inputs,
       outputs: outputs,
       locktime: Locktime(locktime),
+      mwebBytes: mwebBytes,
     );
 
   }
@@ -166,7 +186,7 @@ class Transaction with Writable {
 
     if (isWitness) {
       writer.writeUInt8(0); // Marker
-      writer.writeUInt8(1); // Flag
+      writer.writeUInt8(mwebBytes != null ? 9 : 1); // Flag
     }
 
     writer.writeVarInt(BigInt.from(inputs.length));
@@ -185,6 +205,10 @@ class Transaction with Writable {
       }
     }
 
+    if (mwebBytes != null) {
+      writer.writeSlice(mwebBytes!);
+    }
+
     writer.writeUInt32(locktime.value);
 
   }
@@ -194,6 +218,7 @@ class Transaction with Writable {
     inputs: newInputs,
     outputs: outputs,
     locktime: locktime,
+    mwebBytes: mwebBytes,
   );
 
   T _requireInputOfType<T>(int inputN) {
@@ -346,6 +371,7 @@ class Transaction with Writable {
     ],
     outputs: outputs,
     locktime: locktime,
+    mwebBytes: mwebBytes,
   );
 
   /// Returns a new [Transaction] with the [output] added to the end of the
@@ -370,6 +396,7 @@ class Transaction with Writable {
       inputs: modifiedInputs,
       outputs: [...outputs, output],
       locktime: locktime,
+      mwebBytes: mwebBytes,
     );
 
   }
@@ -394,6 +421,7 @@ class Transaction with Writable {
       ),
       outputs: outputs,
       locktime: locktime,
+      mwebBytes: mwebBytes,
     )
     : this;
 
@@ -414,8 +442,9 @@ class Transaction with Writable {
   String get txid
     => bytesToHex(Uint8List.fromList(legacyHash.reversed.toList()));
 
-  /// If the transaction has any witness inputs.
-  bool get isWitness => inputs.any((input) => input is WitnessInput);
+  /// If the transaction has any witness inputs or carries MWEB data.
+  bool get isWitness
+    => inputs.any((input) => input is WitnessInput) || mwebBytes != null;
 
   bool get isCoinBase
     => inputs.length == 1