From f5bce709d9e0bfbfe733a67b21440e9de61bba42 Mon Sep 17 00:00:00 2001
From: sneurlax <sneurlax@gmail.com>
Date: Thu, 16 Jul 2026 21:53:22 -0800
Subject: [PATCH] feat: support extra payload data in transactions (vExtraData)

Add an optional vExtraData field to Transaction, serialized as a
var-length slice after the locktime. This supports special
transactions carrying an extra payload, such as Firo masternode
(DIP2-style) transactions. The field is carried through input/output
modification copies and legacy sighash computation.

Ported from cypherstack/coinlib commit 22778c0.
---
 .../lib/src/tx/sighash/legacy_signature_hasher.dart    |  1 +
 coinlib/lib/src/tx/transaction.dart                    | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/coinlib/lib/src/tx/sighash/legacy_signature_hasher.dart b/coinlib/lib/src/tx/sighash/legacy_signature_hasher.dart
index daf0396..1ff2f0d 100644
--- a/coinlib/lib/src/tx/sighash/legacy_signature_hasher.dart
+++ b/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,
+      vExtraData: tx.vExtraData,
     );
 
     // Add sighash type onto the end
diff --git a/coinlib/lib/src/tx/transaction.dart b/coinlib/lib/src/tx/transaction.dart
index aefd7bb..36f8049 100644
--- a/coinlib/lib/src/tx/transaction.dart
+++ b/coinlib/lib/src/tx/transaction.dart
@@ -47,6 +47,7 @@ class Transaction with Writable {
   final List<Input> inputs;
   final List<Output> outputs;
   final Locktime locktime;
+  final Uint8List? vExtraData;
 
   /// Constructs a transaction with the given [inputs] and [outputs].
   /// [TransactionTooLarge] will be thrown if the resulting transction exceeds
@@ -59,6 +60,7 @@ class Transaction with Writable {
     required Iterable<Input> inputs,
     required Iterable<Output> outputs,
     this.locktime = Locktime.zero,
+    this.vExtraData,
   })
   : inputs = List.unmodifiable(inputs),
   outputs = List.unmodifiable(outputs)
@@ -187,6 +189,10 @@ class Transaction with Writable {
 
     writer.writeUInt32(locktime.value);
 
+    if (vExtraData != null) {
+      writer.writeVarSlice(vExtraData!);
+    }
+
   }
 
   Transaction _newInputs(List<Input> newInputs) => Transaction(
@@ -194,6 +200,7 @@ class Transaction with Writable {
     inputs: newInputs,
     outputs: outputs,
     locktime: locktime,
+    vExtraData: vExtraData,
   );
 
   T _requireInputOfType<T>(int inputN) {
@@ -346,6 +353,7 @@ class Transaction with Writable {
     ],
     outputs: outputs,
     locktime: locktime,
+    vExtraData: vExtraData,
   );
 
   /// Returns a new [Transaction] with the [output] added to the end of the
@@ -370,6 +378,7 @@ class Transaction with Writable {
       inputs: modifiedInputs,
       outputs: [...outputs, output],
       locktime: locktime,
+      vExtraData: vExtraData,
     );
 
   }
@@ -394,6 +403,7 @@ class Transaction with Writable {
       ),
       outputs: outputs,
       locktime: locktime,
+      vExtraData: vExtraData,
     )
     : this;