From 9f8836279057dbf2356bd1d8c9d14a4b754e299a Mon Sep 17 00:00:00 2001
From: Stephan February <stephan@frobots.io>
Date: Fri, 13 Oct 2023 12:02:27 +0800
Subject: [PATCH] Bugfix: Bigint amount incorrecly encoded during sighash

- Fixes a bug in the buffer conversion to a bigintLE during sighash preimage calculation
---
 lib/src/encoding/utils.dart            | 13 +++++++++++
 lib/src/sighash.dart                   |  2 +-
 test/transaction/transaction_test.dart | 32 ++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/lib/src/encoding/utils.dart b/lib/src/encoding/utils.dart
index 20d6a50..b029b10 100644
--- a/lib/src/encoding/utils.dart
+++ b/lib/src/encoding/utils.dart
@@ -173,6 +173,19 @@ BigInt readVarInt(Uint8List buffer) {
   }
 }
 
+
+List<int> encodeBigIntLE(BigInt number) {
+  int size = 8;
+
+  var result = Uint8List(size);
+  for (int i = 0; i < size; i++) {
+    result[size - i - 1] = (number & _byteMask).toInt();
+    number = number >> 8;
+  }
+
+  return result.reversed.toList();
+}
+
 Uint8List encodeBigIntSV(BigInt number) {
   int size = (number.bitLength + 7) >> 3;
 
diff --git a/lib/src/sighash.dart b/lib/src/sighash.dart
index 7607f61..9208f74 100644
--- a/lib/src/sighash.dart
+++ b/lib/src/sighash.dart
@@ -324,7 +324,7 @@ class Sighash {
         writer.write(subscript.buffer);
 
         // value of the output spent by this input (8-byte little endian)
-        writer.write(encodeBigIntSV(satoshis));
+        writer.write(encodeBigIntLE(satoshis));
 
         // nSequence of the input (4-byte little endian)
         var sequenceNumber = input.sequenceNumber;
diff --git a/test/transaction/transaction_test.dart b/test/transaction/transaction_test.dart
index 7484a66..d40705a 100644
--- a/test/transaction/transaction_test.dart
+++ b/test/transaction/transaction_test.dart
@@ -880,4 +880,36 @@ main() {
   test('print address', () async {
     print(Address.fromPublicKey(privateKey.publicKey, NetworkType.TEST));
   });
+
+
+  test('must create this expected serialized txn', () async {
+
+  });
+
+  test('verify encoding and field size on output amount during sighash preimage calc ', (){
+    var bobWif = "cStLVGeWx7fVYKKDXYWVeEbEcPZEC4TD73DjQpHCks2Y8EAjVDSS";
+    var fundingTxHex = "02000000010000000000000000000000000000000000000000000000000000000000000000ffffffff03570101ffffffff0100f2052a01000000232103f5e57bb21f772e16eee6593f8b324fae3c226045dfc8e221397a643adf9eff4cac00000000";
+    var expectedTxHex = "0200000001ea5a4152878ad9ff6f464d0415a3404c36c4f67973e137bc0147f5fe7473ddc6000000006b4830450221008a413fca7bf901c86bc29e6032f9bdf66700e3e10e5c44d0e072ae515d12c90702202bf76989fba58e7423cfe2eef85b2cd60abaa6dce98c0795ece5220c98a567a041210330aff1a7e5417393f90eb1bf221c86686e0e3ba25d2696aaa20da549b7d4b3f9ffffffff0200e1f505000000001976a914650c4adb156f19e36a755c820d892cda108299c488acc4101024010000001976a914650c4adb156f19e36a755c820d892cda108299c488ac00000000";
+    var fundingTx = Transaction.fromHex(fundingTxHex);
+
+    var sighashType = SighashType.SIGHASH_FORKID.value | SighashType.SIGHASH_ALL.value;
+    var signingKey = SVPrivateKey.fromWIF(bobWif);
+    var txSigner = TransactionSigner(sighashType, signingKey);
+
+    var ownerAddress = signingKey.toAddress(networkType: NetworkType.TEST);
+    var unlocker = P2PKHUnlockBuilder(signingKey.publicKey);
+    var locker = P2PKHLockBuilder.fromPublicKey(signingKey.publicKey, networkType: NetworkType.TEST);
+    var builder = TransactionBuilder();
+    var signedTx = builder
+        .spendToLockBuilder(locker, BigInt.from(100000000))
+        .spendFromTxnWithSigner(txSigner, fundingTx, 0, TransactionInput.MAX_SEQ_NUMBER, unlocker)
+        .sendChangeToPKH(ownerAddress)
+        .withFeePerKb(1000)
+        .withOption(TransactionOption.DISABLE_DUST_OUTPUTS)
+        .build(true);
+
+    var actualTxHex = signedTx.serialize();
+
+    expect(expectedTxHex, equals(actualTxHex));
+  });
 }
\ No newline at end of file