Set anti-fee-sniping locktime on bitcoin sends

Tip + ~10% backdate (Core DiscourageFeeSniping) via shared helper, wired into normal sends (path A) and payjoin/PSBT (path B). 0 when unsynced.

Author: bc1cindy

cw_bitcoin/lib/bitcoin_wallet.dart

@@ -14,6 +14,7 @@ import 'package:cw_bitcoin/electrum_derivations.dart';
 import 'package:cw_bitcoin/electrum_transaction_info.dart';
 import 'package:cw_bitcoin/electrum_wallet.dart';
 import 'package:cw_bitcoin/electrum_wallet_snapshot.dart';
+import 'package:cw_bitcoin/locktime.dart';
 import 'package:cw_bitcoin/hardware/bitcoin_hardware_wallet_service.dart';
 import 'package:cw_bitcoin/lightning/lightning_wallet.dart';
 import 'package:cw_bitcoin/hardware/bitcoin_ledger_service.dart';
@@ -30,6 +31,7 @@ import 'package:cw_core/encryption_file_utils.dart';
 import 'package:cw_core/output_info.dart';
 import 'package:cw_core/payjoin_session.dart';
 import 'package:cw_core/pending_transaction.dart';
+import 'package:cw_core/sync_status.dart';
 import 'package:cw_core/unspent_coin_type.dart';
 import 'package:cw_core/unspent_coins_info.dart';
 import 'package:cw_core/utils/print_verbose.dart';
@@ -442,8 +444,17 @@ abstract class BitcoinWalletBase extends ElectrumWallet with Store {
       ));
     }
 
+    final locktime = antiFeeSnipingLocktime(
+      chainTip: await getCurrentChainTip(),
+      synced: syncStatus is SyncedSyncStatus,
+    );
+
     return PSBTTransactionBuild(
-            inputs: psbtReadyInputs, outputs: outputs, enableRBF: enableRBF, cwOutputs: cwOutputs)
+            inputs: psbtReadyInputs,
+            outputs: outputs,
+            enableRBF: enableRBF,
+            cwOutputs: cwOutputs,
+            locktime: locktime)
         .psbt;
   }
 

cw_bitcoin/lib/electrum_wallet.dart

@@ -5,6 +5,7 @@ import 'dart:isolate';
 
 import 'package:bitcoin_base/bitcoin_base.dart';
 import 'package:cw_bitcoin/lightning/lightning_wallet.dart';
+import 'package:cw_bitcoin/locktime.dart';
 import 'package:cw_core/hardware/hardware_wallet_service.dart';
 import 'package:cw_core/root_dir.dart';
 import 'package:cw_core/utils/proxy_wrapper.dart';
@@ -459,6 +460,15 @@ abstract class ElectrumWalletBase
     return currentChainTip ?? 0;
   }
 
+  /// Anti-fee-sniping locktime (Core `DiscourageFeeSniping`), LE-encoded for
+  /// `BitcoinTransactionBuilder`.
+  Future<List<int>> _antiFeeSnipingLocktime() async {
+    return locktimeToBytes(antiFeeSnipingLocktime(
+      chainTip: await getCurrentChainTip(),
+      synced: syncStatus is SyncedSyncStatus,
+    ));
+  }
+
   @override
   BitcoinWalletKeys get keys {
     String? wif;
@@ -1521,6 +1531,8 @@ abstract class ElectrumWalletBase
           });
       }
 
+      final locktime = await _antiFeeSnipingLocktime();
+
       BasedBitcoinTransacationBuilder txb;
       if (network is BitcoinCashNetwork) {
         txb = ForkedTransactionBuilder(
@@ -1541,6 +1553,7 @@ abstract class ElectrumWalletBase
           memo: estimatedTx.memo,
           outputOrdering: BitcoinOrdering.none,
           enableRBF: !estimatedTx.spendsUnconfirmedTX,
+          locktime: locktime,
         );
       }
 
@@ -2331,6 +2344,7 @@ abstract class ElectrumWalletBase
         memo: memo,
         outputOrdering: BitcoinOrdering.none,
         enableRBF: true,
+        locktime: await _antiFeeSnipingLocktime(),
       );
 
       final transaction = txb.buildTransaction((txDigest, utxo, publicKey, sighash) {

cw_bitcoin/lib/locktime.dart

@@ -0,0 +1,33 @@
+import 'dart:math';
+
+typedef RandomBelow = int Function(int max);
+
+final Random _rng = Random();
+int _defaultRandomBelow(int max) => _rng.nextInt(max);
+
+/// Bitcoin Core `DiscourageFeeSniping` locktime policy.
+///
+/// locktime = chainTip; ~10% of the time backdate by rand(0..99), floored at 0.
+/// Not synced or unknown tip returns 0 to avoid emitting a unique stale-height
+/// fingerprint.
+int antiFeeSnipingLocktime({
+  required int chainTip,
+  required bool synced,
+  RandomBelow? randomBelow,
+}) {
+  final rand = randomBelow ?? _defaultRandomBelow;
+  if (!synced || chainTip <= 0) return 0;
+  var locktime = chainTip;
+  if (rand(10) == 0) {
+    locktime = max(0, locktime - rand(100));
+  }
+  return locktime;
+}
+
+/// Little-endian 4-byte encoding for `BtcTransaction.locktime`.
+List<int> locktimeToBytes(int locktime) => [
+      locktime & 0xff,
+      (locktime >> 8) & 0xff,
+      (locktime >> 16) & 0xff,
+      (locktime >> 24) & 0xff,
+    ];

cw_bitcoin/lib/psbt/transaction_builder.dart

@@ -11,8 +11,9 @@ class PSBTTransactionBuild {
   final PsbtV2 psbt = PsbtV2();
 
   PSBTTransactionBuild(
-      {required List<PSBTReadyUtxoWithAddress> inputs, required List<BitcoinBaseOutput> outputs, required List<OutputInfo> cwOutputs, bool enableRBF = true}) {
+      {required List<PSBTReadyUtxoWithAddress> inputs, required List<BitcoinBaseOutput> outputs, required List<OutputInfo> cwOutputs, bool enableRBF = true, int locktime = 0}) {
     psbt.setGlobalTxVersion(2);
+    psbt.setGlobalFallbackLocktime(locktime);
     psbt.setGlobalInputCount(inputs.length);
     psbt.setGlobalOutputCount(outputs.length);
 

cw_bitcoin/test/locktime_test.dart

@@ -0,0 +1,50 @@
+import 'package:cw_bitcoin/locktime.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('antiFeeSnipingLocktime', () {
+    test('returns 0 when not synced', () {
+      expect(antiFeeSnipingLocktime(chainTip: 800000, synced: false), 0);
+    });
+
+    test('returns 0 when chainTip <= 0', () {
+      expect(antiFeeSnipingLocktime(chainTip: 0, synced: true), 0);
+    });
+
+    test('returns tip when the 1-in-10 branch does not fire', () {
+      expect(
+        antiFeeSnipingLocktime(chainTip: 800000, synced: true, randomBelow: (_) => 1),
+        800000,
+      );
+    });
+
+    test('backdates by rand(0..99) when the 1-in-10 branch fires', () {
+      var calls = 0;
+      int rng(int _) => (calls++ == 0) ? 0 : 37;
+      expect(
+        antiFeeSnipingLocktime(chainTip: 800000, synced: true, randomBelow: rng),
+        799963,
+      );
+    });
+
+    test('backdate floors at 0 for low tips', () {
+      var calls = 0;
+      int rng(int _) => (calls++ == 0) ? 0 : 99;
+      expect(antiFeeSnipingLocktime(chainTip: 50, synced: true, randomBelow: rng), 0);
+    });
+  });
+
+  group('locktimeToBytes', () {
+    test('encodes 0 as four zero bytes', () {
+      expect(locktimeToBytes(0), [0, 0, 0, 0]);
+    });
+
+    test('encodes a block height little-endian', () {
+      expect(locktimeToBytes(800000), [0x00, 0x35, 0x0C, 0x00]);
+    });
+
+    test('encodes max 32-bit value', () {
+      expect(locktimeToBytes(0xFFFFFFFF), [0xFF, 0xFF, 0xFF, 0xFF]);
+    });
+  });
+}