wallet: randomise locktime of transactions a bit. also check if stale. (#4967)

electrum/blockchain.py

@@ -426,6 +426,11 @@ class Blockchain(util.PrintError):
             return None
         return deserialize_header(h, height)
 
+    def header_at_tip(self) -> Optional[dict]:
+        """Return latest header."""
+        height = self.height()
+        return self.read_header(height)
+
     def get_hash(self, height: int) -> str:
         def is_height_checkpoint():
             within_cp_range = height <= constants.net.max_checkpoint()

electrum/tests/test_wallet_vertical.py

@@ -1033,7 +1033,6 @@ class TestWalletSending(TestCaseForTestnet):
 
         class NetworkMock:
             relay_fee = 1000
-            def get_local_height(self): return 1325785
             def run_from_another_thread(self, coro):
                 loop = asyncio.get_event_loop()
                 return loop.run_until_complete(coro)
@@ -1046,7 +1045,7 @@ class TestWalletSending(TestCaseForTestnet):
         privkeys = ['93NQ7CFbwTPyKDJLXe97jczw33fiLijam2SCZL3Uinz1NSbHrTu', ]
         network = NetworkMock()
         dest_addr = 'tb1q3ws2p0qjk5vrravv065xqlnkckvzcpclk79eu2'
-        tx = sweep(privkeys, network, config=None, recipient=dest_addr, fee=5000)
+        tx = sweep(privkeys, network, config=None, recipient=dest_addr, fee=5000, locktime=1325785)
 
         tx_copy = Transaction(tx.serialize())
         self.assertEqual('010000000129349e5641d79915e9d0282fdbaee8c3df0b6731bab9d70bf626e8588bde24ac010000004847304402206bf0d0a93abae0d5873a62ebf277a5dd2f33837821e8b93e74d04e19d71b578002201a6d729bc159941ef5c4c9e5fe13ece9fc544351ba531b00f68ba549c8b38a9a01fdffffff01b82e0f00000000001600148ba0a0bc12b51831f58c7ea8607e76c5982c071fd93a1400',

electrum/wallet.py

@@ -125,7 +125,8 @@ def sweep_preparations(privkeys, network: 'Network', imax=100):
     return inputs, keypairs
 
 
-def sweep(privkeys, network: 'Network', config: 'SimpleConfig', recipient, fee=None, imax=100):
+def sweep(privkeys, network: 'Network', config: 'SimpleConfig', recipient, fee=None, imax=100,
+          *, locktime=None):
     inputs, keypairs = sweep_preparations(privkeys, network, imax)
     total = sum(i.get('value') for i in inputs)
     if fee is None:
@@ -138,7 +139,8 @@ def sweep(privkeys, network: 'Network', config: 'SimpleConfig', recipient, fee=N
         raise Exception(_('Not enough funds on address.') + '\nTotal: %d satoshis\nFee: %d\nDust Threshold: %d'%(total, fee, dust_threshold(network)))
 
     outputs = [TxOutput(TYPE_ADDRESS, recipient, total - fee)]
-    locktime = network.get_local_height()
+    if locktime is None:
+        locktime = get_locktime_for_new_transaction(network)
 
     tx = Transaction.from_io(inputs, outputs, locktime=locktime)
     tx.set_rbf(True)
@@ -146,6 +148,26 @@ def sweep(privkeys, network: 'Network', config: 'SimpleConfig', recipient, fee=N
     return tx
 
 
+def get_locktime_for_new_transaction(network: 'Network') -> int:
+    # if no network or not up to date, just set locktime to zero
+    if not network:
+        return 0
+    chain = network.blockchain()
+    header = chain.header_at_tip()
+    if not header:
+        return 0
+    STALE_DELAY = 8 * 60 * 60  # in seconds
+    if header['timestamp'] + STALE_DELAY < time.time():
+        return 0
+    # discourage "fee sniping"
+    locktime = chain.height()
+    # sometimes pick locktime a bit further back, to help privacy
+    # of setups that need more time (offline/multisig/coinjoin/...)
+    if random.randint(0, 9) == 0:
+        locktime = max(0, locktime - random.randint(0, 99))
+    return locktime
+
+
 
 class CannotBumpFee(Exception): pass
 
@@ -692,7 +714,7 @@ class Abstract_Wallet(AddressSynchronizer):
             tx = Transaction.from_io(coins, outputs[:])
 
         # Timelock tx to current height.
-        tx.locktime = self.get_local_height()
+        tx.locktime = get_locktime_for_new_transaction(self.network)
         run_hook('make_unsigned_transaction', self, tx)
         return tx
 
@@ -794,7 +816,7 @@ class Abstract_Wallet(AddressSynchronizer):
                     continue
         if delta > 0:
             raise CannotBumpFee(_('Cannot bump fee') + ': ' + _('could not find suitable outputs'))
-        locktime = self.get_local_height()
+        locktime = get_locktime_for_new_transaction(self.network)
         tx_new = Transaction.from_io(inputs, outputs, locktime=locktime)
         return tx_new
 
@@ -814,7 +836,7 @@ class Abstract_Wallet(AddressSynchronizer):
         inputs = [item]
         out_address = self.get_unused_address() or address
         outputs = [TxOutput(TYPE_ADDRESS, out_address, value - fee)]
-        locktime = self.get_local_height()
+        locktime = get_locktime_for_new_transaction(self.network)
         return Transaction.from_io(inputs, outputs, locktime=locktime)
 
     def add_input_sig_info(self, txin, address):