wallet: Avoid leaking locktime fingerprint when anti-fee-sniping

src/wallet/wallet.cpp

@@ -2516,6 +2516,65 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
     return true;
 }
 
+static bool IsCurrentForAntiFeeSniping(interfaces::Chain::Lock& locked_chain)
+{
+    if (IsInitialBlockDownload()) {
+        return false;
+    }
+    constexpr int64_t MAX_ANTI_FEE_SNIPING_TIP_AGE = 8 * 60 * 60; // in seconds
+    if (chainActive.Tip()->GetBlockTime() < (GetTime() - MAX_ANTI_FEE_SNIPING_TIP_AGE)) {
+        return false;
+    }
+    return true;
+}
+
+/**
+ * Return a height-based locktime for new transactions (uses the height of the
+ * current chain tip unless we are not synced with the current chain
+ */
+static uint32_t GetLocktimeForNewTransaction(interfaces::Chain::Lock& locked_chain)
+{
+    uint32_t locktime;
+    // Discourage fee sniping.
+    //
+    // For a large miner the value of the transactions in the best block and
+    // the mempool can exceed the cost of deliberately attempting to mine two
+    // blocks to orphan the current best block. By setting nLockTime such that
+    // only the next block can include the transaction, we discourage this
+    // practice as the height restricted and limited blocksize gives miners
+    // considering fee sniping fewer options for pulling off this attack.
+    //
+    // A simple way to think about this is from the wallet's point of view we
+    // always want the blockchain to move forward. By setting nLockTime this
+    // way we're basically making the statement that we only want this
+    // transaction to appear in the next block; we don't want to potentially
+    // encourage reorgs by allowing transactions to appear at lower heights
+    // than the next block in forks of the best chain.
+    //
+    // Of course, the subsidy is high enough, and transaction volume low
+    // enough, that fee sniping isn't a problem yet, but by implementing a fix
+    // now we ensure code won't be written that makes assumptions about
+    // nLockTime that preclude a fix later.
+    if (IsCurrentForAntiFeeSniping(locked_chain)) {
+        locktime = chainActive.Height();
+
+        // Secondly occasionally randomly pick a nLockTime even further back, so
+        // that transactions that are delayed after signing for whatever reason,
+        // e.g. high-latency mix networks and some CoinJoin implementations, have
+        // better privacy.
+        if (GetRandInt(10) == 0)
+            locktime = std::max(0, (int)locktime - GetRandInt(100));
+    } else {
+        // If our chain is lagging behind, we can't discourage fee sniping nor help
+        // the privacy of high-latency transactions. To avoid leaking a potentially
+        // unique "nLockTime fingerprint", set nLockTime to a constant.
+        locktime = 0;
+    }
+    assert(locktime <= (unsigned int)chainActive.Height());
+    assert(locktime < LOCKTIME_THRESHOLD);
+    return locktime;
+}
+
 OutputType CWallet::TransactionChangeType(OutputType change_type, const std::vector<CRecipient>& vecSend)
 {
     // If -changetype is specified, always use that change type.
@@ -2570,37 +2629,8 @@ bool CWallet::CreateTransaction(interfaces::Chain::Lock& locked_chain, const std
 
     CMutableTransaction txNew;
 
-    // Discourage fee sniping.
+    txNew.nLockTime = GetLocktimeForNewTransaction(locked_chain);
-    //
-    // For a large miner the value of the transactions in the best block and
-    // the mempool can exceed the cost of deliberately attempting to mine two
-    // blocks to orphan the current best block. By setting nLockTime such that
-    // only the next block can include the transaction, we discourage this
-    // practice as the height restricted and limited blocksize gives miners
-    // considering fee sniping fewer options for pulling off this attack.
-    //
-    // A simple way to think about this is from the wallet's point of view we
-    // always want the blockchain to move forward. By setting nLockTime this
-    // way we're basically making the statement that we only want this
-    // transaction to appear in the next block; we don't want to potentially
-    // encourage reorgs by allowing transactions to appear at lower heights
-    // than the next block in forks of the best chain.
-    //
-    // Of course, the subsidy is high enough, and transaction volume low
-    // enough, that fee sniping isn't a problem yet, but by implementing a fix
-    // now we ensure code won't be written that makes assumptions about
-    // nLockTime that preclude a fix later.
-    txNew.nLockTime = chainActive.Height();
 
-    // Secondly occasionally randomly pick a nLockTime even further back, so
-    // that transactions that are delayed after signing for whatever reason,
-    // e.g. high-latency mix networks and some CoinJoin implementations, have
-    // better privacy.
-    if (GetRandInt(10) == 0)
-        txNew.nLockTime = std::max(0, (int)txNew.nLockTime - GetRandInt(100));
-
-    assert(txNew.nLockTime <= (unsigned int)chainActive.Height());
-    assert(txNew.nLockTime < LOCKTIME_THRESHOLD);
     FeeCalculation feeCalc;
     CAmount nFeeNeeded;
     int nBytes;

test/functional/test_runner.py

@@ -174,6 +174,7 @@ BASE_SCRIPTS = [
     'wallet_fallbackfee.py',
     'feature_minchainwork.py',
     'rpc_getblockstats.py',
+    'wallet_create_tx.py',
     'p2p_fingerprint.py',
     'feature_uacomment.py',
     'wallet_coinbase_category.py',

test/functional/wallet_create_tx.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+# Copyright (c) 2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+)
+
+
+class CreateTxWalletTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = False
+        self.num_nodes = 1
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def run_test(self):
+        self.log.info('Check that we have some (old) blocks and that anti-fee-sniping is disabled')
+        assert_equal(self.nodes[0].getblockchaininfo()['blocks'], 200)
+        txid = self.nodes[0].sendtoaddress(self.nodes[0].getnewaddress(), 1)
+        tx = self.nodes[0].decoderawtransaction(self.nodes[0].gettransaction(txid)['hex'])
+        assert_equal(tx['locktime'], 0)
+
+        self.log.info('Check that anti-fee-sniping is enabled when we mine a recent block')
+        self.nodes[0].generate(1)
+        txid = self.nodes[0].sendtoaddress(self.nodes[0].getnewaddress(), 1)
+        tx = self.nodes[0].decoderawtransaction(self.nodes[0].gettransaction(txid)['hex'])
+        assert 0 < tx['locktime'] <= 201
+
+
+if __name__ == '__main__':
+    CreateTxWalletTest().main()