Merge pull request #3883 from dgenr8/first_double_spend

Relay and alert user to double spends

Author: gavinandresen

contrib/debian/manpages/bitcoin-qt.1

@@ -139,6 +139,9 @@ Execute command when the best block changes (%s in cmd is replaced by block hash
 \fB\-walletnotify=\fR<cmd>
 Execute command when a wallet transaction changes (%s in cmd is replaced by TxID)
 .TP
+\fB\-respendnotify=\fR<cmd>
+Execute command when a network tx respends wallet tx input (%s=respend TxID, %t=wallet TxID)
+.TP
 \fB\-alertnotify=\fR<cmd>
 Execute command when a relevant alert is received (%s in cmd is replaced by message)
 .TP

doc/release-notes.md

@@ -19,3 +19,49 @@ estimate.
 Statistics used to estimate fees and priorities are saved in the
 data directory in the 'fee_estimates.dat' file just before
 program shutdown, and are read in at startup.
+
+Double-Spend Relay and Alerts
+=============================
+VERY IMPORTANT: *It has never been safe, and remains unsafe, to rely*
+*on unconfirmed transactions.*
+
+Relay
+-----
+When an attempt is seen on the network to spend the same unspent funds
+more than once, it is no longer ignored.  Instead, it is broadcast, to
+serve as an alert.  This broadcast is subject to protections against
+denial-of-service attacks.
+
+Wallets and other bitcoin services should alert their users to
+double-spends that affect them.  Merchants and other users may have
+enough time to withhold goods or services when payment becomes
+uncertain, until confirmation.
+
+Bitcoin Core Wallet Alerts
+--------------------------
+The Bitcoin Core wallet now makes respend attempts visible in several
+ways.
+
+If you are online, and a respend affecting one of your wallet
+transactions is seen, a notification is immediately issued to the
+command registered with `-respendnotify=<cmd>`.  Additionally, if
+using the GUI:
+ - An alert box is immediately displayed.
+ - The affected wallet transaction is highlighted in red until it is
+   confirmed (and it may never be confirmed).
+
+A `respendsobserved` array is added to `gettransaction`, `listtransactions`,
+and `listsinceblock` RPC results.
+
+Warning
+-------
+*If you rely on an unconfirmed transaction, these change do VERY*
+*LITTLE to protect you from a malicious double-spend, because:*
+
+ - You may learn about the respend too late to avoid doing whatever
+   you were being paid for
+ - Using other relay rules, a double-spender can craft his crime to
+   resist broadcast
+ - Miners can choose which conflicting spend to confirm, and some
+   miners may not confirm the first acceptable spend they see
+

src/bloom.cpp

@@ -94,6 +94,13 @@ bool CBloomFilter::contains(const uint256& hash) const
     return contains(data);
 }
 
+void CBloomFilter::clear()
+{
+    vData.assign(vData.size(),0);
+    isFull = false;
+    isEmpty = true;
+}
+
 bool CBloomFilter::IsWithinSizeConstraints() const
 {
     return vData.size() <= MAX_BLOOM_FILTER_SIZE && nHashFuncs <= MAX_HASH_FUNCS;

src/bloom.h

@@ -78,6 +78,8 @@ class CBloomFilter
     bool contains(const COutPoint& outpoint) const;
     bool contains(const uint256& hash) const;
 
+    void clear();
+
     // True if the size is <= MAX_BLOOM_FILTER_SIZE and the number of hash functions is <= MAX_HASH_FUNCS
     // (catch a filter which was just deserialized which was too big)
     bool IsWithinSizeConstraints() const;

src/core.cpp

@@ -119,6 +119,22 @@ CTransaction& CTransaction::operator=(const CTransaction &tx) {
     return *this;
 }
 
+bool CTransaction::IsEquivalentTo(const CTransaction& tx) const
+{
+    if (nVersion   != tx.nVersion   ||
+        nLockTime  != tx.nLockTime  ||
+        vin.size() != tx.vin.size() ||
+        vout       != tx.vout)
+        return false;
+    for (unsigned int i = 0; i < vin.size(); i++)
+    {
+        if (vin[i].nSequence != tx.vin[i].nSequence ||
+            vin[i].prevout   != tx.vin[i].prevout)
+            return false;
+    }
+    return true;
+}
+
 int64_t CTransaction::GetValueOut() const
 {
     int64_t nValueOut = 0;

src/core.h

@@ -256,6 +256,9 @@ class CTransaction
         return hash;
     }
 
+    // True if only scriptSigs are different
+    bool IsEquivalentTo(const CTransaction& tx) const;
+
     // Return sum of txouts.
     int64_t GetValueOut() const;
     // GetValueIn() is a method on CCoinsViewCache, because

src/init.cpp

@@ -260,6 +260,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += "  -upgradewallet         " + _("Upgrade wallet to latest format") + " " + _("on startup") + "\n";
     strUsage += "  -wallet=<file>         " + _("Specify wallet file (within data directory)") + " " + _("(default: wallet.dat)") + "\n";
     strUsage += "  -walletnotify=<cmd>    " + _("Execute command when a wallet transaction changes (%s in cmd is replaced by TxID)") + "\n";
+    strUsage += "  -respendnotify=<cmd>   " + _("Execute command when a network tx respends wallet tx input (%s=respend TxID, %t=wallet TxID)") + "\n";
     strUsage += "  -zapwallettxes=<mode>  " + _("Delete all wallet transactions and only recover those part of the blockchain through -rescan on startup") + "\n";
     strUsage += "                         " + _("(default: 1, 1 = keep tx meta data e.g. account owner and payment request information, 2 = drop tx meta data)") + "\n";
 #endif
@@ -1175,6 +1176,7 @@ bool AppInit2(boost::thread_group& threadGroup)
     LogPrintf("mapAddressBook.size() = %u\n",  pwalletMain ? pwalletMain->mapAddressBook.size() : 0);
 #endif
 
+    RegisterInternalSignals();
     StartNode(threadGroup);
     if (fServer)
         StartRPCThreads();

src/main.cpp

@@ -7,6 +7,7 @@
 
 #include "addrman.h"
 #include "alert.h"
+#include "bloom.h"
 #include "chainparams.h"
 #include "checkpoints.h"
 #include "checkqueue.h"
@@ -124,6 +125,10 @@ namespace {
 
 } // anon namespace
 
+// Forward reference functions defined here:
+static const unsigned int MAX_DOUBLESPEND_BLOOM = 1000;
+static void RelayDoubleSpend(const COutPoint& outPoint, const CTransaction& doubleSpend, bool fInBlock, CBloomFilter& filter);
+
 //////////////////////////////////////////////////////////////////////////////
 //
 // dispatching functions
@@ -146,10 +151,25 @@ struct CMainSignals {
     boost::signals2::signal<void (const uint256 &)> Inventory;
     // Tells listeners to broadcast their data.
     boost::signals2::signal<void ()> Broadcast;
+    // Notifies listeners of detection of a double-spent transaction. Arguments are outpoint that is
+    // double-spent, first transaction seen, double-spend transaction, and whether the second double-spend
+    // transaction was first seen in a block.
+    // Note: only notifies if the previous transaction is in the memory pool; if previous transction was in a block,
+    // then the double-spend simply fails when we try to lookup the inputs in the current UTXO set.
+    boost::signals2::signal<void (const COutPoint&, const CTransaction&, bool)> DetectedDoubleSpend;
 } g_signals;
 
 } // anon namespace
 
+void RegisterInternalSignals() {
+    static CBloomFilter doubleSpendFilter;
+    seed_insecure_rand();
+    doubleSpendFilter = CBloomFilter(MAX_DOUBLESPEND_BLOOM, 0.01, insecure_rand(), BLOOM_UPDATE_NONE);
+
+    g_signals.DetectedDoubleSpend.connect(boost::bind(RelayDoubleSpend, _1, _2, _3, doubleSpendFilter));
+}
+
+
 void RegisterWallet(CWalletInterface* pwalletIn) {
     g_signals.SyncTransaction.connect(boost::bind(&CWalletInterface::SyncTransaction, pwalletIn, _1, _2));
     g_signals.EraseTransaction.connect(boost::bind(&CWalletInterface::EraseFromWallet, pwalletIn, _1));
@@ -872,6 +892,21 @@ int64_t GetMinFee(const CTransaction& tx, unsigned int nBytes, bool fAllowFree,
     return nMinFee;
 }
 
+// Exponentially limit the rate of nSize flow to nLimit.  nLimit unit is thousands-per-minute.
+bool RateLimitExceeded(double& dCount, int64_t& nLastTime, int64_t nLimit, unsigned int nSize)
+{
+    static CCriticalSection csLimiter;
+    int64_t nNow = GetTime();
+
+    LOCK(csLimiter);
+
+    dCount *= pow(1.0 - 1.0/600.0, (double)(nNow - nLastTime));
+    nLastTime = nNow;
+    if (dCount >= nLimit*10*1000)
+        return true;
+    dCount += nSize;
+    return false;
+}
 
 bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransaction &tx, bool fLimitFree,
                         bool* pfMissingInputs, bool fRejectInsaneFee)
@@ -906,9 +941,10 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     for (unsigned int i = 0; i < tx.vin.size(); i++)
     {
         COutPoint outpoint = tx.vin[i].prevout;
-        if (pool.mapNextTx.count(outpoint))
+        // Does tx conflict with a member of the pool, and is it not equivalent to that member?
+        if (pool.mapNextTx.count(outpoint) && !tx.IsEquivalentTo(*pool.mapNextTx[outpoint].ptx))
         {
-            // Disable replacement feature for now
+            g_signals.DetectedDoubleSpend(outpoint, tx, false);
             return false;
         }
     }
@@ -980,23 +1016,15 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         // be annoying or make others' transactions take longer to confirm.
         if (fLimitFree && nFees < CTransaction::minRelayTxFee.GetFee(nSize))
         {
-            static CCriticalSection csFreeLimiter;
             static double dFreeCount;
-            static int64_t nLastTime;
-            int64_t nNow = GetTime();
-
-            LOCK(csFreeLimiter);
+            static int64_t nLastFreeTime;
+            static int64_t nFreeLimit = GetArg("-limitfreerelay", 15);
 
-            // Use an exponentially decaying ~10-minute window:
-            dFreeCount *= pow(1.0 - 1.0/600.0, (double)(nNow - nLastTime));
-            nLastTime = nNow;
-            // -limitfreerelay unit is thousand-bytes-per-minute
-            // At default rate it would take over a month to fill 1GB
-            if (dFreeCount >= GetArg("-limitfreerelay", 15)*10*1000)
+            if (RateLimitExceeded(dFreeCount, nLastFreeTime, nFreeLimit, nSize))
                 return state.DoS(0, error("AcceptToMemoryPool : free transaction rejected by rate limiter"),
                                  REJECT_INSUFFICIENTFEE, "insufficient priority");
+
             LogPrint("mempool", "Rate limit dFreeCount: %g => %g\n", dFreeCount, dFreeCount+nSize);
-            dFreeCount += nSize;
         }
 
         if (fRejectInsaneFee && nFees > CTransaction::minRelayTxFee.GetFee(nSize) * 10000)
@@ -1019,6 +1047,48 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     return true;
 }
 
+static void RelayDoubleSpend(const COutPoint& outPoint, const CTransaction& doubleSpend, bool fInBlock, CBloomFilter& filter)
+{
+    // Relaying double-spend attempts to our peers lets them detect when
+    // somebody might be trying to cheat them. However, blindly relaying
+    // every double-spend across the entire network gives attackers
+    // a denial-of-service attack: just generate a stream of double-spends
+    // re-spending the same (limited) set of outpoints owned by the attacker.
+    // So, we use a bloom filter and only relay (at most) the first double
+    // spend for each outpoint. False-positives ("we have already relayed")
+    // are OK, because if the peer doesn't hear about the double-spend
+    // from us they are very likely to hear about it from another peer, since
+    // each peer uses a different, randomized bloom filter.
+
+    if (fInBlock || filter.contains(outPoint)) return;
+
+    // Apply an independent rate limit to double-spend relays
+    static double dRespendCount;
+    static int64_t nLastRespendTime;
+    static int64_t nRespendLimit = GetArg("-limitrespendrelay", 100);
+    unsigned int nSize = ::GetSerializeSize(doubleSpend, SER_NETWORK, PROTOCOL_VERSION);
+
+    if (RateLimitExceeded(dRespendCount, nLastRespendTime, nRespendLimit, nSize))
+    {
+        LogPrint("mempool", "Double-spend relay rejected by rate limiter\n");
+        return;
+    }
+
+    LogPrint("mempool", "Rate limit dRespendCount: %g => %g\n", dRespendCount, dRespendCount+nSize);
+
+    // Clear the filter on average every MAX_DOUBLE_SPEND_BLOOM
+    // insertions
+    if (insecure_rand()%MAX_DOUBLESPEND_BLOOM == 0)
+        filter.clear();
+
+    filter.insert(outPoint);
+
+    RelayTransaction(doubleSpend);
+
+    // Share conflict with wallet
+    g_signals.SyncTransaction(doubleSpend, NULL);
+}
+
 
 int CMerkleTx::GetDepthInMainChainINTERNAL(CBlockIndex* &pindexRet) const
 {

src/main.h

@@ -108,6 +108,9 @@ struct CNodeStateStats;
 
 struct CBlockTemplate;
 
+/** Set up internal signal handlers **/
+void RegisterInternalSignals();
+
 /** Register a wallet to receive updates from core */
 void RegisterWallet(CWalletInterface* pwalletIn);
 /** Unregister a wallet from core */

src/qt/guiconstants.h

@@ -23,6 +23,10 @@ static const int STATUSBAR_ICONSIZE = 16;
 #define COLOR_NEGATIVE QColor(255, 0, 0)
 /* Transaction list -- bare address (without label) */
 #define COLOR_BAREADDRESS QColor(140, 140, 140)
+/* Transaction list -- has conflicting transactions */
+#define COLOR_HASCONFLICTING Qt::white;
+/* Transaction list -- has conflicting transactions - background */
+#define COLOR_HASCONFLICTING_BG QColor(192, 0, 0)
 
 /* Tooltips longer than this (in characters) are converted into rich text,
    so that they can be word-wrapped.