Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations

The lock-time code currently uses CBlock::nTime as the cutoff point for time based locked transactions. This has the unfortunate outcome of creating a perverse incentive for miners to lie about the time of a block in order to collect more fees by including transactions that by wall clock determination have not yet matured. By using CBlockIndex::GetMedianTimePast from the prior block instead, the self-interested miner no longer gains from generating blocks with fraudulent timestamps. Users can compensate for this change by simply adding an hour (3600 seconds) to their time-based lock times.

If enforced, this would be a soft-fork change. This commit only adds the functionality on an unexecuted code path, without changing the behaviour of Bitcoin Core.

src/main.cpp

@@ -775,7 +775,9 @@ int64_t CheckLockTime(const CTransaction &tx, int flags)
     // Timestamps on the other hand don't get any special treatment,
     // because we can't know what timestamp the next block will have,
     // and there aren't timestamp applications where it matters.
-    const int64_t nBlockTime = GetAdjustedTime();
+    int64_t nBlockTime = GetAdjustedTime();
+    if (flags & LOCKTIME_MEDIAN_TIME_PAST)
+        nBlockTime -= ((CBlockIndex::nMedianTimeSpan + 1) >> 1) * Params().GetConsensus().nPowTargetSpacing;
 
     return LockTime(tx, flags, pCoinsView, nBlockHeight, nBlockTime);
 }
@@ -1051,10 +1053,15 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         view.SetBackend(dummy);
         }
 
+        int nLockTimeFlags = 0;
+        int64_t nLockTimeCutoff = (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST)
+                                ? chainActive.Tip()->GetMedianTimePast()
+                                : GetAdjustedTime();
+
         // Only accept nLockTime-using transactions that can be mined in the next
         // block; we don't want our mempool filled up with transactions that can't
         // be mined yet.
-        if (LockTime(tx, 0, &view, chainActive.Height() + 1, GetAdjustedTime()))
+        if (LockTime(tx, nLockTimeFlags, &view, chainActive.Height() + 1, nLockTimeCutoff))
             return state.DoS(0, error("AcceptToMemoryPool: non-final"),
                              REJECT_NONSTANDARD, "non-final");
 
@@ -2862,7 +2869,10 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
     // Check that all transactions are finalized
     BOOST_FOREACH(const CTransaction& tx, block.vtx) {
         int nLockTimeFlags = 0;
-        if (LockTime(tx, nLockTimeFlags, pcoinsTip, nHeight, block.GetBlockTime()))
+        int64_t nLockTimeCutoff = (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST)
+                                ? pindexPrev->GetMedianTimePast()
+                                : block.GetBlockTime();
+        if (LockTime(tx, nLockTimeFlags, pcoinsTip, nHeight, nLockTimeCutoff))
             return state.DoS(10, error("%s: contains a non-final transaction", __func__), REJECT_INVALID, "bad-txns-nonfinal");
     }
 

src/main.h

@@ -331,6 +331,9 @@ bool IsStandardTx(const CTransaction& tx, std::string& reason);
 enum {
     /* Interpret sequence numbers as relative lock-time constraints. */
     LOCKTIME_VERIFY_SEQUENCE = (1 << 0),
+
+    /* Use GetMedianTimePast() instead of nTime for end point timestamp. */
+    LOCKTIME_MEDIAN_TIME_PAST = (1 << 1),
 };
 
 /**

src/miner.cpp

@@ -145,6 +145,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         CBlockIndex* pindexPrev = chainActive.Tip();
         const int nHeight = pindexPrev->nHeight + 1;
         pblock->nTime = GetAdjustedTime();
+        const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();
         CCoinsViewCache view(pcoinsTip);
 
         // Priority order to process transactions
@@ -159,7 +160,13 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
              mi != mempool.mapTx.end(); ++mi)
         {
             const CTransaction& tx = mi->second.GetTx();
-            if (tx.IsCoinBase() || LockTime(tx, 0, &view, nHeight, pblock->nTime))
+
+            int nLockTimeFlags = 0;
+            int64_t nLockTimeCutoff = (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST)
+                                    ? nMedianTimePast
+                                    : pblock->GetBlockTime();
+
+            if (tx.IsCoinBase() || LockTime(tx, nLockTimeFlags, &view, nHeight, nLockTimeCutoff))
                 continue;
 
             COrphan* porphan = NULL;