Fix Art Forz's time-traveller attack, where a 51% attack can change d…

…ifficulty at will.

src/main.cpp

@@ -1637,9 +1637,16 @@ unsigned int static GetNextWorkRequired(const CBlockIndex* pindexLast, const CBl
         else if ( dAdjustmentFactor < kLimiterDown )
             dAdjustmentFactor = kLimiterDown;
     } else {
+        // This fixes an issue where a 51% attack can change difficulty at will.
+        // Go back the full period unless it's the first retarget after genesis.
+        // Code courtesy of Art Forz
+        int blockstogoback = nInterval-1;
+        if ((pindexLast->nHeight+1) != nInterval)
+            blockstogoback = nInterval;
+
         // Go back by what we want to be 14 days worth of blocks
         const CBlockIndex* pindexFirst = pindexLast;
-        for (int i = 0; pindexFirst && i < nInterval-1; i++)
+        for (int i = 0; pindexFirst && i < blockstogoback; i++)
             pindexFirst = pindexFirst->pprev;
         assert(pindexFirst);