Litecoin: Fix zeitgeist2 attack thanks to Lolcust and ArtForz. 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.

src/main.cpp

@@ -1136,9 +1136,15 @@ unsigned int static GetNextWorkRequired(const CBlockIndex* pindexLast, const CBl
         return pindexLast->nBits;
     }
 
+    // Litecoin: 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);