Reject ztxs with duplicate zkproofs

This is a greatly simplified and slightly tweaked version of
af2e3713e2

Their version will detect duplicate zkproofs across transactions while
this code will only detect duplicate zkproofs in a single ztx. If dupes
are found, the tx will be denied entry into the mempool.

This provides most of the benefit (increased CPU cost to attackers) with the
least code change and no annoyance to full node operators. Detecting
duplicate zkproofs across transactions requires a one-time reindex of
all of history, which means significant downtime for nodes.

Since Hush + HSCs have a much more strict policy on number of shielded
outputs and shielded inputs, only detecting duplicate zkproofs in
individual ztxs seems sufficient for now.

No correctly functioning node or wallet will ever create duplicate
zkproofs, so there is no worry of this accidentally affecting normal
users. Currently this is not a consensus rule but it could become one
in the future.

src/consensus/validation.h

@@ -29,6 +29,8 @@ static const unsigned char REJECT_MALFORMED = 0x01;
 static const unsigned char REJECT_INVALID = 0x10;
 static const unsigned char REJECT_OBSOLETE = 0x11;
 static const unsigned char REJECT_DUPLICATE = 0x12;
+static const unsigned char REJECT_DUPLICATE_OUTPUT_PROOF = 0x13;
+static const unsigned char REJECT_DUPLICATE_SPEND_PROOF  = 0x14;
 static const unsigned char REJECT_NONSTANDARD = 0x40;
 static const unsigned char REJECT_DUST = 0x41;
 static const unsigned char REJECT_INSUFFICIENTFEE = 0x42;

src/main.cpp

@@ -1751,7 +1751,31 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     {
         return error("AcceptToMemoryPool: CheckTransaction failed");
     }
-    
+
+    // Reject duplicate output proofs in a single ztx in mempool
+    // Migrate this to CheckTransaction() to make it a consensus requirement
+    {
+        set<libzcash::GrothProof> vSaplingOutputProof;
+        BOOST_FOREACH(const OutputDescription& output, tx.vShieldedOutput)
+        {
+            if (vSaplingOutputProof.count(output.zkproof))
+                return state.Invalid(error("AcceptToMemoryPool: duplicate output proof"),REJECT_DUPLICATE_OUTPUT_PROOF, "bad-txns-duplicate-output-proof");
+            vSaplingOutputProof.insert(output.zkproof);
+        }
+    }
+
+    // Reject duplicate spend proofs in a single ztx in mempool
+    // Migrate this to CheckTransaction() to make it a consensus requirement
+    {
+        set<libzcash::GrothProof> vSaplingSpendProof;
+        BOOST_FOREACH(const SpendDescription& spend, tx.vShieldedSpend)
+        {
+            if (vSaplingSpendProof.count(spend.zkproof))
+                return state.Invalid(error("AcceptToMemoryPool: duplicate spend proof"),REJECT_DUPLICATE_SPEND_PROOF, "bad-txns-duplicate-spend-proof");
+            vSaplingSpendProof.insert(spend.zkproof);
+        }
+    }
+
     // DoS level set to 10 to be more forgiving.
     // Check transaction contextually against the set of consensus rules which apply in the next block to be mined.
     if (!ContextualCheckTransaction(0,0,0,tx, state, nextBlockHeight, (dosLevel == -1) ? 10 : dosLevel))