Antispam defenses

This code is inspired by
db292a49dd
with various improvements that will be documented below.

The largest improvement is that this code will defend against a spammer using shielded inputs (zins)
or shielded outputs (zouts) while the Pirate code only defends against zout spam.

We wrote a new RPC called z_getstats to study exactly what the distribution of shielded inputs (zins)
and shielded outputs (zouts) look like on HUSH mainnet. Sietch will never make a ztx that contains
more than 9 zouts and so transactions with 10 or more zouts are extremely rare. They correspond to custom
transactions created via code or CLI or mining pool payouts. We allow at most one of these per block. If
there are two, one will remain in the mempool and be mined in the subsequent block. Our code is more strict,
as Pirate will allow up to 6 of these transactions in a single block.

Transactions with many shielded inputs do occur normally when users spend many small shielded unspent outputs
(zutxos) in one transaction, but we determined that a cutoff of 50 zins is quite rare. Between blocks
14000000 and 15000000 only 27 ztxs had 50 or more zins, which is 0.03% . We allow at most one of these
per block and if there are more, they will wait to be mined in a subsequent block.

Also note that a transaction can match both criteria of having large zins and large zouts, so for instance,
if there is a transaction with 50 zins and 10 zouts, it counts towards both requirements and no other
transactions with >=50 zins or >=10 zouts will be mined in that block.

If >=200 transactions with either large zins or large zouts are broadcast to the network it will take at least
200 blocks for them to be mined and so via existing rules for ztx expiration they will expire and be removed
from the mempool, since by default all ztxs expire after 200 blocks. Since normal ztxs that match these
criteria are very rare, the only case when this might happen is during a spam attack and so the attackers
transactions expiring is another part of these defenses.

Other improvements are that we log txids of transactions with large zins or zouts and we do not support a
command line option to turn this protection off. This forces a potential attacker to compile their own custom
code if they want to subvert these protections on their own node and blocks they mine.

Similar to Pirate, these changes are not consensus changes but may be made consensus requirements
in the future.

These protections are not specific to HUSH and are enabled for all HSC's, including DragonX.

src/miner.cpp

@@ -279,11 +279,16 @@ CBlockTemplate* CreateNewBlock(CPubKey _pk,const CScript& _scriptPubKeyIn, int32
         vecPriority.reserve(mempool.mapTx.size() + 1);
 
         //fprintf(stderr,"%s: going to add txs from mempool\n", __func__);
-        // now add transactions from the mem pool
+        // now add transactions from the mempool
         int32_t Notarizations = 0; uint64_t txvalue;
+        uint32_t large_zins  = 0;                  // number of ztxs with large number of inputs in block
+        uint32_t large_zouts = 0;                  // number of ztxs with large number of outputs in block
+        const uint32_t LARGE_ZINS_MAX        = 1;  // max ztxs with large zins per block
+        const uint32_t LARGE_ZOUTS_MAX       = 1;  // max ztxs with large zouts per block
+        const uint32_t LARGE_ZINS_THRESHOLD  = 50; // min number of zins to be considered large
+        const uint32_t LARGE_ZOUTS_THRESHOLD = 10; // min number of zouts to be considered large
         for (CTxMemPool::indexed_transaction_set::iterator mi = mempool.mapTx.begin();
-             mi != mempool.mapTx.end(); ++mi)
-        {
+             mi != mempool.mapTx.end(); ++mi) {
             const CTransaction& tx = mi->GetTx();
 
             int64_t nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
@@ -466,6 +471,18 @@ CBlockTemplate* CreateNewBlock(CPubKey _pk,const CScript& _scriptPubKeyIn, int32
             // fprintf(stderr,"%s: compared first tx from priority queue\n", __func__);
             vecPriority.pop_back();
 
+            if(tx.vShieldedSpend.size() >= LARGE_ZINS_THRESHOLD && large_zins >= LARGE_ZINS_MAX) {
+                LogPrintf("%s: skipping ztx %s with %d zins because there are already %d ztxs with large zins\n",
+                    __func__, tx.GetHash().ToString().c_str(), tx.vShieldedSpend.size(), LARGE_ZINS_MAX);
+                continue;
+            }
+
+            if(tx.vShieldedOutput.size() >= LARGE_ZOUTS_THRESHOLD && large_zouts >= LARGE_ZOUTS_MAX) {
+                LogPrintf("%s: skipping ztx %s with %d zouts because there are already %d ztxs with large zouts\n",
+                    __func__, tx.GetHash().ToString().c_str(), tx.vShieldedOutput.size(), LARGE_ZOUTS_MAX);
+                continue;
+            }
+
             // Size limits
             unsigned int nTxSize = ::GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION);
             // fprintf(stderr,"%s: nTxSize = %u\n", __func__, nTxSize);
@@ -576,6 +593,18 @@ CBlockTemplate* CreateNewBlock(CPubKey _pk,const CScript& _scriptPubKeyIn, int32
             nBlockSigOps += nTxSigOps;
             nFees += nTxFees;
 
+            if(tx.vShieldedOutput.size() >= LARGE_ZOUTS_THRESHOLD) {
+                large_zouts++;
+                LogPrintf("%s: txid=%s has large zouts=%d (%d large zouts in block)\n", __func__, tx.GetHash().ToString().c_str(),
+                    tx.vShieldedOutput.size(), large_zouts );
+            }
+
+            if(tx.vShieldedSpend.size() >= LARGE_ZINS_THRESHOLD) {
+                large_zins++;
+                LogPrintf("%s: txid=%s has large zins=%d (%d large zouts in block)\n", __func__, tx.GetHash().ToString().c_str(),
+                    tx.vShieldedSpend.size(), large_zins );
+            }
+
             if (fPrintPriority)
             {
                 LogPrintf("priority %.1f fee %s txid %s\n",dPriority, feeRate.ToString(), tx.GetHash().ToString());