Make transactions larger than 100K non-standard

Extremely large transactions with lots of inputs can cost the network
almost as much to process as they cost the sender in fees.

We would never create transactions larger than 100K big; this change
makes transactions larger than 100K non-standard, so they are not
relayed/mined by default. This is most important for miners that might
create blocks larger than 250K big, who could be vulnerable to a
make-your-blocks-so-expensive-to-verify-they-get-orphaned attack.

src/main.cpp

@@ -362,6 +362,14 @@ bool CTransaction::IsStandard() const
     if (!IsFinal())
         return false;
 
+    // Extremely large transactions with lots of inputs can cost the network
+    // almost as much to process as they cost the sender in fees, because
+    // computing signature hashes is O(ninputs*txsize). Limiting transactions
+    // to MAX_STANDARD_TX_SIZE mitigates CPU exhaustion attacks.
+    unsigned int sz = this->GetSerializeSize(SER_NETWORK, CTransaction::CURRENT_VERSION);
+    if (sz >= MAX_STANDARD_TX_SIZE)
+        return false;
+
     BOOST_FOREACH(const CTxIn& txin, vin)
     {
         // Biggest 'standard' txin is a 3-signature 3-of-3 CHECKMULTISIG