main: cap signature operations per block (#20)

MAX_SIZE bounds a block's bytes, not what it costs to check. Every
OP_CHECKSIG is an elliptic-curve verification, and blocks may be 32 MB
here, so one packed with signature operations takes every node minutes of
CPU to reject -- while at this chain's difficulty it stays cheap to mine.
There was no counter at all.

CheckBlock now sums signature operations across the block and rejects
past 20000, the figure Bitcoin settled on. OP_CHECKMULTISIG counts as 20
since it may verify that many.

Consensus change. Real blocks here carry one or two operations each --
coinbase pay-to-pubkey and standard pay-to-pubkey-hash spends -- so the
ceiling is four orders of magnitude above anything in the chain, but it
still needs verifying against the full history before merge.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/script.h b/src/script.h
index 27c1b13..26bb65a 100644
--- a/src/script.h
+++ b/src/script.h
@@ -536,6 +536,29 @@ public:
     }
 
 
+    // Signature operations in this script. Each one costs every validating
+    // node an elliptic-curve verification, so blocks are capped on this count
+    // and not only on their size -- a byte of OP_CHECKSIG is far more
+    // expensive to check than a byte of anything else.
+    unsigned int GetSigOpCount() const
+    {
+        unsigned int n = 0;
+        const_iterator pc = begin();
+        opcodetype opcode;
+        vector<unsigned char> vchPushValue;
+        while (pc < end())
+        {
+            if (!GetOp(pc, opcode, vchPushValue))
+                break;
+            if (opcode == OP_CHECKSIG || opcode == OP_CHECKSIGVERIFY)
+                n++;
+            else if (opcode == OP_CHECKMULTISIG || opcode == OP_CHECKMULTISIGVERIFY)
+                n += 20;
+        }
+        return n;
+    }
+
+
     void FindAndDelete(const CScript& b)
     {
         iterator pc = begin();