[UTXO-BUG] CRIT-1: Conservation law bypass via negative/zero-value outputs

apply_transaction() never validates that output value_nrtc is positive.
A negative-value output reduces output_total, allowing an attacker to
create outputs exceeding input_total while the conservation check passes.

Attack: 100 RTC input -> [+200 RTC, -100 RTC] outputs
  output_total = 200 + (-100) = 100 <= input_total = 100  -> PASSES
  Attacker now has 200 RTC from a 100 RTC input.

Fix: validate every output has integer value_nrtc > 0 before the
conservation check. Also rejects zero-value dust and float types.

Tests added:
- test_negative_value_output_rejected
- test_zero_value_output_rejected
- test_float_value_nrtc_rejected

Bounty: #2819 (Critical, 200 RTC)

Author: ArokyaMatthew

node/test_utxo_db.py

@@ -355,6 +355,69 @@ def test_mempool_empty_inputs_rejected_for_transfer(self):
         ok = self.db.mempool_add(tx)
         self.assertFalse(ok)
 
+    # -- bounty #2819: negative / zero value outputs -------------------------
+
+    def test_negative_value_output_rejected(self):
+        """Negative value_nrtc on an output bypasses conservation law.
+
+        Attack: 100 RTC input → [+200 RTC, -100 RTC] outputs.
+        output_total = 200 + (-100) = 100 <= input_total = 100, PASSES.
+        Attacker mints 100 RTC from nothing.
+        """
+        self._apply_coinbase('alice', 100 * UNIT)
+        boxes = self.db.get_unspent_for_address('alice')
+
+        ok = self.db.apply_transaction({
+            'tx_type': 'transfer',
+            'inputs': [{'box_id': boxes[0]['box_id'],
+                         'spending_proof': 'sig'}],
+            'outputs': [
+                {'address': 'attacker', 'value_nrtc': 200 * UNIT},
+                {'address': 'burn',     'value_nrtc': -100 * UNIT},
+            ],
+            'fee_nrtc': 0,
+        }, block_height=10)
+
+        self.assertFalse(ok)
+        # Balance must be unchanged
+        self.assertEqual(self.db.get_balance('alice'), 100 * UNIT)
+        self.assertEqual(self.db.get_balance('attacker'), 0)
+
+    def test_zero_value_output_rejected(self):
+        """Zero-value outputs are meaningless dust that bloats the UTXO set."""
+        self._apply_coinbase('alice', 100 * UNIT)
+        boxes = self.db.get_unspent_for_address('alice')
+
+        ok = self.db.apply_transaction({
+            'tx_type': 'transfer',
+            'inputs': [{'box_id': boxes[0]['box_id'],
+                         'spending_proof': 'sig'}],
+            'outputs': [
+                {'address': 'bob',  'value_nrtc': 100 * UNIT},
+                {'address': 'dust', 'value_nrtc': 0},
+            ],
+            'fee_nrtc': 0,
+        }, block_height=10)
+
+        self.assertFalse(ok)
+
+    def test_float_value_nrtc_rejected(self):
+        """value_nrtc must be an integer; floats cause silent truncation."""
+        self._apply_coinbase('alice', 100 * UNIT)
+        boxes = self.db.get_unspent_for_address('alice')
+
+        ok = self.db.apply_transaction({
+            'tx_type': 'transfer',
+            'inputs': [{'box_id': boxes[0]['box_id'],
+                         'spending_proof': 'sig'}],
+            'outputs': [
+                {'address': 'bob', 'value_nrtc': 99.5 * UNIT},
+            ],
+            'fee_nrtc': 0,
+        }, block_height=10)
+
+        self.assertFalse(ok)
+
 
 class TestCoinSelect(unittest.TestCase):
 

node/utxo_db.py

@@ -341,6 +341,15 @@ def apply_transaction(self, tx: dict, block_height: int,
                 return False
 
             output_total = sum(o['value_nrtc'] for o in outputs)
+
+            # Every output must carry a strictly positive value.
+            # Without this, a negative-value output lowers output_total,
+            # letting an attacker create more value than the inputs hold.
+            for o in outputs:
+                if not isinstance(o['value_nrtc'], int) or o['value_nrtc'] <= 0:
+                    conn.execute("ROLLBACK")
+                    return False
+
             if fee < 0:
                 conn.execute("ROLLBACK")
                 return False