Updated the signature S value checks in accordance with the recent yello paper.

Author: Nerd

ethereum/processblock.py

@@ -10,6 +10,7 @@
 from ethereum.exceptions import *
 from ethereum.utils import safe_ord, normalize_address, mk_contract_address
 from ethereum import transactions
+import ethereum.config as config
 
 sys.setrecursionlimit(100000)
 
@@ -80,6 +81,8 @@ def rp(what, actual, target):
     # (1) The transaction signature is valid;
     if not tx.sender:  # sender is set and validated on Transaction initialization
         raise UnsignedTransaction(tx)
+    if block.number >= config.default_config["HOMESTEAD_FORK_BLKNUM"]:
+            tx.check_low_s()
 
     # (2) the transaction nonce is valid (equivalent to the
     #     sender account's current nonce);

ethereum/tests/test_transactions.py

@@ -4,6 +4,7 @@
 from rlp.utils import decode_hex, encode_hex, str_to_bytes
 import ethereum.testutils as testutils
 from ethereum.testutils import fixture_to_bytes
+import ethereum.config as config
 import sys
 import json
 import os
@@ -24,6 +25,9 @@ def run_test(filename, testname, testdata):
         rlpdata = decode_hex(testdata["rlp"][2:])
         o = {}
         tx = rlp.decode(rlpdata, transactions.Transaction)
+        blknum = int(testdata["blocknumber"])
+        if blknum >= config.default_config["HOMESTEAD_FORK_BLKNUM"]:
+            tx.check_low_s()
         o["sender"] = tx.sender
         o["transaction"] = {
             "data": b'0x' * (len(tx.data) > 0) + encode_hex(tx.data),

ethereum/transactions.py

@@ -170,6 +170,13 @@ def __repr__(self):
     def __structlog__(self):
         return encode_hex(self.hash)
 
+    # This method should be called for block numbers >= HOMESTEAD_FORK_BLKNUM only.
+    # The >= operator is replaced by > because the integer division N/2 always produces the value
+    # which is by 0.5 less than the real N/2
+    def check_low_s(self):
+        if self.s > N/2 or self.s == 0:
+            raise InvalidTransaction("Invalid signature S value!")
+
 
 UnsignedTransaction = Transaction.exclude(['v', 'r', 's'])