Make test framework produce lowS signatures

Author: jl2012

qa/rpc-tests/test_framework/key.py

@@ -75,6 +75,9 @@
 # this specifies the curve used with ECDSA.
 NID_secp256k1 = 714 # from openssl/obj_mac.h
 
+SECP256K1_ORDER = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141
+SECP256K1_ORDER_HALF = SECP256K1_ORDER // 2
+
 # Thx to Sam Devlin for the ctypes magic 64-bit fix.
 def _check_result(val, func, args):
     if val == 0:
@@ -147,7 +150,7 @@ def get_ecdh_key(self, other_pubkey, kdf=lambda k: hashlib.sha256(k).digest()):
         r = self.get_raw_ecdh_key(other_pubkey)
         return kdf(r)
 
-    def sign(self, hash):
+    def sign(self, hash, low_s = True):
         # FIXME: need unit tests for below cases
         if not isinstance(hash, bytes):
             raise TypeError('Hash must be bytes instance; got %r' % hash.__class__)
@@ -159,7 +162,25 @@ def sign(self, hash):
         mb_sig = ctypes.create_string_buffer(sig_size0.value)
         result = ssl.ECDSA_sign(0, hash, len(hash), mb_sig, ctypes.byref(sig_size0), self.k)
         assert 1 == result
-        return mb_sig.raw[:sig_size0.value]
+        assert mb_sig.raw[0] == 0x30
+        assert mb_sig.raw[1] == sig_size0.value - 2
+        total_size = mb_sig.raw[1]
+        assert mb_sig.raw[2] == 2
+        r_size = mb_sig.raw[3]
+        assert mb_sig.raw[4 + r_size] == 2
+        s_size = mb_sig.raw[5 + r_size]
+        s_value = int.from_bytes(mb_sig.raw[6+r_size:6+r_size+s_size], byteorder='big')
+        if (not low_s) or s_value <= SECP256K1_ORDER_HALF:
+            return mb_sig.raw[:sig_size0.value]
+        else:
+            low_s_value = SECP256K1_ORDER - s_value
+            low_s_bytes = (low_s_value).to_bytes(33, byteorder='big')
+            while len(low_s_bytes) > 1 and low_s_bytes[0] == 0 and low_s_bytes[1] < 0x80:
+                low_s_bytes = low_s_bytes[1:]
+            new_s_size = len(low_s_bytes)
+            new_total_size_byte = (total_size + new_s_size - s_size).to_bytes(1,byteorder='big')
+            new_s_size_byte = (new_s_size).to_bytes(1,byteorder='big')
+            return b'\x30' + new_total_size_byte + mb_sig.raw[2:5+r_size] + new_s_size_byte + low_s_bytes
 
     def verify(self, hash, sig):
         """Verify a DER signature"""