use integer division for canonicalization of signatures

fixes #353

Author: tomato42

src/ecdsa/test_pyecdsa.py

@@ -520,6 +520,23 @@ def test_sigencode_der_canonize(self):
         self.assertEqual(r, new_r)
         self.assertEqual(order - s, new_s)
 
+    def test_sigencode_der_canonize_with_close_to_half_order(self):
+        r = 13
+        order = SECP112r1.order
+        s = order // 2 + 1
+
+        regular_encode = sigencode_der(r, s, order)
+        canonical_encode = sigencode_der_canonize(r, s, order)
+
+        self.assertNotEqual(regular_encode, canonical_encode)
+
+        new_r, new_s = sigdecode_der(
+            sigencode_der_canonize(r, s, order), order
+        )
+
+        self.assertEqual(r, new_r)
+        self.assertEqual(order - s, new_s)
+
     def test_sig_decode_strings_with_invalid_count(self):
         with self.assertRaises(MalformedSignature):
             sigdecode_strings([b"one", b"two", b"three"], 0xFF)

src/ecdsa/util.py

@@ -304,6 +304,23 @@ def sigencode_der(r, s, order):
     return der.encode_sequence(der.encode_integer(r), der.encode_integer(s))
 
 
+def _canonize(s, order):
+    """
+    Internal function for ensuring that the ``s`` value of a signature is in
+    the "canonical" format.
+
+    :param int s: the second parameter of ECDSA signature
+    :param int order: the order of the curve over which the signatures was
+        computed
+
+    :return: canonical value of s
+    :rtype: int
+    """
+    if s > order // 2:
+        s = order - s
+    return s
+
+
 def sigencode_strings_canonize(r, s, order):
     """
     Encode the signature to a pair of strings in a tuple
@@ -326,8 +343,7 @@ def sigencode_strings_canonize(r, s, order):
     :return: raw encoding of ECDSA signature
     :rtype: tuple(bytes, bytes)
     """
-    if s > order / 2:
-        s = order - s
+    s = _canonize(s, order)
     return sigencode_strings(r, s, order)
 
 
@@ -350,8 +366,7 @@ def sigencode_string_canonize(r, s, order):
     :return: raw encoding of ECDSA signature
     :rtype: bytes
     """
-    if s > order / 2:
-        s = order - s
+    s = _canonize(s, order)
     return sigencode_string(r, s, order)
 
 
@@ -381,8 +396,7 @@ def sigencode_der_canonize(r, s, order):
     :return: DER encoding of ECDSA signature
     :rtype: bytes
     """
-    if s > order / 2:
-        s = order - s
+    s = _canonize(s, order)
     return sigencode_der(r, s, order)