[py3] Fixed #17040 -- ported django.utils.crypto.constant_time_compare.

This is a private API; adding a type check is acceptable.

django/utils/crypto.py

@@ -24,6 +24,7 @@ except NotImplementedError:
 
 from django.conf import settings
 from django.utils.encoding import smart_bytes
+from django.utils import six
 from django.utils.six.moves import xrange
 
 
@@ -81,13 +82,19 @@ def get_random_string(length=12,
 
 def constant_time_compare(val1, val2):
     """
-    Returns True if the two strings are equal, False otherwise.
+    Returns True if the two bytestrings are equal, False otherwise.
 
     The time taken is independent of the number of characters that match.
     """
+    if not (isinstance(val1, bytes) and isinstance(val2, bytes)):
+        raise TypeError("constant_time_compare only supports bytes")
     if len(val1) != len(val2):
         return False
     result = 0
+    if six.PY3:
+        for x, y in zip(val1, val2):
+            result |= x ^ y
+    else:
         for x, y in zip(val1, val2):
             result |= ord(x) ^ ord(y)
     return result == 0

tests/regressiontests/utils/crypto.py

@@ -6,7 +6,17 @@ import timeit
 import hashlib
 
 from django.utils import unittest
-from django.utils.crypto import pbkdf2
+from django.utils.crypto import constant_time_compare, pbkdf2
+
+
+class TestUtilsCryptoMisc(unittest.TestCase):
+
+    def test_constant_time_compare(self):
+        # It's hard to test for constant time, just test the result.
+        self.assertTrue(constant_time_compare(b'spam', b'spam'))
+        self.assertFalse(constant_time_compare(b'spam', b'eggs'))
+        with self.assertRaises(TypeError):
+            constant_time_compare('spam', 'spam')
 
 
 class TestUtilsCryptoPBKDF2(unittest.TestCase):

tests/regressiontests/utils/tests.py

@@ -6,7 +6,7 @@ from __future__ import absolute_import
 from .archive import TestBzip2Tar, TestGzipTar, TestTar, TestZip
 from .baseconv import TestBaseConv
 from .checksums import TestUtilsChecksums
-from .crypto import TestUtilsCryptoPBKDF2
+from .crypto import TestUtilsCryptoMisc, TestUtilsCryptoPBKDF2
 from .datastructures import (DictWrapperTests, ImmutableListTests,
     MergeDictTests, MultiValueDictTests, SortedDictTests)
 from .dateformat import DateFormatTests