diff --git a/tests/test_util.py b/tests/test_util.py
new file mode 100644
index 0000000..a0d5db2
--- /dev/null
+++ b/tests/test_util.py
@@ -0,0 +1,48 @@
+# -*- coding:utf-8 -*-
+#
+# Copyright 2015 Hewlett-Packard Development Company, L.P.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+
+import unittest
+
+import mock
+
+from anchor import util
+
+
+class UtilTests(unittest.TestCase):
+
+    @mock.patch('hmac.compare_digest', create=True)
+    def test_compare_with_hmac(self, compare_digest):
+        compare_digest.return_value = True
+        self.assertTrue(util.constant_time_compare("", ""))
+
+    @mock.patch('hmac.compare_digest', create=True)
+    def test_compare_with_shim_eq(self, compare_digest):
+        compare_digest.side_effect = AttributeError(
+            "'hmac' has no attribute 'compare_digest'")
+        self.assertTrue(util.constant_time_compare("abc", "abc"))
+
+    @mock.patch('hmac.compare_digest', create=True)
+    def test_compare_with_shim_ne(self, compare_digest):
+        compare_digest.side_effect = AttributeError(
+            "'hmac' has no attribute 'compare_digest'")
+        self.assertFalse(util.constant_time_compare("abc", "def"))
+
+    @mock.patch('hmac.compare_digest', create=True)
+    def test_compare_with_shim_different_len(self, compare_digest):
+        compare_digest.side_effect = AttributeError(
+            "'hmac' has no attribute 'compare_digest'")
+        self.assertFalse(util.constant_time_compare("abc", ""))