diff --git a/letsencrypt/client/tests/crypto_util_test.py b/letsencrypt/client/tests/crypto_util_test.py
new file mode 100644
index 000000000..88fea4c11
--- /dev/null
+++ b/letsencrypt/client/tests/crypto_util_test.py
@@ -0,0 +1,50 @@
+"""Tests for letsencrypt.client.crypto_util."""
+import unittest
+
+
+class CreateSigTest(unittest.TestCase):
+    """Tests for letsencrypt.client.crypto_util.create_sig."""
+
+    def setUp(self):
+        self.privkey = """-----BEGIN RSA PRIVATE KEY-----
+MIIBOgIBAAJBAKx1c7RR7R/drnBSQ/zfx1vQLHUbFLh1AQQQ5R8DZUXd36efNK79
+vukFhN9HFoHZiUvOjm0c+pVE6K+EdE/twuUCAwEAAQJAMbrEnJCrQe8YqAbw1/Bn
+elAzIamndfE3U8bTavf9sgFpS4HL83rhd6PDbvx81ucaJAT/5x048fM/nFl4fzAc
+mQIhAOF/a9o3EIsDKEmUl+Z1OaOiUxDF3kqWSmALEsmvDhwXAiEAw8ljV5RO/rUp
+Zu2YMDFq3MKpyyMgBIJ8CxmGRc6gCmMCIGRQzkcmhfqBrhOFwkmozrqIBRIKJIjj
+8TRm2LXWZZ2DAiAqVO7PztdNpynugUy4jtbGKKjBrTSNBRGA7OHlUgm0dQIhALQq
+6oGU29Vxlvt3k0vmiRKU4AVfLyNXIGtcWcNG46h/
+-----END RSA PRIVATE KEY-----"""
+        self.nonce = '\xec\xd6\xf2oYH\xeb\x13\xd5#q\xe0\xdd\xa2\x92\xa9'
+        self.b64nonce = '7Nbyb1lI6xPVI3Hg3aKSqQ'
+        self.signature = {
+            'nonce': self.b64nonce,
+            'alg': 'RS256',
+            'jwk': {
+                'kty': 'RSA',
+                'e': 'AQAB',
+                'n': 'rHVztFHtH92ucFJD_N_HW9AsdRsUuHUBBBDlHwNlRd3fp5'
+                     '80rv2-6QWE30cWgdmJS86ObRz6lUTor4R0T-3C5Q',
+            },
+            'sig': 'SUPYKucUnhlTt8_sMxLiigOYdf_wlOLXPI-o7aRLTsOquVjDd6r'
+                   'AX9AFJHk-bCMQPJbSzXKjG6H1IWbvxjS2Ew',
+        }
+
+    def _call(self, *args, **kwargs):
+        from letsencrypt.client.crypto_util import create_sig
+        return create_sig(*args, **kwargs)
+
+    def test_it(self):
+        self.assertEqual(
+            self._call('message', self.privkey, self.nonce), self.signature)
+
+    def test_random_nonce(self):
+        signature = self._call('message', self.privkey)
+        sig = signature.pop('sig')
+        nonce = signature.pop('nonce')
+        del self.signature['sig']
+        del self.signature['nonce']
+        self.assertEqual(signature, self.signature)
+
+if __name__ == '__main__':
+    unittest.main()