Add a check for the cryptography version before attempting to use it.

google/auth/crypt/_cryptography_rsa.py

@@ -19,6 +19,8 @@
 ``rsa`` library.
 """
 
+import pkg_resources
+
 import cryptography.exceptions
 from cryptography.hazmat import backends
 from cryptography.hazmat.primitives import hashes
@@ -29,6 +31,17 @@
 from google.auth import _helpers
 from google.auth.crypt import base
 
+_IMPORT_ERROR_MSG = (
+    'cryptography>=1.4.0 is required to use cryptography-based RSA '
+    'implementation.')
+
+try:  # pragma: NO COVER
+    release = pkg_resources.get_distribution('cryptography').parsed_version
+    if release < pkg_resources.parse_version('1.4.0'):
+        raise ImportError(_IMPORT_ERROR_MSG)
+except pkg_resources.DistributionNotFound:  # pragma: NO COVER
+    raise ImportError(_IMPORT_ERROR_MSG)
+
 
 _CERTIFICATE_MARKER = b'-----BEGIN CERTIFICATE-----'
 _BACKEND = backends.default_backend()