Added backend check to hmac primitives

1 files changed, 8 insertions, 1 deletions

diff --git a/cryptography/hazmat/primitives/hmac.py b/cryptography/hazmat/primitives/hmac.py
index 76d658aa..9e1d8d02 100644
--- a/cryptography/hazmat/primitives/hmac.py
+++ b/cryptography/hazmat/primitives/hmac.py
@@ -16,13 +16,20 @@ from __future__ import absolute_import, division, print_function
 import six
 
 from cryptography import utils
-from cryptography.exceptions import AlreadyFinalized, InvalidSignature
+from cryptography.exceptions import (
+    AlreadyFinalized, InvalidSignature, UnsupportedInterface)
+
+from cryptography.hazmat.backends.interfaces import HMACBackend
 from cryptography.hazmat.primitives import constant_time, interfaces
 
 
 @utils.register_interface(interfaces.HashContext)
 class HMAC(object):
     def __init__(self, key, algorithm, backend, ctx=None):
+        if not isinstance(backend, HMACBackend):
+            raise UnsupportedInterface(
+                "Backend object does not implement HMACBackend")
+
         if not isinstance(algorithm, interfaces.HashAlgorithm):
             raise TypeError("Expected instance of interfaces.HashAlgorithm.")
         self.algorithm = algorithm