Added backend check to hmac primitives

3 files changed, 28 insertions, 5 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
diff --git a/docs/hazmat/primitives/hmac.rst b/docs/hazmat/primitives/hmac.rst
index 6ca9e167..ce4e8803 100644
--- a/docs/hazmat/primitives/hmac.rst
+++ b/docs/hazmat/primitives/hmac.rst
@@ -56,6 +56,10 @@ of a message.
         :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
         provider.
 
+    :raises cryptography.exceptions.UnsupportedInterface: This is raised if the
+        provided ``backend`` does not implement
+        :class:`~cryptography.hazmat.backends.interfaces.HMACBackend`
+
     .. method:: update(msg)
 
         :param bytes msg: The bytes to hash and authenticate.
diff --git a/tests/hazmat/primitives/test_hmac.py b/tests/hazmat/primitives/test_hmac.py
index 88bed52c..24988e76 100644
--- a/tests/hazmat/primitives/test_hmac.py
+++ b/tests/hazmat/primitives/test_hmac.py
@@ -21,8 +21,10 @@ import six
 
 from cryptography import utils
 from cryptography.exceptions import (
-    AlreadyFinalized, UnsupportedHash, InvalidSignature
-)
+    AlreadyFinalized, UnsupportedHash, InvalidSignature,
+    UnsupportedInterface)
+
+from cryptography.hazmat.backends.interfaces import HMACBackend
 from cryptography.hazmat.primitives import hashes, hmac, interfaces
 
 from .utils import generate_base_hmac_test
@@ -52,8 +54,11 @@ class TestHMAC(object):
             h.update(six.u("\u00FC"))
 
     def test_copy_backend_object(self):
-        pretend_hmac = pretend.stub()
-        pretend_backend = pretend.stub(hmacs=pretend_hmac)
+        @utils.register_interface(HMACBackend)
+        class PretendBackend(object):
+            pass
+
+        pretend_backend = PretendBackend()
         copied_ctx = pretend.stub()
         pretend_ctx = pretend.stub(copy=lambda: copied_ctx)
         h = hmac.HMAC(b"key", hashes.SHA1(), backend=pretend_backend,
@@ -104,3 +109,10 @@ class TestHMAC(object):
     def test_unsupported_hash(self, backend):
         with pytest.raises(UnsupportedHash):
             hmac.HMAC(b"key", UnsupportedDummyHash(), backend)
+
+
+def test_invalid_backend():
+    pretend_backend = object()
+
+    with pytest.raises(UnsupportedInterface):
+        hmac.HMAC(b"key", hashes.SHA1(), pretend_backend)