Merge pull request #804 from Ayrx/add-backend-check-to-hashes

Added backend check to hash primitives

3 files changed, 27 insertions, 3 deletions

diff --git a/cryptography/hazmat/primitives/hashes.py b/cryptography/hazmat/primitives/hashes.py
index bee188b3..409f564e 100644
--- a/cryptography/hazmat/primitives/hashes.py
+++ b/cryptography/hazmat/primitives/hashes.py
@@ -16,13 +16,18 @@ from __future__ import absolute_import, division, print_function
 import six
 
 from cryptography import utils
-from cryptography.exceptions import AlreadyFinalized
+from cryptography.exceptions import AlreadyFinalized, UnsupportedInterface
+from cryptography.hazmat.backends.interfaces import HashBackend
 from cryptography.hazmat.primitives import interfaces
 
 
 @utils.register_interface(interfaces.HashContext)
 class Hash(object):
     def __init__(self, algorithm, backend, ctx=None):
+        if not isinstance(backend, HashBackend):
+            raise UnsupportedInterface(
+                "Backend object does not implement HashBackend")
+
         if not isinstance(algorithm, interfaces.HashAlgorithm):
             raise TypeError("Expected instance of interfaces.HashAlgorithm.")
         self.algorithm = algorithm
diff --git a/docs/hazmat/primitives/cryptographic-hashes.rst b/docs/hazmat/primitives/cryptographic-hashes.rst
index 627ca7bd..b7eee2f5 100644
--- a/docs/hazmat/primitives/cryptographic-hashes.rst
+++ b/docs/hazmat/primitives/cryptographic-hashes.rst
@@ -45,6 +45,10 @@ Message Digests
         :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
         provider.
 
+    :raises cryptography.exceptions.UnsupportedInterface: This is raised if the
+        provided ``backend`` does not implement
+        :class:`~cryptography.hazmat.backends.interfaces.HashBackend`
+
     .. method:: update(data)
 
         :param bytes data: The bytes to be hashed.
diff --git a/tests/hazmat/primitives/test_hashes.py b/tests/hazmat/primitives/test_hashes.py
index fc53d635..d74d1d82 100644
--- a/tests/hazmat/primitives/test_hashes.py
+++ b/tests/hazmat/primitives/test_hashes.py
@@ -20,7 +20,11 @@ import pytest
 import six
 
 from cryptography import utils
-from cryptography.exceptions import AlreadyFinalized, UnsupportedHash
+from cryptography.exceptions import (
+    AlreadyFinalized, UnsupportedHash, UnsupportedInterface
+)
+
+from cryptography.hazmat.backends.interfaces import HashBackend
 from cryptography.hazmat.primitives import hashes, interfaces
 
 from .utils import generate_base_hash_test
@@ -39,7 +43,11 @@ class TestHashContext(object):
             m.update(six.u("\u00FC"))
 
     def test_copy_backend_object(self):
-        pretend_backend = pretend.stub()
+        @utils.register_interface(HashBackend)
+        class PretendBackend(object):
+            pass
+
+        pretend_backend = PretendBackend()
         copied_ctx = pretend.stub()
         pretend_ctx = pretend.stub(copy=lambda: copied_ctx)
         h = hashes.Hash(hashes.SHA1(), backend=pretend_backend,
@@ -171,3 +179,10 @@ class TestMD5(object):
         digest_size=16,
         block_size=64,
     )
+
+
+def test_invalid_backend():
+    pretend_backend = object()
+
+    with pytest.raises(UnsupportedInterface):
+        hashes.Hash(hashes.SHA1(), pretend_backend)