Added backend check to cipher primitives

3 files changed, 22 insertions, 1 deletions

diff --git a/cryptography/hazmat/primitives/ciphers/base.py b/cryptography/hazmat/primitives/ciphers/base.py
index d366e4cf..2c804cac 100644
--- a/cryptography/hazmat/primitives/ciphers/base.py
+++ b/cryptography/hazmat/primitives/ciphers/base.py
@@ -16,12 +16,18 @@ from __future__ import absolute_import, division, print_function
 from cryptography import utils
 from cryptography.exceptions import (
     AlreadyFinalized, NotYetFinalized, AlreadyUpdated,
-)
+    UnsupportedInterface)
+
+from cryptography.hazmat.backends.interfaces import CipherBackend
 from cryptography.hazmat.primitives import interfaces
 
 
 class Cipher(object):
     def __init__(self, algorithm, mode, backend):
+        if not isinstance(backend, CipherBackend):
+            raise UnsupportedInterface(
+                "Backend object does not implement CipherBackend")
+
         if not isinstance(algorithm, interfaces.CipherAlgorithm):
             raise TypeError("Expected interface of interfaces.CipherAlgorithm")
 
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 741091b2..71a1064e 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -56,6 +56,10 @@ an "encrypt-then-MAC" formulation as `described by Colin Percival`_.
         :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
         provider.
 
+    :raises cryptography.exceptions.UnsupportedInterface: This is raised if the
+        provided ``backend`` does not implement
+        :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+
     .. method:: encryptor()
 
         :return: An encrypting
diff --git a/tests/hazmat/primitives/test_ciphers.py b/tests/hazmat/primitives/test_ciphers.py
index d9f83535..dfed63a8 100644
--- a/tests/hazmat/primitives/test_ciphers.py
+++ b/tests/hazmat/primitives/test_ciphers.py
@@ -14,9 +14,13 @@
 from __future__ import absolute_import, division, print_function
 
 import binascii
+from cryptography.hazmat.primitives.ciphers.modes import ECB
 
 import pytest
 
+from cryptography.exceptions import UnsupportedInterface
+
+from cryptography.hazmat.primitives import ciphers
 from cryptography.hazmat.primitives.ciphers.algorithms import (
     AES, Camellia, TripleDES, Blowfish, ARC4, CAST5, IDEA
 )
@@ -120,3 +124,10 @@ class TestIDEA(object):
     def test_invalid_key_size(self):
         with pytest.raises(ValueError):
             IDEA(b"\x00" * 17)
+
+
+def test_invalid_backend():
+    pretend_backend = object()
+
+    with pytest.raises(UnsupportedInterface):
+        ciphers.Cipher(AES(b"AAAAAAAAAAAAAAAA"), ECB, pretend_backend)