5 files changed, 60 insertions, 10 deletions
diff --git a/cryptography/hazmat/backends/__init__.py b/cryptography/hazmat/backends/__init__.py
index 59d1bc6c..ae78822c 100644
--- a/cryptography/hazmat/backends/__init__.py
+++ b/cryptography/hazmat/backends/__init__.py
@@ -13,23 +13,42 @@
 
 from __future__ import absolute_import, division, print_function
 
-from cryptography.hazmat.backends import openssl
 from cryptography.hazmat.backends.multibackend import MultiBackend
 from cryptography.hazmat.bindings.commoncrypto.binding import (
     Binding as CommonCryptoBinding
 )
+from cryptography.hazmat.bindings.openssl.binding import (
+    Binding as OpenSSLBinding
+)
+
+
+_available_backends_list = None
+
 
-_ALL_BACKENDS = []
+def _available_backends():
+    global _available_backends_list
 
-if CommonCryptoBinding.is_available():
-    from cryptography.hazmat.backends import commoncrypto
-    _ALL_BACKENDS.append(commoncrypto.backend)
+    if _available_backends_list is None:
+        _available_backends_list = []
 
-_ALL_BACKENDS.append(openssl.backend)
+        if CommonCryptoBinding.is_available():
+            from cryptography.hazmat.backends import commoncrypto
+            _available_backends_list.append(commoncrypto.backend)
 
+        if OpenSSLBinding.is_available():
+            from cryptography.hazmat.backends import openssl
+            _available_backends_list.append(openssl.backend)
 
-_default_backend = MultiBackend(_ALL_BACKENDS)
+    return _available_backends_list
+
+
+_default_backend = None
 
 
 def default_backend():
+    global _default_backend
+
+    if _default_backend is None:
+        _default_backend = MultiBackend(_available_backends())
+
     return _default_backend
diff --git a/cryptography/hazmat/primitives/asymmetric/rsa.py b/cryptography/hazmat/primitives/asymmetric/rsa.py
index dfb43340..cbef8e32 100644
--- a/cryptography/hazmat/primitives/asymmetric/rsa.py
+++ b/cryptography/hazmat/primitives/asymmetric/rsa.py
@@ -16,6 +16,8 @@ from __future__ import absolute_import, division, print_function
 import six
 
 from cryptography import utils
+from cryptography.exceptions import UnsupportedInterface
+from cryptography.hazmat.backends.interfaces import RSABackend
 from cryptography.hazmat.primitives import interfaces
 
 
@@ -41,6 +43,10 @@ class RSAPublicKey(object):
         self._modulus = modulus
 
     def verifier(self, signature, padding, algorithm, backend):
+        if not isinstance(backend, RSABackend):
+            raise UnsupportedInterface(
+                "Backend object does not implement RSABackend")
+
         return backend.create_rsa_verification_ctx(self, signature, padding,
                                                    algorithm)
 
@@ -128,9 +134,17 @@ class RSAPrivateKey(object):
 
     @classmethod
     def generate(cls, public_exponent, key_size, backend):
+        if not isinstance(backend, RSABackend):
+            raise UnsupportedInterface(
+                "Backend object does not implement RSABackend")
+
         return backend.generate_rsa_private_key(public_exponent, key_size)
 
     def signer(self, padding, algorithm, backend):
+        if not isinstance(backend, RSABackend):
+            raise UnsupportedInterface(
+                "Backend object does not implement RSABackend")
+
         return backend.create_rsa_signature_ctx(self, padding, algorithm)
 
     @property
diff --git a/cryptography/hazmat/primitives/ciphers/base.py b/cryptography/hazmat/primitives/ciphers/base.py
index d366e4cf..1275019e 100644
--- a/cryptography/hazmat/primitives/ciphers/base.py
+++ b/cryptography/hazmat/primitives/ciphers/base.py
@@ -15,13 +15,18 @@ from __future__ import absolute_import, division, print_function
 
 from cryptography import utils
 from cryptography.exceptions import (
-    AlreadyFinalized, NotYetFinalized, AlreadyUpdated,
+    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/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/cryptography/hazmat/primitives/hmac.py b/cryptography/hazmat/primitives/hmac.py
index 76d658aa..0bcbb3cd 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