Merge branch 'master' into urandom-engine

* master:
  PKCS #1 RSA test vector loader
  Removed pointless anchor
  Docs need virtualenv as well
  Everything about bash is the worst
  Some reST markup nonsense
  Fix for OS X
  More clearly describe the behavior of constant_time.bytes_eq
  Run the doc tests under OS X
  Made OpenSSL's derive_pbkdf2_hmac raise the right exception
  Document which backends implement which itnerfaces. Fixes #538
  pep8
  Fixed a typo in the docs
  Make the default backend be a multi-backend

Conflicts:
	tests/hazmat/backends/test_openssl.py

2 files changed, 10 insertions, 3 deletions

diff --git a/cryptography/hazmat/backends/__init__.py b/cryptography/hazmat/backends/__init__.py
index d1b95f2a..41d260a8 100644
--- a/cryptography/hazmat/backends/__init__.py
+++ b/cryptography/hazmat/backends/__init__.py
@@ -12,6 +12,7 @@
 # limitations under the License.
 
 from cryptography.hazmat.backends import openssl
+from cryptography.hazmat.backends.multibackend import MultiBackend
 from cryptography.hazmat.bindings.commoncrypto.binding import (
     Binding as CommonCryptoBinding
 )
@@ -23,5 +24,8 @@ if CommonCryptoBinding.is_available():
     _ALL_BACKENDS.append(commoncrypto.backend)
 
 
+_default_backend = MultiBackend(_ALL_BACKENDS)
+
+
 def default_backend():
-    return openssl.backend
+    return _default_backend
diff --git a/cryptography/hazmat/backends/openssl/backend.py b/cryptography/hazmat/backends/openssl/backend.py
index 31ffe0d1..f4b5c3ac 100644
--- a/cryptography/hazmat/backends/openssl/backend.py
+++ b/cryptography/hazmat/backends/openssl/backend.py
@@ -217,8 +217,11 @@ class Backend(object):
             )
             assert res == 1
         else:
-            # OpenSSL < 1.0.0
-            assert isinstance(algorithm, hashes.SHA1)
+            if not isinstance(algorithm, hashes.SHA1):
+                raise UnsupportedAlgorithm(
+                    "This version of OpenSSL only supports PBKDF2HMAC with "
+                    "SHA1"
+                )
             res = self._lib.PKCS5_PBKDF2_HMAC_SHA1(
                 key_material,
                 len(key_material),