Initial work on a prioritized multibackend for which implements CipherBackend

2 files changed, 49 insertions, 3 deletions

diff --git a/cryptography/hazmat/backends/__init__.py b/cryptography/hazmat/backends/__init__.py
index cb1fee90..b4ef210d 100644
--- a/cryptography/hazmat/backends/__init__.py
+++ b/cryptography/hazmat/backends/__init__.py
@@ -12,16 +12,17 @@
 # limitations under the License.
 
 from cryptography.hazmat.backends import openssl
+from cryptography.hazmat.backends.multibackend import PrioritizedMultiBackend
 from cryptography.hazmat.bindings.commoncrypto.binding import (
-    Binding as CCBinding
+    Binding as CommonCryptoBinding
 )
 
 _ALL_BACKENDS = [openssl.backend]
 
-if CCBinding.is_available():
+if CommonCryptoBinding.is_available():
     from cryptography.hazmat.backends import commoncrypto
     _ALL_BACKENDS.append(commoncrypto.backend)
 
 
 def default_backend():
-    return openssl.backend
+    return PrioritizedMultiBackend(_ALL_BACKENDS)
diff --git a/cryptography/hazmat/backends/multibackend.py b/cryptography/hazmat/backends/multibackend.py
new file mode 100644
index 00000000..9b91ff45
--- /dev/null
+++ b/cryptography/hazmat/backends/multibackend.py
@@ -0,0 +1,45 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import absolute_import, division, print_function
+
+from cryptography import utils
+from cryptography.exceptions import UnsupportedAlgorithm
+from cryptography.hazmat.backends.interfaces import CipherBackend
+
+
+@utils.register_interface(CipherBackend)
+class PrioritizedMultiBackend(object):
+    name = "multibackend"
+
+    def __init__(self, backends):
+        self._backends = backends
+
+    def cipher_supported(self, algorithm, mode):
+        return any(b.cipher_supported(algorithm, mode) for b in self._backends)
+
+    def create_symmetric_encryption_ctx(self, algorithm, mode):
+        for b in self._backends:
+            try:
+                return b.create_symmetric_encryption_ctx(algorithm, mode)
+            except UnsupportedAlgorithm:
+                pass
+        raise UnsupportedAlgorithm
+
+    def create_symmetric_decryption_ctx(self, algorithm, mode):
+        for b in self._backends:
+            try:
+                return b.create_symmetric_decryption_ctx(algorithm, mode)
+            except UnsupportedAlgorithm:
+                pass
+        raise UnsupportedAlgorithm