Document and implement the public API for when the backend doesn't support the requested algorithm

2 files changed, 22 insertions, 3 deletions

diff --git a/cryptography/exceptions.py b/cryptography/exceptions.py
new file mode 100644
index 00000000..1821ceb7
--- /dev/null
+++ b/cryptography/exceptions.py
@@ -0,0 +1,15 @@
+# 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.
+
+class NoSuchAlgorithm(Exception):
+    pass
diff --git a/cryptography/hazmat/bindings/openssl/backend.py b/cryptography/hazmat/bindings/openssl/backend.py
index fc73dd39..ce8c6a55 100644
--- a/cryptography/hazmat/bindings/openssl/backend.py
+++ b/cryptography/hazmat/bindings/openssl/backend.py
@@ -18,6 +18,7 @@ import sys
 
 import cffi
 
+from cryptography.exceptions import NoSuchAlgorithm
 from cryptography.hazmat.primitives import interfaces
 from cryptography.hazmat.primitives.block.ciphers import (
     AES, Blowfish, Camellia, CAST5, TripleDES,
@@ -128,9 +129,12 @@ class _CipherContext(object):
         ctx = self._backend.ffi.gc(ctx, self._backend.lib.EVP_CIPHER_CTX_free)
 
         registry = self._backend.ciphers._cipher_registry
-        evp_cipher = registry[type(cipher), type(mode)](
-            self._backend, cipher, mode
-        )
+        try:
+            adapter = registry[type(cipher), type(mode)]
+        except KeyError:
+            raise NoSuchAlgorithm
+
+        evp_cipher = adapter(self._backend, cipher, mode)
         assert evp_cipher != self._backend.ffi.NULL
         if isinstance(mode, interfaces.ModeWithInitializationVector):
             iv_nonce = mode.initialization_vector