Merge pull request #531 from reaperhulk/pbkdf2-commoncrypto

PBKDF2HMAC (CommonCrypto)
diff --git a/cryptography/hazmat/backends/commoncrypto/backend.py b/cryptography/hazmat/backends/commoncrypto/backend.py
index 8792c8a..e5d4ee0 100644
--- a/cryptography/hazmat/backends/commoncrypto/backend.py
+++ b/cryptography/hazmat/backends/commoncrypto/backend.py
@@ -99,20 +99,10 @@
         }
 
     def hash_supported(self, algorithm):
-        try:
-            self._hash_mapping[algorithm.name]
-        except KeyError:
-            return False
-        else:
-            return True
+        return algorithm.name in self._hash_mapping
 
     def hmac_supported(self, algorithm):
-        try:
-            self._supported_hmac_algorithms[algorithm.name]
-        except KeyError:
-            return False
-        else:
-            return True
+        return algorithm.name in self._supported_hmac_algorithms
 
     def create_hash_ctx(self, algorithm):
         return _HashContext(self, algorithm)
@@ -121,11 +111,7 @@
         return _HMACContext(self, key, algorithm)
 
     def cipher_supported(self, cipher, mode):
-        try:
-            self._cipher_registry[type(cipher), type(mode)]
-        except KeyError:
-            return False
-        return True
+        return (type(cipher), type(mode)) in self._cipher_registry
 
     def create_symmetric_encryption_ctx(self, cipher, mode):
         if isinstance(mode, GCM):
diff --git a/docs/hazmat/primitives/key-derivation-functions.rst b/docs/hazmat/primitives/key-derivation-functions.rst
index 529f441..551dbd6 100644
--- a/docs/hazmat/primitives/key-derivation-functions.rst
+++ b/docs/hazmat/primitives/key-derivation-functions.rst
@@ -80,7 +80,7 @@
         keys. See OWASP's `Password Storage Cheat Sheet`_ for more
         detailed recommendations if you intend to use this for password storage.
     :param backend: A
-        :class:`~cryptography.hazmat.backends.interfaces.CipherBackend`
+        :class:`~cryptography.hazmat.backends.interfaces.PBKDF2HMACBackend`
         provider.
 
     .. method:: derive(key_material)