Refactor MasterKeyProvider.master_keys_for_data_key (#200)

* refactor MasterKeyProvider.decrypt_data_key and break out master key location to master_keys_for_data_key

* simplify MasterKey.owns_data_key

* autoformat

* fix linting issues

Author: mattsb42-aws

src/aws_encryption_sdk/key_providers/base.py

@@ -26,8 +26,17 @@
     MasterKeyProviderError,
 )
 from aws_encryption_sdk.internal.str_ops import to_bytes
+from aws_encryption_sdk.structures import DataKey  # pylint: disable=unused-import
+from aws_encryption_sdk.structures import EncryptedDataKey  # pylint: disable=unused-import
+from aws_encryption_sdk.structures import RawDataKey  # pylint: disable=unused-import
 from aws_encryption_sdk.structures import MasterKeyInfo
 
+try:  # Python 3.5.0 and 3.5.1 have incompatible typing modules
+    from typing import Iterable, Union  # noqa pylint: disable=unused-import
+except ImportError:  # pragma: no cover
+    # We only actually need these imports when running the mypy checks
+    pass
+
 _LOGGER = logging.getLogger(__name__)
 
 
@@ -212,6 +221,35 @@ def master_key_for_decrypt(self, key_info):
         self._decrypt_key_index[key_info] = decrypt_master_key
         return decrypt_master_key
 
+    def master_keys_for_data_key(self, data_key):
+        # type: (Union[DataKey, EncryptedDataKey, RawDataKey]) -> Iterable[MasterKey]
+        """Locates the correct master keys from children for the specified data key.
+
+        :param data_key: Data key for which to locate owning master keys
+        :type data_key: :class:`EncryptedDataKey`, :class:`RawDataKey`, or :class:`DataKey`
+        :returns: Masters key that own data key
+        :rtype: iterator of :class:`MasterKey`
+        :raises UnknownIdentityError: if unable to locate the correct master key
+        """
+        for member in [self] + self._members:
+            if member.provider_id != data_key.key_provider.provider_id:
+                continue
+
+            _LOGGER.debug("attempting to locate master key from key provider: %s", member.provider_id)
+
+            if isinstance(member, MasterKey):
+                if member.owns_data_key(data_key):
+                    _LOGGER.debug("using existing master key")
+                    yield member
+
+            if self.vend_masterkey_on_decrypt:
+                try:
+                    _LOGGER.debug("attempting to add master key: %s", data_key.key_provider.key_info)
+                    yield member.master_key_for_decrypt(data_key.key_provider.key_info)
+                except InvalidKeyIdError:
+                    _LOGGER.debug("master key %s not available in provider", data_key.key_provider.key_info)
+                    continue
+
     def decrypt_data_key(self, encrypted_data_key, algorithm, encryption_context):
         """Iterates through all currently added Master Keys and Master Key Providers
         to attempt to decrypt data key.
@@ -225,42 +263,25 @@ def decrypt_data_key(self, encrypted_data_key, algorithm, encryption_context):
         :rtype: aws_encryption_sdk.structures.DataKey
         :raises DecryptKeyError: if unable to decrypt encrypted data key
         """
-        data_key = None
-        master_key = None
         _LOGGER.debug("starting decrypt data key attempt")
-        for member in [self] + self._members:
-            if member.provider_id == encrypted_data_key.key_provider.provider_id:
-                _LOGGER.debug("attempting to locate master key from key provider: %s", member.provider_id)
-                if isinstance(member, MasterKey):
-                    _LOGGER.debug("using existing master key")
-                    master_key = member
-                elif self.vend_masterkey_on_decrypt:
-                    try:
-                        _LOGGER.debug("attempting to add master key: %s", encrypted_data_key.key_provider.key_info)
-                        master_key = member.master_key_for_decrypt(encrypted_data_key.key_provider.key_info)
-                    except InvalidKeyIdError:
-                        _LOGGER.debug(
-                            "master key %s not available in provider", encrypted_data_key.key_provider.key_info
-                        )
-                        continue
-                else:
-                    continue
-                try:
-                    _LOGGER.debug(
-                        "attempting to decrypt data key with provider %s", encrypted_data_key.key_provider.key_info
-                    )
-                    data_key = master_key.decrypt_data_key(encrypted_data_key, algorithm, encryption_context)
-                except (IncorrectMasterKeyError, DecryptKeyError) as error:
-                    _LOGGER.debug(
-                        "%s raised when attempting to decrypt data key with master key %s",
-                        repr(error),
-                        master_key.key_provider,
-                    )
-                    continue
-                break  # If this point is reached without throwing any errors, the data key has been decrypted
-        if not data_key:
-            raise DecryptKeyError("Unable to decrypt data key")
-        return data_key
+        for master_key in self.master_keys_for_data_key(encrypted_data_key):
+            try:
+                _LOGGER.debug(
+                    "attempting to decrypt data key with provider %s", encrypted_data_key.key_provider.key_info
+                )
+                return master_key.decrypt_data_key(encrypted_data_key, algorithm, encryption_context)
+
+            # MasterKeyProvider.decrypt_data_key throws DecryptKeyError
+            # but MasterKey.decrypt_data_key throws IncorrectMasterKeyError
+            except (IncorrectMasterKeyError, DecryptKeyError) as error:
+                _LOGGER.debug(
+                    "%s raised when attempting to decrypt data key with master key %s",
+                    repr(error),
+                    master_key.key_provider,
+                )
+                continue
+
+        raise DecryptKeyError("Unable to decrypt data key")
 
     def decrypt_data_key_from_list(self, encrypted_data_keys, algorithm, encryption_context):
         """Receives a list of encrypted data keys and returns the first one which this provider is able to decrypt.
@@ -356,9 +377,7 @@ def owns_data_key(self, data_key):
         :returns: Boolean statement of ownership
         :rtype: bool
         """
-        if data_key.key_provider == self.key_provider:
-            return True
-        return False
+        return data_key.key_provider == self.key_provider
 
     def master_keys_for_encryption(self, encryption_context, plaintext_rostream, plaintext_length=None):
         """Returns self and a list containing self, to match the format of output for a Master Key Provider.