olm: Allow decryption functions to define how to handle unicode decode errors.

This patch changes the decryption functions not to fail if there was an
unicode decode error while converting the decrypted bytes plaintext into
a native python string.

Characters that cannot be decoded as unicode are now replaced with the
unicode replacement character (U+FFFD).

The old behaviour of raising an UnicodeDecodeError can be achieved by
passing the "strict" error handling scheme to the decrypt function.

1 files changed, 15 insertions, 6 deletions

diff --git a/python/olm/session.py b/python/olm/session.py
index cba9be0..cf66582 100644
--- a/python/olm/session.py
+++ b/python/olm/session.py
@@ -40,7 +40,7 @@ from future.utils import bytes_to_native_str
 # pylint: disable=no-name-in-module
 from _libolm import ffi, lib  # type: ignore
 
-from ._compat import URANDOM, to_bytearray, to_bytes
+from ._compat import URANDOM, to_bytearray, to_bytes, to_native_str
 from ._finalize import track_for_finalization
 
 # This is imported only for type checking purposes
@@ -273,8 +273,8 @@ class Session(object):
         else:  # pragma: no cover
             raise ValueError("Unknown message type")
 
-    def decrypt(self, message):
-        # type: (_OlmMessage) -> str
+    def decrypt(self, message, errors="replace"):
+        # type: (_OlmMessage, str) -> str
         """Decrypts a message using the session. Returns the plaintext string
         on success. Raises OlmSessionError on failure. If the base64 couldn't
         be decoded then the error message will be "INVALID_BASE64". If the
@@ -285,7 +285,14 @@ class Session(object):
 
         Args:
             message(OlmMessage): The Olm message that will be decrypted. It can
-            be either an OlmPreKeyMessage or an OlmMessage.
+                be either an OlmPreKeyMessage or an OlmMessage.
+            unicode_errors(str, optional): The error handling scheme to use for
+                unicode decoding errors. The default is "replace" meaning that
+                the character that was unable to decode will be replaced with
+                the unicode replacement character (U+FFFD). Other possible
+                values are "strict", "ignore" and "xmlcharrefreplace" as well
+                as any other name registered with codecs.register_error that
+                can handle UnicodeEncodeErrors.
         """
         if not message.ciphertext:
             raise ValueError("Ciphertext can't be empty")
@@ -311,8 +318,10 @@ class Session(object):
             plaintext_buffer, max_plaintext_length
         )
         self._check_error(plaintext_length)
-        plaintext = bytes_to_native_str(
-            ffi.unpack(plaintext_buffer, plaintext_length))
+        plaintext = to_native_str(
+            ffi.unpack(plaintext_buffer, plaintext_length),
+            errors=errors
+        )
 
         # clear out copies of the plaintext
         lib.memset(plaintext_buffer, 0, max_plaintext_length)