python: Turn the signature buffer into a bytearray.

This is a workaround for a bug where signature verification would
overwrite the variable holding the signature.

This only happens on python2.

Signed-off-by: Damir Jelić <poljar@termina.org.uk>

python/olm/utility.py

@@ -81,14 +81,15 @@ class _Utility(object):
 
         byte_key = to_bytes(key)
         byte_message = to_bytearray(message)
-        byte_signature = to_bytes(signature)
+        byte_signature = to_bytearray(signature)
 
         try:
             cls._check_error(
                 lib.olm_ed25519_verify(cls._utility, byte_key, len(byte_key),
                                        ffi.from_buffer(byte_message),
                                        len(byte_message),
-                                       byte_signature, len(byte_signature)))
+                                       ffi.from_buffer(byte_signature),
+                                       len(byte_signature)))
         finally:
             # clear out copies of the message, which may be a plaintext
             if byte_message is not message:

python/tests/account_test.py

@@ -98,3 +98,19 @@ class TestClass(object):
 
         with pytest.raises(OlmVerifyError):
             ed25519_verify(signing_key, message, signature)
+
+    @given(text())
+    def test_signature_verification_twice(self, message):
+        alice = Account()
+
+        signature = alice.sign(message)
+        signing_key = alice.identity_keys["ed25519"]
+
+        assert signature
+        assert signing_key
+
+        ed25519_verify(signing_key, message, signature)
+        assert signature == alice.sign(message)
+
+        ed25519_verify(signing_key, message, signature)
+        assert signature == alice.sign(message)