Ah yes, this is another one of the WIP questions. OpenSSL's convention is that if finalizing the decrypt context returns 0 AND you're using an AEAD cipher then that means the tag validation has failed. Of course, there are quite a few other failures that also return 0 just to complicate the issue. Assuming I can figure out how to determine the difference between tag validation failure and other failures, I'd like to have it raise a TagValidationError at that point.