I have one last concern: what is the character set of an OpenSSL password? I see you are using PyUnicode_AsEncodedString(x, NULL, NULL), which basically returns a utf8-encoded bytestring. Since the OpenSSL doc don't specify anything, we could accept it as a best-effort thing.
Or perhaps we should use PyUnicode_EncodeFSDefault(), which uses the "filesystem encoding", which is usually the same as the current terminal (assuming the password is typed on a terminal).