Fernet key must be 32 url-safe base64-encoded bytes.

Raise code

 typing.Union[bytes, str],
        backend: typing.Optional[Backend] = None,
    ):
        backend = _get_backend(backend)

        key = base64.urlsafe_b64decode(key)
        if len(key) != 32:
            raise ValueError(
                "Fernet key must be 32 url-safe base64-encoded bytes."
            )

        self._signing_key = key[:16]
        self._encryption_key = key[16:]
        self._backend = backend

    @classme