def encrypt_string(self, content: str, key: int = 0) -> str:

        # precondition
        assert isinstance(key, int) and isinstance(content, str)

        key = key or self.__key or 1

        # make sure key can be any size
        while key > 255:
            key -= 255

        # This will be returned
        ans = ""

        for ch in content:
            ans += chr(ord(ch) ^ key)

        return ans