docs: def get_hmac(self, key):
          """Returns the keyed HMAC for authentication of this state data.
  
          :param key: the key for the keyed hash function
          """
          h = HMAC.new(key, None, SHA256)
          h.update(str(self.index).encode())
          h.update(self.seed)
          h.update(str(self.n).encode())
          h.update(self.root)
          h.update(str(self.timestamp).encode())
          return h.digest()