code: func (sc *Client) SubscribeMessages(ctx context.Context, criteria whisper.Criteria, ch chan<- *whisper.Message) (ethereum.Subscription, error) {
  	return sc.c.ShhSubscribe(ctx, ch, "messages", criteria)
  }
docs: // SubscribeMessages subscribes to messages that match the given criteria. This method
  // is only supported on bi-directional connections such as websockets and IPC.
  // NewMessageFilter uses polling and is supported over HTTP.