code: func (e *Envelope) OpenSymmetric(key []byte) (msg *ReceivedMessage, err error) {
  	msg = &ReceivedMessage{Raw: e.Data}
  	err = msg.decryptSymmetric(key)
  	if err != nil {
  		msg = nil
  	}
  	return msg, err
  }
docs: // OpenSymmetric tries to decrypt an envelope, potentially encrypted with a particular key.