code: func (whisper *Whisper) AddSymKeyDirect(key []byte) (string, error) {
  	if len(key) != aesKeyLength {
  		return "", fmt.Errorf("wrong key size: %d", len(key))
  	}
  
  	id, err := GenerateRandomID()
  	if err != nil {
  		return "", fmt.Errorf("failed to generate ID: %s", err)
  	}
  
  	whisper.keyMu.Lock()
  	defer whisper.keyMu.Unlock()
  
  	if whisper.symKeys[id] != nil {
  		return "", fmt.Errorf("failed to generate unique ID")
  	}
  	whisper.symKeys[id] = key
  	return id, nil
  }
docs: // AddSymKeyDirect stores the key, and returns its id.