code: func (whisper *Whisper) AddSymKeyFromPassword(password string) (string, error) {
  	id, err := GenerateRandomID()
  	if err != nil {
  		return "", fmt.Errorf("failed to generate ID: %s", err)
  	}
  	if whisper.HasSymKey(id) {
  		return "", fmt.Errorf("failed to generate unique ID")
  	}
  
  	// kdf should run no less than 0.1 seconds on an average computer,
  	// because it's an once in a session experience
  	derived := pbkdf2.Key([]byte(password), nil, 65356, aesKeyLength, sha256.New)
  	if err != nil {
  		return "", err
  	}
  
  	whisper.keyMu.Lock()
  	defer whisper.keyMu.Unlock()
  
  	// double check is necessary, because deriveKeyMaterial() is very slow
  	if whisper.symKeys[id] != nil {
  		return "", fmt.Errorf("critical error: failed to generate unique ID")
  	}
  	whisper.symKeys[id] = derived
  	return id, nil
  }
docs: // AddSymKeyFromPassword generates the key from password, stores it, and returns its id.