code: function exportKs (password, privateKey, salt, iv, type, option = {}) {
    if (!password || !privateKey || !type) {
      throw new Error('Missing parameter! Make sure password, privateKey and type provided')
    }
  
    if (!crypto.isPrivateKey(privateKey)) {
      throw new Error('given Private Key is not a valid Private Key string');
    }
  
    salt = salt || util.bytesToHexUpperString(crypto.generateSalt());
    iv = iv || util.bytesToHexUpperString(crypto.generateIv())
  
    return crypto.marshal(
      crypto.deriveKey(password, salt, option), privateKey,
      util.strToBuffer(salt), util.strToBuffer(iv), type, option
    );
  }
docs: PRIVATE METHODS
   helper function to export keystore