code: private static Path getPrivateKey(FileSystem fs, String privateKeyPath) throws IOException {
      Path keyPath = new Path(privateKeyPath);
      FileStatus fileStatus = fs.getFileStatus(keyPath);
      Preconditions.checkArgument(USER_READ_PERMISSION_ONLY.equals(fileStatus.getPermission()),
                                  "Private key file should only have read only permission only on user. " + keyPath);
      return keyPath;
    }
docs: Before retrieving private key, it makes sure that original private key's permission is read only on the owner.
  This is a way to ensure to keep private key private.
  @param fs
  @param privateKeyPath
  @return
  @throws IOException