code: func PrivateKeyFromFile(file string) (interface{}, error) {
  	data, err := ioutil.ReadFile(file)
  	if err != nil {
  		return nil, err
  	}
  	key, err := ParsePrivateKeyPEM(data)
  	if err != nil {
  		return nil, fmt.Errorf("error reading private key file %s: %v", file, err)
  	}
  	return key, nil
  }
docs: // PrivateKeyFromFile returns the private key in rsa.PrivateKey or ecdsa.PrivateKey format from a given PEM-encoded file.
  // Returns an error if the file could not be read or if the private key could not be parsed.