code: func fileBackedSigner(root *Root, policy *config.Signing) (signer.Signer, bool, error) {
  	keyFile := root.Config["key-file"]
  	certFile := root.Config["cert-file"]
  
  	if keyFile == "" {
  		return nil, false, nil
  	}
  
  	signer, err := local.NewSignerFromFile(certFile, keyFile, policy)
  	return signer, true, err
  }
docs: // fileBackedSigner determines whether a file-backed local signer is supported.