code: func WritePrivateKey(outFile afero.File, key *rsa.PrivateKey) error {
  	b := &pem.Block{
  		Type:  "RSA PRIVATE KEY",
  		Bytes: x509.MarshalPKCS1PrivateKey(key),
  	}
  
  	return pem.Encode(outFile, b)
  }
docs: // WritePrivateKey writes the private.key private key file