code: func authFromIdentity(k *client.Key) (ssh.AuthMethod, error) {
  	signer, err := sshutils.NewSigner(k.Priv, k.Cert)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	return client.NewAuthMethodForCert(signer), nil
  }
docs: // authFromIdentity returns a standard ssh.Authmethod for a given identity file