code: func SharedCertificateExists(cfg *kubeadmapi.ClusterConfiguration) (bool, error) {
  
  	if err := validateCACertAndKey(certKeyLocation{cfg.CertificatesDir, kubeadmconstants.CACertAndKeyBaseName, "", "CA"}); err != nil {
  		return false, err
  	}
  
  	if err := validatePrivatePublicKey(certKeyLocation{cfg.CertificatesDir, "", kubeadmconstants.ServiceAccountKeyBaseName, "service account"}); err != nil {
  		return false, err
  	}
  
  	if err := validateCACertAndKey(certKeyLocation{cfg.CertificatesDir, kubeadmconstants.FrontProxyCACertAndKeyBaseName, "", "front-proxy CA"}); err != nil {
  		return false, err
  	}
  
  	// in case of local/stacked etcd
  	if cfg.Etcd.External == nil {
  		if err := validateCACertAndKey(certKeyLocation{cfg.CertificatesDir, kubeadmconstants.EtcdCACertAndKeyBaseName, "", "etcd CA"}); err != nil {
  			return false, err
  		}
  	}
  
  	return true, nil
  }
docs: // SharedCertificateExists verifies if the shared certificates - the certificates that must be
  // equal across control-plane nodes: ca.key, ca.crt, sa.key, sa.pub + etcd/ca.key, etcd/ca.crt if local/stacked etcd