code: func NewSigner(keyBytes, certBytes []byte) (ssh.Signer, error) {
  	keySigner, err := ssh.ParsePrivateKey(keyBytes)
  	if err != nil {
  		return nil, trace.Wrap(err, "failed to parse SSH private key")
  	}
  
  	pubkey, _, _, _, err := ssh.ParseAuthorizedKey(certBytes)
  	if err != nil {
  		return nil, trace.Wrap(err, "failed to parse SSH certificate")
  	}
  
  	cert, ok := pubkey.(*ssh.Certificate)
  	if !ok {
  		return nil, trace.BadParameter("expected SSH certificate, got %T ", pubkey)
  	}
  
  	return ssh.NewCertSigner(cert, keySigner)
  }
docs: // NewSigner returns new ssh Signer from private key + certificate pair.  The
  // signer can be used to create "auth methods" i.e. login into Teleport SSH
  // servers.