public function genUid() {
    $rand = $this['crypt/rand'];

    $hex = bin2hex($rand->Bytes(32));
    $str = substr($hex,0,16) . '-' . substr($hex,16,8) . '-' . substr($hex,24,8) . '-' . substr($hex,32,8) . '-' . substr($hex,40,24);
    return $str;
  }