TYPO3 - get signed storage in TypoScript

setup.typoscript

page.10.variables {
  usersPid = USER
  usersPid.userFunc = Vendor\Extension\User\SignedStorageFolders->getSignedStorageFolders
  usersPid.pidList = 42
  usersPid.additionalSecret = TYPO3\CMS\Frontend\Authentication\FrontendUserAuthentication
}

SignedStorageFolders.php

<?php
namespace Vendor\Extension\User;

use TYPO3\CMS\Core\Utility\GeneralUtility;

class SignedStorageFolders
{
    public function getSignedStorageFolders(string $content, array $config): string
    {
        $pidList = $config['pidList'];
        $additionalSecret = $config['additionalSecret'] ?? '';

        return sprintf(
            '%s@%s',
            $pidList,
            GeneralUtility::hmac($pidList, $additionalSecret)
        );
    }
}