public static function hmacBase64($data, $key) {
    // $data and $key being strings here is necessary to avoid empty string
    // results of the hash function if they are not scalar values. As this
    // function is used in security-critical contexts like token validation it
    // is important that it never returns an empty string.
    if (!is_scalar($data) || !is_scalar($key)) {
      throw new \InvalidArgumentException('Both parameters passed to \Drupal\Component\Utility\Crypt::hmacBase64 must be scalar values.');
    }

    $hmac = base64_encode(hash_hmac('sha256', $data, $key, TRUE));
    // Modify the hmac so it's safe to use in URLs.
    return str_replace(['+', '/', '='], ['-', '_', ''], $hmac);
  }