Signature.php

<?php
if($_COOKIE == null
  || !isset($_COOKIE["DataCookie"])
  || !isset($_COOKIE["Signature"])) {
  echo "Login failed!";
  return;
}

parse_str($_COOKIE["DataCookie"], $parse);
if ($parse == null
  || !isset($parse["logged_in"])
  || $parse["logged_in"] ===  "0"
  || !isset($parse["id"])) {
  echo "Login failed : invalid parameters!";
  return;
}
$salt = getenv('SIGNATURE_FLAG');
$data = "logged_in={$parse["logged_in"]}&id={$parse["id"]}";
$signature = md5($salt. $data);

if($signature !== $_COOKIE["Signature"]) {
  echo "Login failed : invalid signature!";
  return;
}
?>