php cookie tamper-proofing

Question

I use this code on my website:

<?php
$pass = "61e7680d2ac47e5b9e3c82118fae6e3cfcddff285ac75bb82872bb01f24ac657";
function valCookie(){
    if (isset($_COOKIE['session'])){
        $cookie = json_decode(hex2bin($_COOKIE['session']), true);
        global $pass;
        $hash = hash('sha256', $_SERVER['REMOTE_ADDR'] . $cookie['uid'] . 
        $cookie['expiry'] . $pass);
        $uid = $cookie['uid'];
        if ((hash_unique($hash, $cookie['hash'])) && ($cookie['expiry'] > time())){
            return $uid; //return user id.
            }
        }
    }
function hashCookie($uid, $expiry){
    global $pass;
    $cookie['uid'] = $uid;
    $cookie['expiry'] = $expiry;
    $cookie['hash'] = hash('sha256', $_SERVER['REMOTE_ADDR'] . $cookie['uid'] . 
    $cookie['expiry'] . $pass);
    $hexCookie = bin2hex(json_encode($cookie));
    setcookie("session", $hexCookie, $expiry);
    if(strlen($uid)){
        return true;
        }
    }
?>

Is it safe to use this to tamper-proof my cookies? I include the time in the hashing to expire the cookie. is this a secure way of doing it?

Answer 1

It's generally a bad idea to roll your own crypto .

Is it safe to use this to tamper-proof my cookies? I include the time in the hashing to expire the cookie. is this a secure way of doing it?

No, you're using a SHA256 hash of values that can largely be provided by attackers instead of HMAC-SHA256. Without HMAC, SHA256 is vulnerable to length-extension attacks .

Instead, consider (in order of preference):

1. PASETO , which provides tamper-resistant tokens with a high security margin. The only downside is that they're not immune to replay attacks.
2. JWT , with HS256. The linked library will allow you to securely only ever allow HS256, thereby side-stepping 99.9% of JWT security fails .
3. Halite's Cookie class . Halite is a usability wrapper for libsodium, a modern cryptography library that now ships with PHP 7.2.