Proper way to use "Remember me" functionality in PHP

Question

Short

Working on login system and trying to implement remember me feature.

Recently, l did research about this subject, read bunch of articles, posts, stories, novels, fairy tales (calling them so, because some of them doesn't contain even 1 line of code, just loads of words) about, cookie vulnerabilities such as fixation, hijacking ... etc.

And decided to achieve following targets

1. To set time delay between login attempts (to prevent bruteforce attacks) and to limit attempts count
2. To regenerate session id on nearly every operation

But I really confused about my main problem: which way is proper, for "remember me" feature? to use cookies/session/database?

And please explain your idea on code.(I can't understand clearly without code)

Detailed

Currently, my code looks like that

During sign-in I'm using following function to set cookies and session

protected function validateUser($userid, $ckey=0, $rememmber=0) {
    session_start();
    session_regenerate_id(true); //this is a security measure
    $_SESSION['user_id'] = $userid;
    $_SESSION['HTTP_USER_AGENT'] = md5($_SERVER['HTTP_USER_AGENT']);
    if (isset($remember) && $rememmber == 'on') {
        setcookie("user_id", $_SESSION['user_id'], time() + 60 * 60 * 24 * COOKIE_TIME_OUT, "/");
        setcookie("user_key", sha1($ckey), time() + 60 * 60 * 24 * COOKIE_TIME_OUT, "/");
    }
    return true;
}

Then on secure user pages, checking for user_id using user_id to fetch all important data about user from db

public function protect() {
        session_start();

        /* Secure against Session Hijacking by checking user agent */
        if (isset($_SESSION['HTTP_USER_AGENT'])) {
            if ($_SESSION['HTTP_USER_AGENT'] != md5($_SERVER['HTTP_USER_AGENT'])) {
                $this->signout();
                exit;
            }
        }

// before we allow sessions, we need to check authentication key - ckey and ctime stored in database

        /* If session not set, check for cookies set by Remember me */
        if (!isset($_SESSION['user_id'])) {
            if (isset($_COOKIE['user_id']) && isset($_COOKIE['user_key'])) {
                /* we double check cookie expiry time against stored in database */

                $cookie_user_id = $_COOKIE['user_id'];
                               $stmt = $this->db->prepare("select `ckey`,`ctime` from `users` where `id` =?") or die($this->db->error);
            $stmt->bind_param("i", $cookie_user_id) or die(htmlspecialchars($stmt->error));
            $stmt->execute() or die(htmlspecialchars($stmt->error));
            $stmt->bind_result($ckey, $ctime) or die($stmt->error);
            $stmt->close() or die(htmlspecialchars($stmt->error));
                // coookie expiry
                if ((time() - $ctime) > 60 * 60 * 24 * COOKIE_TIME_OUT) {
                    $this->signout();
                }
                /* Security check with untrusted cookies - dont trust value stored in cookie.       
                  /* We also do authentication check of the `ckey` stored in cookie matches that stored in database during login */

                if (!empty($ckey) && is_numeric($_COOKIE['user_id']) && $_COOKIE['key'] == sha1($ckey)) {
                    session_regenerate_id(); //against session fixation attacks.

                    $_SESSION['user_id'] = $_COOKIE['user_id'];
                    $_SESSION['HTTP_USER_AGENT'] = md5($_SERVER['HTTP_USER_AGENT']);
                } else {
                    $this->signout();
                }
            } else {
                if ($page != 'main') {
                    header('Location:' . wsurl);
                    exit();
                }
            }
        }

Answer 1

To set time delay between login attempts (to prevent bruteforce attacks) and to limit attempts count

So you're providing a method for DOS by account?

To regenerate session id on nearly every operation

erm, no. That's actually likely to defeat the object. You should always generate a new id when the current id is expired or when the user is authenticated - otherwise leave it alone.

But I really confused about my main problem: which way is proper, for "remember me" feature? to use cookies/session/database?

Since you need to retain a token on the client, that means cookies (unless you fancy writing something really complicated using local storage). Since you don't want to expose data via the cookie / make forgery simple that means it should be a random value. And in order to reconcile the stored random value, that means storing data serverside - probably in a database since it must be possible to reference the data based on the user id or based on the random value.

While you could just use a non-expiring (or very long lived) session, I'd stay away from this - the data will snowball - and it's a good idea to renew the session data once in a while.

You also need to cater for the scenario where a user wants you to remember her on 2 different computers. If you only hold a single 'remember me' token for each account, then either you'll have to use the same value at both clients or delete the old token when you create a new one (i.e. user can only be remembered on one machine).

please explain your idea on code. I can't understand clearly without code

No. I get paid to write code; If you want me to write the code for you then you'll need to pay me. And the code will take up much more space and time than the description above.