Django password in PHP

Question

I have a problem, because I have a database with users and theirs passwords were secured with Django (pbkdf2). So '123' looks like this:

pbkdf2_sha256$20000$MflWfLXbejfO$tNrjk42YE9ZXkg7IvXY5fikbC+H52Ipd2mf7m0azttk=

Now I need to use this passwords in PHP project and I don't have any idea how to compare them.

I strongly disagree @Sayse - it sounds like he's replacing django with PHP, so the answer shouldn't be "run both" — Fitblip
– Fitblip, Commented Sep 3, 2016 at 21:32

Scott Arciszewski · Accepted Answer · 2016-09-03 21:11:47Z

14

pbkdf2_sha256$20000$MflWfLXbejfO$tNrjk42YE9ZXkg7IvXY5fikbC+H52Ipd2mf7m0azttk=

Let's break this down. The $ are separators:

pbkdf2_sh256 means PBKDF2-SHA256, i.e. hash_pbkf2('sha256', ...)
20000 is the iteration count
MflWfLXbejfO is the salt
tNrjk42YE9ZXkg7IvXY5fikbC+H52Ipd2mf7m0azttk= is likely the hash.

This is all the information you need to validate the hash from PHP. You just need:

hash_pbkdf2() to generate a new hash from the password provided by the user
hash_equals() to compare the generated hash with the stored one

This function should work (PHP 7+):

/**
 * Verify a Django password (PBKDF2-SHA256)
 *
 * @ref http://stackoverflow.com/a/39311299/2224584
 * @param string $password   The password provided by the user
 * @param string $djangoHash The hash stored in the Django app
 * @return bool
 * @throws Exception
 */
function django_password_verify(string $password, string $djangoHash): bool
{
    $pieces = explode('$', $djangoHash);
    if (count($pieces) !== 4) {
        throw new Exception("Illegal hash format");
    }
    list($header, $iter, $salt, $hash) = $pieces;
    // Get the hash algorithm used:
    if (preg_match('#^pbkdf2_([a-z0-9A-Z]+)$#', $header, $m)) {
        $algo = $m[1];
    } else {
        throw new Exception(sprintf("Bad header (%s)", $header));
    }
    if (!in_array($algo, hash_algos())) {
        throw new Exception(sprintf("Illegal hash algorithm (%s)", $algo));
    }

    $calc = hash_pbkdf2(
        $algo,
        $password,
        $salt,
        (int) $iter,
        32,
        true
    );
    return hash_equals($calc, base64_decode($hash));
}

Demo: https://3v4l.org/WbTpW

If you need legacy PHP 5 support, removing the string prefixes and the : bool from the function definition will make it work on PHP 5.6. I don't advise trying to add backward compatibility for versions of PHP earlier than 5.6; if you find yourself in this situation, you should update your server software instead.

edited Sep 3, 2016 at 21:11

answered Sep 3, 2016 at 21:02

Scott Arciszewski

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3 Comments

Fitblip Over a year ago

Props for using hash_equals - constant time comparisons should always be used for comparing things in a secure way. I think in this case though you can safely use a normal string comparison, since the output of PBKDF2 isn't guessable from it's input, so there's no way to influence the timing attack (and therefore shouldn't be a side-channel).

Scott Arciszewski Over a year ago

Correct, there's no practical attacks on password hashing functions that don't use hash_equals(); I do so for consistency and to establish good habits in people who reference my code snippets. :)

Fitblip Over a year ago

Which is definitely the way to go about things :). Never hurts to do a constant time comparison always (sans a few cycles for hashing), and then you never screw it up!

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