PBKDF2 Password Hashing for PHP

Question

Thanks to some great advice on a recent post I'm implementing PBKDF2 from https://defuse.ca/php-pbkdf2.htm into a small PHP image gallery I'm building to teach myself some PHP.

I understand that you store the salt and the hash in the database and then reconstruct them to match when the user enters their password. What I don't understand is how the validate_password function on the above website works when it generates a different, unique salt for the same password.

For example, I created a test repeatedly generating a password of just my name (andrew). Each time the final hash was different, presumably due to the salt being unique? However, it did not matter which generated hash I compared the password too, it validated it. (It obviously didn't validate a totally different password like Andrew56 or ndrew1).

Can anyone explain to my simple self why this is the case? It seems like I don't even need to store the salt? I hope this isn't off topic.

<?php
/*
 * Password hashing with PBKDF2.
 * Author: havoc AT defuse.ca
 * www: https://defuse.ca/php-pbkdf2.htm
 */

// These constants may be changed without breaking existing hashes.
define("PBKDF2_HASH_ALGORITHM", "sha256");
define("PBKDF2_ITERATIONS", 1000);
define("PBKDF2_SALT_BYTES", 24);
define("PBKDF2_HASH_BYTES", 24);

define("HASH_SECTIONS", 4);
define("HASH_ALGORITHM_INDEX", 0);
define("HASH_ITERATION_INDEX", 1);
define("HASH_SALT_INDEX", 2);
define("HASH_PBKDF2_INDEX", 3);

function create_hash($password)
{
// format: algorithm:iterations:salt:hash
$salt = base64_encode(mcrypt_create_iv(PBKDF2_SALT_BYTES, MCRYPT_DEV_URANDOM));
return PBKDF2_HASH_ALGORITHM . ":" . PBKDF2_ITERATIONS . ":" .  $salt . ":" . 
    base64_encode(pbkdf2(
        PBKDF2_HASH_ALGORITHM,
        $password,
        $salt,
        PBKDF2_ITERATIONS,
        PBKDF2_HASH_BYTES,
        true
    ));
}

function validate_password($password, $good_hash)
{
$params = explode(":", $good_hash);
if(count($params) < HASH_SECTIONS)
   return false; 
$pbkdf2 = base64_decode($params[HASH_PBKDF2_INDEX]);
return slow_equals(
    $pbkdf2,
    pbkdf2(
        $params[HASH_ALGORITHM_INDEX],
        $password,
        $params[HASH_SALT_INDEX],
        (int)$params[HASH_ITERATION_INDEX],
        strlen($pbkdf2),
        true
    )
);
}

// Compares two strings $a and $b in length-constant time.
function slow_equals($a, $b)
{
$diff = strlen($a) ^ strlen($b);
for($i = 0; $i < strlen($a) && $i < strlen($b); $i++)
{
    $diff |= ord($a[$i]) ^ ord($b[$i]);
}
return $diff === 0; 
}

/*
 * PBKDF2 key derivation function as defined by RSA's PKCS #5:         https://www.ietf.org/rfc/rfc2898.txt
 * $algorithm - The hash algorithm to use. Recommended: SHA256
 * $password - The password.
 * $salt - A salt that is unique to the password.
 * $count - Iteration count. Higher is better, but slower. Recommended: At least 1000.
 * $key_length - The length of the derived key in bytes.
 * $raw_output - If true, the key is returned in raw binary format. Hex encoded otherwise.
 * Returns: A $key_length-byte key derived from the password and salt.
 *
 * Test vectors can be found here: https://www.ietf.org/rfc/rfc6070.txt
 *
 * This implementation of PBKDF2 was originally created by https://defuse.ca
 * With improvements by http://www.variations-of-shadow.com
 */
function pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output = false)
{
$algorithm = strtolower($algorithm);
if(!in_array($algorithm, hash_algos(), true))
    die('PBKDF2 ERROR: Invalid hash algorithm.');
if($count <= 0 || $key_length <= 0)
    die('PBKDF2 ERROR: Invalid parameters.');

$hash_length = strlen(hash($algorithm, "", true));
$block_count = ceil($key_length / $hash_length);

$output = "";
for($i = 1; $i <= $block_count; $i++) {
    // $i encoded as 4 bytes, big endian.
    $last = $salt . pack("N", $i);
    // first iteration
    $last = $xorsum = hash_hmac($algorithm, $last, $password, true);
    // perform the other $count - 1 iterations
    for ($j = 1; $j < $count; $j++) {
        $xorsum ^= ($last = hash_hmac($algorithm, $last, $password, true));
    }
    $output .= $xorsum;
}

if($raw_output)
    return substr($output, 0, $key_length);
else
    return bin2hex(substr($output, 0, $key_length));
}
?>

Answer 1

I found this looking for a PBKDF2 implementation for an older PHP version. For anyone else happening to hit this page, if you're using this method directly:

pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output = false)

Then, it does NOT include the salt with the output string, just the hash, which is what I was desiring.

The create_hash method is what the Original Poster was using and, with his settings above, that would return:

sha256:1000:$salt:$hash

This allows you to save it all in one db column, which is easy to use. However, I would note that you "might" still want to split it up if you care about storage space. With a hexidecimal salt and password, you can reduce the storage space it takes in half by storing it in a binary column...eg in MySQL this is done with SET binaryColumn=UNHEX('0F0F') , and then SELECT HEX(binaryColumn) AS binaryColumn . It's more work, so it comes down to your own preference and goals, but thought I'd throw it out there.