PBKDF2 for Older Versions of PHP - Based on @defuse's Code

pbkdf2.compat.php

<?php
/*
 * Password Hashing with PBKDF2 (http://crackstation.net/hashing-security.htm).
 * Copyright (c) 2013, Taylor Hornby
 * All rights reserved.
 *
 * Modified to Work with Older Versions of PHP
 * Copyright (c) 2014, Kijin Sung
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without 
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice, 
 * this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 * this list of conditions and the following disclaimer in the documentation 
 * and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE 
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
 * POSSIBILITY OF SUCH DAMAGE.
 */

// These constants may be changed without breaking existing hashes.

define('PBKDF2_COMPAT_HASH_ALGORITHM', 'SHA256');
define('PBKDF2_COMPAT_ITERATIONS', 12000);
define('PBKDF2_COMPAT_SALT_BYTES', 24);
define('PBKDF2_COMPAT_HASH_BYTES', 24);

// Calculates a hash from the given password.

function create_hash($password, $force_compat = false)
{
    // Generate the salt.
    
    if (function_exists('mcrypt_create_iv')) {
        $salt = base64_encode(mcrypt_create_iv(PBKDF2_COMPAT_SALT_BYTES, MCRYPT_DEV_URANDOM));
    } elseif (file_exists('/dev/urandom') && $fp = @fopen('/dev/urandom', 'r')) {
        $salt = base64_encode(fread($fp, PBKDF2_COMPAT_SALT_BYTES));
    } else {
        $salt = '';
        for ($i = 0; $i < PBKDF2_COMPAT_SALT_BYTES; $i += 2) {
            $salt .= pack('S', mt_rand(0, 65535));
        }
        $salt = base64_encode(substr($salt, 0, PBKDF2_COMPAT_SALT_BYTES));
    }
    
    // Determine the best supported algorithm and iteration count.
    
    $algo = strtolower(PBKDF2_COMPAT_HASH_ALGORITHM);
    $iterations = PBKDF2_COMPAT_ITERATIONS;
    if ($force_compat || !function_exists('hash_algos') || !in_array($algo, hash_algos())) {
        $algo = false;                         // This flag will be detected by pbkdf2_default()
        $iterations = round($iterations / 5);  // PHP 4 is very slow. Don't cause too much server load.
    }
    
    // Return format: algorithm:iterations:salt:hash
    
    $pbkdf2 = pbkdf2_default($algo, $password, $salt, $iterations, PBKDF2_COMPAT_HASH_BYTES);
    $prefix = $algo ? $algo : 'sha1';
    return $prefix . ':' . $iterations . ':' . $salt . ':' . base64_encode($pbkdf2);
}

// Checks whether a password matches a previously calculated hash

function validate_password($password, $hash)
{
    // Split the hash into 4 parts.
    
    $params = explode(':', $hash);
    if (count($params) < 4) return false;
    
    // Recalculate the hash and compare it with the original.
    
    $pbkdf2 = base64_decode($params[3]);
    $pbkdf2_check = pbkdf2_default($params[0], $password, $params[2], (int)$params[1], strlen($pbkdf2));
    return slow_equals($pbkdf2, $pbkdf2_check);
}

// Checks whether a hash needs upgrading.

function needs_upgrade($hash)
{
    // Get the current algorithm and iteration count.
    
    $params = explode(':', $hash);
    if (count($params) < 4) return true;
    $algo = $params[0];
    $iterations = (int)$params[1];
    
    // Compare the current hash with the best supported options.
    
    if (!function_exists('hash_algos') || !in_array($algo, hash_algos())) {
        return false;
    } elseif ($algo === strtolower(PBKDF2_COMPAT_HASH_ALGORITHM) && $iterations >= PBKDF2_COMPAT_ITERATIONS) {
        return false;
    } else {
        return 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
// 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_default($algo, $password, $salt, $count, $key_length)
{
    // Sanity check.
    
    if ($count <= 0 || $key_length <= 0) {
        trigger_error('PBKDF2 ERROR: Invalid parameters.', E_USER_ERROR);
    }
    
    // Check if we should use the fallback function.
    
    if (!$algo) return pbkdf2_fallback($password, $salt, $count, $key_length);
    
    // Check if the selected algorithm is available.
    
    $algo = strtolower($algo);
    if (!function_exists('hash_algos') || !in_array($algo, hash_algos())) {
        if ($algo === 'sha1') {
            return pbkdf2_fallback($password, $salt, $count, $key_length);
        } else {
            trigger_error('PBKDF2 ERROR: Hash algorithm not supported.', E_USER_ERROR);
        }
    }
    
    // Use built-in function if available.
    
    if (function_exists('hash_pbkdf2')) {
        return hash_pbkdf2($algo, $password, $salt, $count, $key_length, true);
    }
    
    // Count the blocks.
    
    $hash_length = strlen(hash($algo, '', true));
    $block_count = ceil($key_length / $hash_length);
    
    // Hash it!
    
    $output = '';
    for ($i = 1; $i <= $block_count; $i++) {
        $last = $salt . pack('N', $i);                               // $i encoded as 4 bytes, big endian.
        $last = $xorsum = hash_hmac($algo, $last, $password, true);  // first iteration.
        for ($j = 1; $j < $count; $j++) {                            // The other $count - 1 iterations.
            $xorsum ^= ($last = hash_hmac($algo, $last, $password, true));
        }
        $output .= $xorsum;
    }
    
    // Truncate and return.
    
    return substr($output, 0, $key_length);
}

// Fallback function using sha1() and a pure-PHP implementation of HMAC.
// The result is identical to the default function when used with SHA-1.
// But it is approximately 1.6x slower than the hash_hmac() function of PHP 5.1.2+,
// And approximately 2.3x slower than the hash_pbkdf2() function of PHP 5.5+.

function pbkdf2_fallback($password, $salt, $count, $key_length)
{
    // Count the blocks.
    
    $hash_length = 20;
    $block_count = ceil($key_length / $hash_length);
    
    // Prepare the HMAC key and padding.
    
    if (strlen($password) > 64) {
        $password = str_pad(sha1($password, true), 64, chr(0));
    } else {
        $password = str_pad($password, 64, chr(0));
    }
    
    $opad = str_repeat(chr(0x5C), 64) ^ $password;
    $ipad = str_repeat(chr(0x36), 64) ^ $password;
    
    // Hash it!
    
    $output = '';
    for ($i = 1; $i <= $block_count; $i++) {
        $last = $salt . pack('N', $i);
        $xorsum = $last = pack('H*', sha1($opad . pack('H*', sha1($ipad . $last))));
        for ($j = 1; $j < $count; $j++) {
            $last = pack('H*', sha1($opad . pack('H*', sha1($ipad . $last))));
            $xorsum ^= $last;
        }
        $output .= $xorsum;
    }
    
    // Truncate and return.
    
    return substr($output, 0, $key_length);
}