<?php

/*
 * Password Hashing With PBKDF2 (http://crackstation.net/hashing-security.htm).
 * Copyright (c) 2013, Taylor Hornby
 * 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.
 */

class Password
{
    // these constants may be changed without breaking existing hashes.
    const PBKDF2_HASH_ALGORITHM = 'sha256';
    const PBKDF2_ITERATIONS     = 1000;
    const PBKDF2_SALT_BYTE_SIZE = 24;
    const PBKDF2_HASH_BYTE_SIZE = 24;
    const HASH_SECTIONS         = 4;
    const HASH_ALGORITHM_INDEX  = 0;
    const HASH_ITERATION_INDEX  = 1;
    const HASH_SALT_INDEX       = 2;
    const HASH_PBKDF2_INDEX     = 3;

    static function createHash($password)
    {
        // format: algorithm:iterations:salt:hash
        $salt = base64_encode(mcrypt_create_iv(self::PBKDF2_SALT_BYTE_SIZE, MCRYPT_DEV_URANDOM));
        return self::PBKDF2_HASH_ALGORITHM . ":" . self::PBKDF2_ITERATIONS . ":" . $salt . ":" .
            base64_encode(self::pbkdf2(
                    self::PBKDF2_HASH_ALGORITHM, $password, $salt, self::PBKDF2_ITERATIONS, self::PBKDF2_HASH_BYTE_SIZE, true
        ));
    }

    static function validatePassword($password, $correct_hash)
    {
        $params = explode(":", $correct_hash);
        if (count($params) < self::HASH_SECTIONS)
            return false;
        $pbkdf2 = base64_decode($params[self::HASH_PBKDF2_INDEX]);
        return self::slowEquals(
                $pbkdf2, self::pbkdf2(
                    $params[self::HASH_ALGORITHM_INDEX], $password, $params[self::HASH_SALT_INDEX], (int) $params[self::HASH_ITERATION_INDEX], strlen($pbkdf2), true
                )
        );
    }

    // compares two strings $a and $b in length-constant time.
    static function slowEquals($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
     */

    static function pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output = false)
    {
        $algorithm = strtolower($algorithm);
        if (!in_array($algorithm, hash_algos(), true))
            trigger_error('PBKDF2 ERROR: Invalid hash algorithm.', E_USER_ERROR);
        if ($count <= 0 || $key_length <= 0)
            trigger_error('PBKDF2 ERROR: Invalid parameters.', E_USER_ERROR);

        if (function_exists("hash_pbkdf2"))
        {
            // The output length is in NIBBLES (4-bits) if $raw_output is false!
            if (!$raw_output)
            {
                $key_length = $key_length * 2;
            }
            return hash_pbkdf2($algorithm, $password, $salt, $count, $key_length, $raw_output);
        }

        $hash_length = strlen(hash($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));
    }

}
