<?php
/* vim: set expandtab tabstop=4 shiftwidth=4 softtabstop=4: */

/**
 * Random Number Generator
 *
 * PHP versions 4 and 5
 *
 * Here's a short example of how to use this library:
 * <code>
 * <?php
 *    include('Crypt/Random.php');
 *
 *    echo bin2hex(crypt_random_string(8));
 * ?>
 * </code>
 *
 * LICENSE: Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 * @category   Crypt
 * @package    Crypt_Random
 * @author     Jim Wigginton <terrafrost@php.net>
 * @copyright  MMVII Jim Wigginton
 * @license    http://www.opensource.org/licenses/mit-license.html  MIT License
 * @version    $Id: Random.php,v 1.9 2010/04/24 06:40:48 terrafrost Exp $
 * @link       http://phpseclib.sourceforge.net
 */

/**
 * Generate a random string.
 *
 * Although microoptimizations are generally discouraged as they impair readability this function is ripe with
 * microoptimizations because this function has the potential of being called a huge number of times.
 * eg. for RSA key generation.
 *
 * @param Integer $length
 * @return String
 * @access public
 */
function crypt_random_string($length) {
    // PHP_OS & "\xDF\xDF\xDF" == strtoupper(substr(PHP_OS, 0, 3)), but a lot faster
    if ((PHP_OS & "\xDF\xDF\xDF") === 'WIN') {
        // method 1. prior to PHP 5.3 this would call rand() on windows hence the function_exists('class_alias') call.
        // ie. class_alias is a function that was introduced in PHP 5.3
        if (function_exists('mcrypt_create_iv') && function_exists('class_alias')) {
            return mcrypt_create_iv($length);
        }
        // method 2. openssl_random_pseudo_bytes was introduced in PHP 5.3.0 but prior to PHP 5.3.4 there was,
        // to quote <http://php.net/ChangeLog-5.php#5.3.4>, "possible blocking behavior". as of 5.3.4
        // openssl_random_pseudo_bytes and mcrypt_create_iv do the exact same thing on Windows. ie. they both
        // call php_win32_get_random_bytes():
        //
        // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/ext/openssl/openssl.c#L5008
        // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/ext/mcrypt/mcrypt.c#L1392
        //
        // php_win32_get_random_bytes() is defined thusly:
        //
        // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/win32/winutil.c#L80
        //
        // we're calling it, all the same, in the off chance that the mcrypt extension is not available
        if (function_exists('openssl_random_pseudo_bytes') && version_compare(PHP_VERSION, '5.3.4', '>=')) {
            return openssl_random_pseudo_bytes($length);
        }
    } else {
        // method 1. the fastest
        if (function_exists('openssl_random_pseudo_bytes')) {
            return openssl_random_pseudo_bytes($length);
        }
        // method 2
        static $fp = true;
        if ($fp === true) {
            // warning's will be output unles the error suppression operator is used. errors such as
            // "open_basedir restriction in effect", "Permission denied", "No such file or directory", etc.
            $fp = @fopen('/dev/urandom', 'rb');
        }
        if ($fp !== true && $fp !== false) { // surprisingly faster than !is_bool() or is_resource()
            return fread($fp, $length);
        }
        // method 3. pretty much does the same thing as method 2 per the following url:
        // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/ext/mcrypt/mcrypt.c#L1391
        // surprisingly slower than method 2. maybe that's because mcrypt_create_iv does a bunch of error checking that we're
        // not doing. regardless, this'll only be called if this PHP script couldn't open /dev/urandom due to open_basedir
        // restrictions or some such
        if (function_exists('mcrypt_create_iv')) {
            return mcrypt_create_iv($length, MCRYPT_DEV_URANDOM);
        }
    }
    // at this point we have no choice but to use a pure-PHP CSPRNG

    // cascade entropy across multiple PHP instances by fixing the session and collecting all
    // environmental variables, including the previous session data and the current session
    // data.
    //
    // mt_rand seeds itself by looking at the PID and the time, both of which are (relatively)
    // easy to guess at. linux uses mouse clicks, keyboard timings, etc, as entropy sources, but
    // PHP isn't low level to be able to use those as sources and on a web server there's not likely
    // going to be a ton of keyboard or mouse action. web servers do have one thing that we can use
    // however. a ton of people visiting the website. obviously you don't want to base your seeding
    // soley on parameters a potential attacker sends but (1) not everything in $_SERVER is controlled
    // by the user and (2) this isn't just looking at the data sent by the current user - it's based
    // on the data sent by all users. one user requests the page and a hash of their info is saved.
    // another user visits the page and the serialization of their data is utilized along with the
    // server envirnment stuff and a hash of the previous http request data (which itself utilizes
    // a hash of the session data before that). certainly an attacker should be assumed to have
    // full control over his own http requests. he, however, is not going to have control over
    // everyone's http requests.
    static $crypto = false, $v;
    if ($crypto === false) {
        // save old session data
        $old_session_id = session_id();
        $old_use_cookies = ini_get('session.use_cookies');
        $old_session_cache_limiter = session_cache_limiter();
        if (isset($_SESSION)) {
            $_OLD_SESSION = $_SESSION;
        }
        if ($old_session_id != '') {
            session_write_close();
        }

        session_id(1);
        ini_set('session.use_cookies', 0);
        session_cache_limiter('');
        session_start();

        $v = $seed = $_SESSION['seed'] = pack('H*', sha1(
            serialize($_SERVER) .
            serialize($_POST) .
            serialize($_GET) .
            serialize($_COOKIE) .
            serialize($_GLOBAL) .
            serialize($_SESSION) .
            serialize($_OLD_SESSION)
        ));
        if (!isset($_SESSION['count'])) {
            $_SESSION['count'] = 0;
        }
        $_SESSION['count']++;

        session_write_close();

        // restore old session data
        if ($old_session_id != '') {
            session_id($old_session_id);
            session_start();
            ini_set('session.use_cookies', $old_use_cookies);
            session_cache_limiter($old_session_cache_limiter);
        } else {
           if (isset($_OLD_SESSION)) {
               $_SESSION = $_OLD_SESSION;
               unset($_OLD_SESSION);
            } else {
                unset($_SESSION);
            }
        }

        // in SSH2 a shared secret and an exchange hash are generated through the key exchange process.
        // the IV client to server is the hash of that "nonce" with the letter A and for the encryption key it's the letter C.
        // if the hash doesn't produce enough a key or an IV that's long enough concat successive hashes of the
        // original hash and the current hash. we'll be emulating that. for more info see the following URL:
        //
        // http://tools.ietf.org/html/rfc4253#section-7.2
        //
        // see the is_string($crypto) part for an example of how to expand the keys
        $key = pack('H*', sha1($seed . 'A'));
        $iv = pack('H*', sha1($seed . 'C'));

        // ciphers are used as per the nist.gov link below. also, see this link:
        //
        // http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator#Designs_based_on_cryptographic_primitives
        switch (true) {
            case class_exists('Crypt_AES'):
                $crypto = new Crypt_AES(CRYPT_AES_MODE_CTR);
                break;
            case class_exists('Crypt_TripleDES'):
                $crypto = new Crypt_TripleDES(CRYPT_DES_MODE_CTR);
                break;
            case class_exists('Crypt_DES'):
                $crypto = new Crypt_DES(CRYPT_DES_MODE_CTR);
                break;
            case class_exists('Crypt_RC4'):
                $crypto = new Crypt_RC4();
                break;
            default:
                $crypto = $seed;
                return crypt_random_string($length);
        }

        $crypto->setKey($key);
        $crypto->setIV($iv);
        $crypto->enableContinuousBuffer();
    }

    if (is_string($crypto)) {
        // the following is based off of ANSI X9.31:
        //
        // http://csrc.nist.gov/groups/STM/cavp/documents/rng/931rngext.pdf
        //
        // OpenSSL uses that same standard for it's random numbers:
        //
        // http://www.opensource.apple.com/source/OpenSSL/OpenSSL-38/openssl/fips-1.0/rand/fips_rand.c
        // (do a search for "ANS X9.31 A.2.4")
        //
        // ANSI X9.31 recommends ciphers be used and phpseclib does use them if they're available (see
        // later on in the code) but if they're not we'll use sha1
        $result = '';
        while (strlen($result) < $length) { // each loop adds 20 bytes
            // microtime() isn't packed as "densely" as it could be but then neither is that the idea.
            // the idea is simply to ensure that each "block" has a unique element to it.
            $i = pack('H*', sha1(microtime()));
            $r = pack('H*', sha1($i ^ $v));
            $v = pack('H*', sha1($r ^ $i));
            $result.= $r;
        }
        return substr($result, 0, $length);
    }

    //return $crypto->encrypt(str_repeat("\0", $length));

    $result = '';
    while (strlen($result) < $length) {
        $i = $crypto->encrypt(microtime());
        $r = $crypto->encrypt($i ^ $v);
        $v = $crypto->encrypt($r ^ $i);
        $result.= $r;
    }
    return substr($result, 0, $length);
}
