<?php

/**
 * Random Number Generator
 *
 * PHP version 5
 *
 * Here's a short example of how to use this library:
 * <code>
 * <?php
 *    include 'vendor/autoload.php';
 *
 *    echo bin2hex(\phpseclib\Crypt\Random::string(8));
 * ?>
 * </code>
 *
 * @category  Crypt
 * @package   Random
 * @author    Jim Wigginton <terrafrost@php.net>
 * @copyright 2007 Jim Wigginton
 * @license   http://www.opensource.org/licenses/mit-license.html  MIT License
 * @link      http://phpseclib.sourceforge.net
 */

namespace phpseclib\Crypt;

use phpseclib\Crypt\AES;
use phpseclib\Crypt\Base;
use phpseclib\Crypt\Blowfish;
use phpseclib\Crypt\DES;
use phpseclib\Crypt\RC4;
use phpseclib\Crypt\TripleDES;
use phpseclib\Crypt\Twofish;

/**
 * Pure-PHP Random Number Generator
 *
 * @package Random
 * @author  Jim Wigginton <terrafrost@php.net>
 * @access  public
 */
class Random
{
    /**
     * 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 int $length
     * @return string
     */
    static function string($length)
    {
        if (version_compare(PHP_VERSION, '7.0.0', '>=')) {
            try {
                return \random_bytes($length);
            } catch (\Throwable $e) {
                // If a sufficient source of randomness is unavailable, random_bytes() will throw an
                // object that implements the Throwable interface (Exception, TypeError, Error).
                // We don't actually need to do anything here. The string() method should just continue
                // as normal. Note, however, that if we don't have a sufficient source of randomness for
                // random_bytes(), most of the other calls here will fail too, so we'll end up using
                // the PHP implementation.
            }
        }

        if (strtoupper(substr(PHP_OS, 0, 3)) === '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 (extension_loaded('mcrypt') && function_exists('class_alias')) {
                return mcrypt_create_iv($length);
            }
            // method 2. openssl_random_pseudo_bytes was introduced in PHP 5.3.0 but prior to PHP 5.3.4 there was,
            // to quote <http://php.net/ChangeLog-5.php#5.3.4>, "possible blocking behavior". as of 5.3.4
            // openssl_random_pseudo_bytes and mcrypt_create_iv do the exact same thing on Windows. ie. they both
            // call php_win32_get_random_bytes():
            //
            // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/ext/openssl/openssl.c#L5008
            // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/ext/mcrypt/mcrypt.c#L1392
            //
            // php_win32_get_random_bytes() is defined thusly:
            //
            // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/win32/winutil.c#L80
            //
            // we're calling it, all the same, in the off chance that the mcrypt extension is not available
            if (extension_loaded('openssl') && version_compare(PHP_VERSION, '5.3.4', '>=')) {
                return openssl_random_pseudo_bytes($length);
            }
        } else {
            // method 1. the fastest
            if (extension_loaded('openssl')) {
                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, $length);
            }
            // method 3. pretty much does the same thing as method 2 per the following url:
            // https://github.com/php/php-src/blob/7014a0eb6d1611151a286c0ff4f2238f92c120d6/ext/mcrypt/mcrypt.c#L1391
            // surprisingly slower than method 2. maybe that's because mcrypt_create_iv does a bunch of error checking that we're
            // not doing. regardless, this'll only be called if this PHP script couldn't open /dev/urandom due to open_basedir
            // restrictions or some such
            if (extension_loaded('mcrypt')) {
                return mcrypt_create_iv($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();
            $_OLD_SESSION = isset($_SESSION) ? $_SESSION : false;
            if ($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($_COOKIE) .
                serialize($GLOBALS) .
                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 ($_OLD_SESSION !== false) {
                    $_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('\phpseclib\Crypt\AES'):
                    $crypto = new AES(Base::MODE_CTR);
                    break;
                case class_exists('\phpseclib\Crypt\Twofish'):
                    $crypto = new Twofish(Base::MODE_CTR);
                    break;
                case class_exists('\phpseclib\Crypt\Blowfish'):
                    $crypto = new Blowfish(Base::MODE_CTR);
                    break;
                case class_exists('\phpseclib\Crypt\TripleDES'):
                    $crypto = new TripleDES(Base::MODE_CTR);
                    break;
                case class_exists('\phpseclib\Crypt\DES'):
                    $crypto = new DES(Base::MODE_CTR);
                    break;
                case class_exists('\phpseclib\Crypt\RC4'):
                    $crypto = new RC4();
                    break;
                default:
                    user_error(__CLASS__ . ' requires at least one symmetric cipher be loaded');
                    return false;
            }

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

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

        // 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")
        $result = '';
        while (strlen($result) < $length) {
            $i = $crypto->encrypt(microtime()); // strlen(microtime()) == 21
            $r = $crypto->encrypt($i ^ $v); // strlen($v) == 20
            $v = $crypto->encrypt($r ^ $i); // strlen($r) == 20
            $result.= $r;
        }
        return substr($result, 0, $length);
    }
}
