function_crypt.php

<?php
/**
 * Created by PhpStorm.
 * User: pfven
 * Date: 2016/7/20
 * Time: 17:19
 */

error_reporting(E_ALL || ~E_NOTICE);
include('config.php');
function random($length, $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz') {
    $hash = '';
    $max = strlen($chars) - 1;
    for($i = 0; $i < $length; $i++)	{
        $hash .= $chars[mt_rand(0, $max)];
    }
    return $hash;
}

function encrypt($txt,$key){  
    
    for($i=0;$i<strlen($txt);$i++){
        $tmp .= chr(ord($txt[$i])+10);
    }
    $txt = $tmp;
    $rnd=random(4);
    $key=md5($rnd.$key);
    $s=0;
    for($i=0;$i<strlen($txt);$i++){
        if($s == 32) $s = 0;
        $ttmp .= $txt[$i] ^ $key[++$s];
    }
    return base64_encode($rnd.$ttmp);
}
function decrypt($txt,$key){

    $txt=base64_decode($txt);
    
    $rnd = substr($txt,0,4); //取前四位 
    $txt = substr($txt,4);   //去从第五位开始后面的 

    $key=md5($rnd.$key);

    $s=0;
    for($i=0;$i<strlen($txt);$i++){
        if($s == 32) $s = 0;
        $tmp .= $txt[$i]^$key[++$s];
    }
    for($i=0;$i<strlen($tmp);$i++){
        $tmp1 .= chr(ord($tmp[$i])-10);
    }
    return $tmp1;

}

$username = decrypt($_COOKIE['user'],$key);

if ($username == 'system'){
    echo $flag;
}else{
    setcookie('user',encrypt('guest',$key));
    echo "It's Works!";
}