GWCTF 2019-WEB-枯燥的抽奖

复现地址：https://buuoj.cn/challenges#[GWCTF%202019]枯燥的抽奖

考察点：php mt_srand() 随机种子

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题目给了一部分字符串，查看源码看到check.php

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<?php
#这不是抽奖程序的源代码！不许看！
header("Content-Type: text/html;charset=utf-8");
session_start();
if(!isset($_SESSION['seed'])){
$_SESSION['seed']=rand(0,999999999);
}

mt_srand($_SESSION['seed']);
$str_long1 = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
$str='';
$len1=20;
for ( $i = 0; $i < $len1; $i++ ){
    $str.=substr($str_long1, mt_rand(0, strlen($str_long1) - 1), 1);       
}
$str_show = substr($str, 0, 10);
echo "<p id='p1'>".$str_show."</p>";


if(isset($_POST['num'])){
    if($_POST['num']===$str){x
        echo "<p id=flag>抽奖，就是那么枯燥且无味，给你flag{xxxxxxxxx}</p>";
    }
    else{
        echo "<p id=flag>没抽中哦，再试试吧</p>";
    }
}
show_source("check.php"); 

mt_srand()函数

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str1='abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
str2='20S7rgtCU9'
str3 = str1[::-1]
length = len(str2)
res=''
for i in range(len(str2)):
    for j in range(len(str1)):
        if str2[i] == str1[j]:
            res+=str(j)+' '+str(j)+' '+'0'+' '+str(len(str1)-1)+' '
            break
print(res)

通过php_mt_seed这个脚本，破解seed

image.png

爆破出后进行解密

<?php

mt_srand(916398275);
$str_long1 = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
$str = '';
$len1 = 20;
for ($i = 0; $i < $len1; $i++) {
    $str .= substr($str_long1, mt_rand(0, strlen($str_long1) - 1), 1);
}
echo "<p id='p1'>" . $str . "</p>";

?>

得出完整的字符串后输入得出flag

image.png