[GWCTF 2019]枯燥的抽奖

 <?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){
        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"); 

伪随机数，主要是不知道种子，先生成解密序列

str1 = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
str2 = 'k15my7a2XC'
res = ''
for char in str2:
    index = str1.find(char)
    res += str(index) + ' ' + str(index) + ' ' + '0' + ' ' + str(len(str1) - 1) + ' '
print(res)

再用php_mt_seed找到种子
找到之后直接代入题目给的源码，得到完整的字符串。