moectf2020 write up for mess

一道简单的编程语言应用题> <

题干给出的加密代码如下

import random
flag = 'moectf{xxxxxxxxxxx}'

digit = ''

for i in flag:
    digit += str(ord(i))


i = 0

while i < len(digit):
    n = random.randint(0, 128)
    if ord('a') <= n <= ord('z') or ord('A') <= n <= ord('Z'):
        digit = digit[0:i] + chr(n) + digit[i:]
    i += 1

with open('puzzle.txt', 'w') as out:
    out.write(digit)

加密后字符串如下

1091111A01ruVJl99hw11Qv6i102xCYC1c2B31DIsz1tm212l11A1l610448re11BQ09549115951n154V895F115d49109h1m1210810j11w2A5

这个时候我们就很容易看出加密过程为字符串转ASCII码转字符串再随机插入英文字母啦

故构造逆向代码如下

#include <iostream>
#include <cctype>
#include <cstring>
using namespace std;
int main(void)
{
    char ch[200] = "1091111A01ruVJl99hw11Qv6i102xCYC1c2B31DIsz1tm212l11A1l610448re11BQ09549115951n154V895F115d49109h1m1210810j11w2A5";
    int len = strlen(ch);
    char str[100]={0};
    for(int i=0;i<len;i++)
        if(isdigit(ch[i]))
            str[strlen(str)] = ch[i];
    len = strlen(str);
    int digit = 0;
    for(int i=0;i<len;i++)
    {
        digit = digit*10;
        digit += str[i]-'0';
        if(digit>='0')
        {
            cout << (char)digit;
            digit = 0;
        }
    }
}

获得flag

moectf{pyth0n_1s_s0_s1mple}

（python is so simple（指我用C++写题（x（wwwwww