pwnable.kr(9) - mistake

## Problem Points: 1pt

We all make mistakes, let's move on.
(don't take this too seriously, no fancy hacking skill is required at all)

This task is based on real event
Thanks to dhmonkey

hint : operator priority

ssh [email protected] -p2222 (pw:guest)

Link

Code

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

#define PW_LEN 10
#define XORKEY 1

void xor(char* s, int len){
        int i;
        for(i=0; i<len; i++){
                s[i] ^= XORKEY;
        }
}

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

        int fd;
        if(fd=open("/home/mistake/password",O_RDONLY,0400) < 0){
                printf("can't open password %d\n", fd);
                return 0;
        }

        printf("do not bruteforce...\n");
        sleep(time(0)%20);

        char pw_buf[PW_LEN+1];
        int len;
        if(!(len=read(fd,pw_buf,PW_LEN) > 0)){
                printf("read error\n");
                close(fd);
                return 0;
        }

        char pw_buf2[PW_LEN+1];
        printf("input password : ");
        scanf("%10s", pw_buf2);

        // xor your input
        xor(pw_buf2, 10);

        if(!strncmp(pw_buf, pw_buf2, PW_LEN)){
                printf("Password OK\n");
                system("/bin/cat flag\n");
        }
        else{
                printf("Wrong Password\n");
        }

        close(fd);
        return 0;
}

Thinking

What the hint of operator priority. It tells you every thing!

Solution

First

at L17, look at fd = open(....) < 0, will you be curious operator = execute first or <?
The answer is < first, so if open() successfully executed, it will return 0. Then fd = 0 < 0 which is fd = 0.

At L27, there is a read() function is waiting you, when the fd, what does that means? When file descriptor is 0, that is standard input, commonly aka Keyboard Input lool.

So…?

The code’s error operator priority cause it ask Keyboard input twice. At Line38 make a Exclusive-OR(XOR) at our second input, if the first input matches second input’s xor result. Then we got flag. Is that simple?

XOR TIME!!!!!

Both first and second input takes a string with length 10. You will not want to process any situation with ASCII is 0, so make sure you do fill in every characters.

#define PW_LEN 10
#define XORKEY 1
...
void xor(char* s, int len){
        int i;
        for(i=0; i<len; i++){
                s[i] ^= XORKEY;
        }
}

The funny thing is, #define doesn’t tells us any type before compile. At this case, s[i] ^= 1, so only last(lowest) bit will be reverse in each character. If we input a B(0x01000010), after xor, it will be C(0x01000011).

Give BBBBBBBBBB and CCCCCCCCCC, hooray!

Interesting thing.

Some how, you may encount a long delay. I bet you did notice about sleep(time(0)%20);