pwnable_kr_mistake.md

Mistake

He say the hint is operator priority.

#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;
}

After see the code, I found that the suspicious part is compare in the if statement.

if(fd=open("/home/mistake/password",O_RDONLY,0400) < 0)

see this first : operator precedence

We know that computer will do compare with 0 first, it will get 0.

if (fd = 0) {}

If you write this way it will do:

fd = 0
if(fd) {}

So fd is 0, for linux file descriptor it represent stdin !!!
It means that you can input the password !!!

Solve it !

I use 0000000000 as password. '0' is 110000 after xor with 1 it will become 110001(49) !