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random - 1 pt

Daddy, teach me how to use random value in programming!

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

random@ubuntu:~$ ls -l

total 20

-r--r----- 1 random_pwn root     49 Jun 30  2014 flag

-r-sr-x--- 1 random_pwn random 8538 Jun 30  2014 random

-rw-r--r-- 1 root       root    301 Jun 30  2014 random.c

random@ubuntu:~$ cat random.c

#include <stdio.h>

int main(){

unsigned int random;

random = rand(); // random value!

unsigned int key=0;

scanf("%d", &key);

if( (key ^ random) == 0xdeadbeef ){

printf("Good!\n");

system("/bin/cat flag");

return 0;

}

printf("Wrong, maybe you should try 2^32 cases.\n");

return 0;

}

random@ubuntu:~$ gdb -q random

Reading symbols from random...(no debugging symbols found)...done.

(gdb) set disassembly-flavor intel

(gdb) disas main

   0x000000000040062c <+56>: xor    eax,DWORD PTR [rbp-0x4]

   0x000000000040062f <+59>: cmp    eax,0xdeadbeef

random@ubuntu:~$ ./random 

3039230856

Good!

Mommy, I thought libc random is unpredictable...