pwnable.kr Random

The source code for the challenge is:

#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;
}

The mistake here is clear, rand() is called without calling srand() first. According to the rand manpage:

If no seed value is provided, the rand() function is automatically seeded with a value of 1.

So, running the program below gives us the key we should enter:

#include <stdio.h>
#include <stdlib.h>
 
int main (int argc, char *argv[])
	unsigned int random = rand();
 
	printf("%d\n", random ^ 0xdeadbeef);
 
	return 0;
}