random - pwnable.kr

Daddy, teach me how to use random value in programming! ssh random@pwnable.kr -p2222 (pw:guest)

three files on the ssh server : random,random.c,flag

downloading for better analysis:

~/Downloads/pwnabl/random
❯ scp -P 2222 random@pwnable.kr:~/random.c ./
random@pwnable.kr's password: 
random.c                         100%  301     0.7KB/s   00:00    

~/Downloads/pwnabl/random 8s
❯ scp -P 2222 random@pwnable.kr:~/random ./  
random@pwnable.kr's password: 
random

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;
}

If random numbers are generated with rand() without first calling srand(), your program will create the same sequence of numbers each time it runs.

since no seed was provided, it will always generate the same no. which we can obtain by compiling and outputing the random no., and then we can pass the if condintion and cat the flag

Last login: Tue Oct 12 13:53:50 2021 from 110.225.44.92
random@pwnable:~$ ./random
3039230856
Good!
Mommy, I thought libc random is unpredictable...
random@pwnable:~$