lottery

Lottery

Description

This lottery is soooooo boring...

Author: @kriive

Solution

The function rand() is called without calling srand() first, so the magic guess number is predictable.

Per the man page of rand(3):

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

One can copy the following to a new program:

// exp.c
#include <stdio.h>
#include <stdlib.h>

int main() {
  long long result = 0;

  for (int i = 0; i < 1000; i++) {
    result += (long long)rand();
  }
  printf("%lld", result);
}
// ❯ gcc -o exp exp.c && ./exp
// 1091191137495

The result is the magic number and it can be submitted to the service to retrieve the flag.

Flag

havceCTF{th1s_is_wh4t_h3ppens_wh3n_u_d0nt_use_d3v_ur4nd0m}