I had a network security class in my university. And there is a challenge for finding a secret number. Here is the code

#include <stdlib.h>
#include <time.h>
#include <stdio.h>

void init() {
    setbuf(stdin, NULL);
    setbuf(stdout, NULL);
}

int main() {
    init();
    srand(time(0));
    int secret  = 0;
    puts("Your secret: ");
    scanf("%d", &secret);
    if(secret == rand()) {
        system("/bin/sh");
    } else {
        puts("failed");
    }
}

I actually could not understand my professor's explanation. Anyone can explain the meaning of this code, and how can i find the secret number?

CodePudding user response：

Technically you shouldn't be able to find the "secret" number - that's the whole point of the exercise: the "secret number" is generated using a strong PRNG seeded with a more-or-less random set of bits coming from the system clock.

Theoretically, if you have complete knowledge about the system - i.e. you both know the specific PRNG implementation used and the exact value used to seed it (the result of the time() call) - you should be able to guess the number simply by seeding another instance of the same PRNG and getting the next random int from it.

In this specific case, as the attacker you have complete control of the execution of the code and you should be able to predict the value returned from time() with very good accuracy (it is only a second resolution), so if you can build a program on the same system, that takes the predicted time value, seeds it to the same srand() implementation and returns the first random int - you can guess the "secret number".

So maybe the point of the exercise is to show that PRNG security trivially depends on knowing to start when no one is looking - otherwise you aren't secure, and possibly to not use time() to seed srand() and instead rely on something actually robust, like /dev/urandom