Pseudo Random number generation in C using libsodium

Question

I am trying to create a sequence of integers which is the same everytime as the generator is seeded however I am struggling to get it working. Right now the sequence is never the same.

#include "sodium.h"
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

main()
{
    if (sodium_init() == -1)
    {
        return 1;
    }

    unsigned int myInts[128];
    char seed[randombytes_SEEDBYTES] = "a seeeeeed";
    printf("%s", seed);

    randombytes_buf_deterministic(myInts, 128, seed);

    for (int i = 0; i < 128; i++)
    {
        printf("\n(%u)", myInts[i]);
    }
}

unrelated: prefer the newline at the end of the *format string* `printf("(%u)\n", myInts[i]);` — pmg, Mar 07 '20 at 18:30
@Yunnosch Hm, `seed` should be fully initialized in any case, or what do you mean exactly? — Ctx, Mar 07 '20 at 18:58
Yeah sorry I didnt get what you meant @Yunnosch. I did also try this to initialize the seed: — Lamebro1, Mar 07 '20 at 19:00
for (int i = 0; i <= randombytes_SEEDBYTES; i++) { seed[i] = 's'; } — Lamebro1, Mar 07 '20 at 19:00
@Yunnosch In this case the rest of the char array has to be initialized with zero-bytes — Ctx, Mar 07 '20 at 19:04
@Yunnosch The c standard requires it at least, so most sane compilers should comply ;) — Ctx, Mar 07 '20 at 19:06
I see (from accepted answers explanation) that I was wrong with my target, but not far off with my hunch. :-) — Yunnosch, Mar 07 '20 at 19:07

score 3 · Accepted Answer · answered Mar 07 '20 at 19:03

The problem is, that here

randombytes_buf_deterministic(myInts, 128, seed);

you generate 128 bytes of pseudorandom-data, but your buffer

unsigned int myInts[128];

is sizeof(int)*128 bytes big. So you have to generate enough data with

randombytes_buf_deterministic(myInts, sizeof(myInts), seed);

to fill the whole buffer with deterministic values. Then it should give the output you expect.

Pseudo Random number generation in C using libsodium

1 Answers1