static int rng_get(void *p_rng, unsigned char *buf, size_t len) {
  (void) p_rng;
  mg_random(buf, len);
  return 0;
}
static int rng_get(void *p_rng, unsigned char *buf, size_t len) {
  (void) p_rng;
  mg_random(buf, len);
  return 0;
}