Modulo Bias about pwgen HOT 1 OPEN

The pw_random_number function can be re-written as follows to remove the modulo bias

/*

• Generate a random number n, where 0 <= n < max_num, using

• /dev/urandom if possible.

• max_num is never greater than 127 for this application
  */
  int pw_random_number(max_num)
  int max_num;
  {
  unsigned char rand_num = 0xff;
  unsigned char mask = 0x7f;
  int i, fd = get_random_fd();
  int lose_counter = 0x1f;
  char *cp = (char *) &rand_num;

  if (max_num <= 0x3f) mask = 0x3f;
  if (max_num <= 0x1f) mask = 0x1f;

  if (fd >= 0) {
  while (rand_num >= (unsigned int)max_num) { // while loop removes modulo bias
  i = read(fd, cp, 1);
  rand_num &= mask;
  if ((i < 0) &&
  ((errno == EINTR) || (errno == EAGAIN)))
  continue;
  if (i <= 0) {
  if (!lose_counter--) break;
  continue;
  }
  }
  close(fd);
  return (rand_num);
  }

  /* We weren't able to use /dev/random, fail hard */

  fprintf(stderr, "No entropy available!\n");
  exit(1);
  }

from pwgen.