nbase_rnd.c

/***************************************************************************
 * nbase_rnd.c -- Some simple routines for obtaining random numbers for    *
 * casual use.  These are pretty secure on systems with /dev/urandom, but  *
 * falls back to poor entropy for seeding on systems without such support. *
 *                                                                         *
 *                   Based on DNET / OpenBSD arc4random().                 *
 *                                                                         *
 * Copyright (c) 2000 Dug Song <dugsong@monkey.org>                        *
 * Copyright (c) 1996 David Mazieres <dm@lcs.mit.edu>                      *
 *                                                                         *
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 ***************************************************************************/

/* $Id$ */

#include "nbase.h"
#include <errno.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#if HAVE_SYS_TIME_H
#include <sys/time.h>
#endif /* HAV_SYS_TIME_H */
#ifdef WIN32
#include <wincrypt.h>
#endif /* WIN32 */

/* data for our random state */
struct nrand_handle {
  u8    i, j, s[256], *tmp;
  int   tmplen;
};
typedef struct nrand_handle nrand_h;

static void nrand_addrandom(nrand_h *rand, u8 *buf, int len) {
  int i;
  u8 si;

  /* Mix entropy in buf with s[]...
   *
   * This is the ARC4 key-schedule.  It is rather poor and doesn't mix
   * the key in very well.  This causes a bias at the start of the stream.
   * To eliminate most of this bias, the first N bytes of the stream should
   * be dropped.
   */
  rand->i--;
  for (i = 0; i < 256; i++) {
    rand->i = (rand->i + 1);
    si = rand->s[rand->i];
    rand->j = (rand->j + si + buf[i % len]);
    rand->s[rand->i] = rand->s[rand->j];
    rand->s[rand->j] = si;
  }
  rand->j = rand->i;
}

static u8 nrand_getbyte(nrand_h *r) {
  u8 si, sj;

  /* This is the core of ARC4 and provides the pseudo-randomness */
  r->i = (r->i + 1);
  si = r->s[r->i];
  r->j = (r->j + si);
  sj = r->s[r->j];
  r->s[r->i] = sj; /* The start of the swap */
  r->s[r->j] = si; /* The other half of the swap */
  return (r->s[(si + sj) & 0xff]);
}

int nrand_get(nrand_h *r, void *buf, size_t len) {
  u8 *p;
  size_t i;

  /* Hand out however many bytes were asked for */
  for (p = buf, i = 0; i < len; i++) {
    p[i] = nrand_getbyte(r);
  }
  return (0);
}

void nrand_init(nrand_h *r) {
  u8 seed[256]; /* Starts out with "random" stack data */
  int i;

  /* Gather seed entropy with best the OS has to offer */
#ifdef WIN32
  HCRYPTPROV hcrypt = 0;

  CryptAcquireContext(&hcrypt, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT);
  CryptGenRandom(hcrypt, sizeof(seed), seed);
  CryptReleaseContext(hcrypt, 0);
#else
  struct timeval *tv = (struct timeval *)seed;
  int *pid = (int *)(seed + sizeof(*tv));
  int fd;

  gettimeofday(tv, NULL); /* fill lowest seed[] with time */
  *pid = getpid();        /* fill next lowest seed[] with pid */

  /* Try to fill the rest of the state with OS provided entropy */
  if ((fd = open("/dev/urandom", O_RDONLY)) != -1 ||
      (fd = open("/dev/arandom", O_RDONLY)) != -1) {
    ssize_t n;
    do {
      errno = 0;
      n = read(fd, seed + sizeof(*tv) + sizeof(*pid),
               sizeof(seed) - sizeof(*tv) - sizeof(*pid));
    } while (n < 0 && errno == EINTR);
    close(fd);
  }
#endif

  /* Fill up our handle with starter values */
  for (i = 0; i < 256; i++) { r->s[i] = i; };
  r->i = r->j = 0;

  nrand_addrandom(r, seed, 128); /* lower half of seed data for entropy */
  nrand_addrandom(r, seed + 128, 128); /* Now use upper half */
  r->tmp = NULL;
  r->tmplen = 0;

  /* This stream will start biased.  Get rid of 1K of the stream */
  nrand_get(r, seed, 256); nrand_get(r, seed, 256);
  nrand_get(r, seed, 256); nrand_get(r, seed, 256);
}

int get_random_bytes(void *buf, int numbytes) {
  static nrand_h state;
  static int state_init = 0;

  /* Initialize if we need to */
  if (!state_init) {
    nrand_init(&state);
    state_init = 1;
  }

  /* Now fill our buffer */
  nrand_get(&state, buf, numbytes);

  return 0;
}

int get_random_int() {
  int i;
  get_random_bytes(&i, sizeof(int));
  return i;
}

unsigned int get_random_uint() {
  unsigned int i;
  get_random_bytes(&i, sizeof(unsigned int));
  return i;
}

u64 get_random_u64() {
  u64 i;
  get_random_bytes(&i, sizeof(i));
  return i;
}


u32 get_random_u32() {
  u32 i;
  get_random_bytes(&i, sizeof(i));
  return i;
}

u16 get_random_u16() {
  u16 i;
  get_random_bytes(&i, sizeof(i));
  return i;
}

u8 get_random_u8() {
  u8 i;
  get_random_bytes(&i, sizeof(i));
  return i;
}

unsigned short get_random_ushort() {
  unsigned short s;
  get_random_bytes(&s, sizeof(unsigned short));
  return s;
}


/* This function is magic ;-)
 *
 * Sometimes Nmap wants to generate IPs that look random
 * but don't have any duplicates.  The strong RC4 generator
 * can't be used for this purpose because it can generate duplicates
 * if you get enough IPs (birthday paradox).
 *
 * This routine exploits the fact that a LCG won't repeat for the
 * entire duration of its period.  An LCG has some pretty bad
 * properties though so this routine does extra work to try to
 * tweak the LCG output so that is has very good statistics but
 * doesn't repeat.  The tweak used was mostly made up on the spot
 * but is generally based on good ideas and has been moderately
 * tested.  See links and reasoning below.
 */
u32 get_random_unique_u32() {
  static u32 state, tweak1, tweak2, tweak3;
  static int state_init = 0;
  u32 output;

  /* Initialize if we need to */
  if (!state_init) {
    get_random_bytes(&state, sizeof(state));
    get_random_bytes(&tweak1, sizeof(tweak1));
    get_random_bytes(&tweak2, sizeof(tweak2));
    get_random_bytes(&tweak3, sizeof(tweak3));

    state_init = 1;
  }

  /* What is this math crap?
   *
   * The whole idea behind this generator is that an LCG can be constructed
   * with a period of exactly 2^32.  As long as the LCG is fed back onto
   * itself the period will be 2^32.  The tweak after the LCG is just
   * a good permutation in GF(2^32).
   *
   * To accomplish the tweak the notion of rounds and round keys from
   * block ciphers has been borrowed.  The only special aspect of this
   * block cipher is that the first round short-circuits the LCG.
   *
   * This block cipher uses three rounds.  Each round is as follows:
   *
   * 1) Affine transform in GF(2^32)
   * 2) Rotate left by round constant
   * 3) XOR with round key
   *
   * For round one the affine transform is used as an LCG.
   */

  /* Reasoning:
   *
   * Affine transforms were chosen both to make a LCG and also
   * to try to introduce non-linearity.
   *
   * The rotate up each round was borrowed from SHA-1 and was introduced
   * to help obscure the obvious short cycles when you truncate an LCG with
   * a power-of-two period like the one used.
   *
   * The XOR with the round key was borrowed from several different
   * published functions (but see Xorshift)
   * and provides a different sequence for the full LCG.
   * There are 3 32 bit round keys.  This generator can
   * generate 2^96 different sequences of period 2^32.
   *
   * This generator was tested with Dieharder.  It did not fail any test.
   */

  /* See:
   *
   * https://en.wikipedia.org/wiki/Galois_field
   * https://en.wikipedia.org/wiki/Affine_cipher
   * https://en.wikipedia.org/wiki/Linear_congruential_generator
   * https://en.wikipedia.org/wiki/Xorshift
   * https://en.wikipedia.org/wiki/Sha-1
   *
   * https://seclists.org/nmap-dev/2009/q3/0695.html
   */


  /* First off, we need to evolve the state with our LCG
   * We'll use the LCG from Numerical Recipes (m=2^32,
   * a=1664525, c=1013904223).  All by itself this generator
   * pretty bad.  We're going to try to fix that without causing
   * duplicates.
   */
  state = (((state * 1664525) & 0xFFFFFFFF) + 1013904223) & 0xFFFFFFFF;

  output = state;

  /* With a normal LCG, we would just output the state.
   * In this case, though, we are going to try to destroy the
   * linear correlation between IPs by approximating a random permutation
   * in GF(2^32) (collision-free)
   */

  /* Then rotate and XOR */
  output = ((output << 7) | (output >> (32 - 7)));
  output = output ^ tweak1; /* This is the round key */

  /* End round 1, start round 2 */

  /* Then put it through an affine transform (glibc constants) */
  output = (((output * 1103515245) & 0xFFFFFFFF) + 12345) & 0xFFFFFFFF;

  /* Then rotate and XOR some more */
  output = ((output << 15) | (output >> (32 - 15)));
  output = output ^ tweak2;

  /* End round 2, start round 3 */

  /* Then put it through another affine transform (Quick C/C++ constants) */
  output = (((output * 214013) & 0xFFFFFFFF) + 2531011) & 0xFFFFFFFF;

  /* Then rotate and XOR some more */
  output = ((output << 5) | (output >> (32 - 5)));
  output = output ^ tweak3;

  return output;
}