/* Copyright (c) 2003, Roger Dingledine

 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.

 * Copyright (c) 2007-2021, The Tor Project, Inc. */

/* See LICENSE for licensing information */

/**

 * \file weakrng.c

 *

 * \brief A weak but fast PRNG based on a linear congruential generator.

 *

 * We don't want to use the platform random(), since some of them are even

 * worse than this.

 **/

#include "lib/intmath/weakrng.h"

#include "lib/err/torerr.h"

#include <stdlib.h>

/** Initialize the insecure RNG <b>rng</b> from a seed value <b>seed</b>. */

void

tor_init_weak_random(tor_weak_rng_t *rng, unsigned seed)

{

  rng->state = (uint32_t)(seed & 0x7fffffff);

}

/** Return a randomly chosen value in the range 0..TOR_WEAK_RANDOM_MAX based

 * on the RNG state of <b>rng</b>.  This entropy will not be cryptographically

 * strong; do not rely on it for anything an adversary should not be able to

 * predict. */

int32_t

tor_weak_random(tor_weak_rng_t *rng)

{

  /* Here's a linear congruential generator. OpenBSD and glibc use these

   * parameters; they aren't too bad, and should have maximal period over the

   * range 0..INT32_MAX. We don't want to use the platform rand() or random(),

   * since some platforms have bad weak RNGs that only return values in the

   * range 0..INT16_MAX, which just isn't enough. */

  rng->state = (rng->state * 1103515245 + 12345) & 0x7fffffff;

  return (int32_t) rng->state;

}

/** Return a random number in the range [0 , <b>top</b>). {That is, the range

 * of integers i such that 0 <= i < top.}  Chooses uniformly.  Requires that

 * top is greater than 0. This randomness is not cryptographically strong; do

 * not rely on it for anything an adversary should not be able to predict. */

int32_t

tor_weak_random_range(tor_weak_rng_t *rng, int32_t top)

{

  /* We don't want to just do tor_weak_random() % top, since random() is often

   * implemented with an LCG whose modulus is a power of 2, and those are

   * cyclic in their low-order bits. */

  int divisor, result;

  raw_assert(top > 0);

  divisor = TOR_WEAK_RANDOM_MAX / top;

  do {

    result = (int32_t)(tor_weak_random(rng) / divisor);

  } while (result >= top);

  return result;

}