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/* Copyright (c) 2003, Roger Dingledine
* Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
* Copyright (c) 2007-2019, 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;
}
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