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61 lines
2.1 KiB
C
61 lines
2.1 KiB
C
/* Copyright (c) 2003, Roger Dingledine
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* Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
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* Copyright (c) 2007-2019, The Tor Project, Inc. */
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/* See LICENSE for licensing information */
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/**
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* \file weakrng.c
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*
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* \brief A weak but fast PRNG based on a linear congruential generator.
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*
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* We don't want to use the platform random(), since some of them are even
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* worse than this.
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**/
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#include "lib/intmath/weakrng.h"
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#include "lib/err/torerr.h"
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#include <stdlib.h>
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/** Initialize the insecure RNG <b>rng</b> from a seed value <b>seed</b>. */
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void
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tor_init_weak_random(tor_weak_rng_t *rng, unsigned seed)
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{
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rng->state = (uint32_t)(seed & 0x7fffffff);
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}
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/** Return a randomly chosen value in the range 0..TOR_WEAK_RANDOM_MAX based
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* on the RNG state of <b>rng</b>. This entropy will not be cryptographically
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* strong; do not rely on it for anything an adversary should not be able to
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* predict. */
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int32_t
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tor_weak_random(tor_weak_rng_t *rng)
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{
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/* Here's a linear congruential generator. OpenBSD and glibc use these
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* parameters; they aren't too bad, and should have maximal period over the
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* range 0..INT32_MAX. We don't want to use the platform rand() or random(),
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* since some platforms have bad weak RNGs that only return values in the
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* range 0..INT16_MAX, which just isn't enough. */
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rng->state = (rng->state * 1103515245 + 12345) & 0x7fffffff;
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return (int32_t) rng->state;
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}
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/** Return a random number in the range [0 , <b>top</b>). {That is, the range
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* of integers i such that 0 <= i < top.} Chooses uniformly. Requires that
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* top is greater than 0. This randomness is not cryptographically strong; do
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* not rely on it for anything an adversary should not be able to predict. */
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int32_t
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tor_weak_random_range(tor_weak_rng_t *rng, int32_t top)
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{
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/* We don't want to just do tor_weak_random() % top, since random() is often
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* implemented with an LCG whose modulus is a power of 2, and those are
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* cyclic in their low-order bits. */
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int divisor, result;
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raw_assert(top > 0);
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divisor = TOR_WEAK_RANDOM_MAX / top;
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do {
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result = (int32_t)(tor_weak_random(rng) / divisor);
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} while (result >= top);
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return result;
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}
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