123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960 |
- /* 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;
- }
|