1 files changed, 60 insertions, 0 deletions
diff --git a/src/lib/intmath/weakrng.c b/src/lib/intmath/weakrng.c
new file mode 100644
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+++ b/src/lib/intmath/weakrng.c
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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;
+}