1 files changed, 40 insertions, 0 deletions
diff --git a/src/common/util.c b/src/common/util.c
index 93e2ba8e14..03840402b7 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -4996,3 +4996,43 @@ tor_check_port_forwarding(const char *filename,
   }
 }
 
+/** 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 it. We
+   * don't want to use windows's rand(), because that returns 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;
+  tor_assert(top > 0);
+  divisor = TOR_WEAK_RANDOM_MAX / top;
+  do {
+    result = (int32_t)(tor_weak_random(rng) / divisor);
+  } while (result >= top);
+  return result;
+}
+