Fix numerous problems with Tor's weak RNG.

We need a weak RNG in a couple of places where the strong RNG is
both needless and too slow.  We had been using the weak RNG from our
platform's libc implementation, but that was problematic (because
many platforms have exceptionally horrible weak RNGs -- like, ones
that only return values between 0 and SHORT_MAX) and because we were
using it in a way that was wrong for LCG-based weak RNGs.  (We were
counting on the low bits of the LCG output to be as random as the
high ones, which isn't true.)

This patch adds a separate type for a weak RNG, adds an LCG
implementation for it, and uses that exclusively where we had been
using the platform weak RNG.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/or/relay.c b/src/or/relay.c
index 5d06fd93fd..cbb2aca10d 100644
--- a/src/or/relay.c
+++ b/src/or/relay.c
@@ -70,6 +70,9 @@ uint64_t stats_n_relay_cells_relayed = 0;
  */
 uint64_t stats_n_relay_cells_delivered = 0;
 
+/** Used to tell which stream to read from first on a circuit. */
+static tor_weak_rng_t stream_choice_rng = TOR_WEAK_RNG_INIT;
+
 /** Update digest from the payload of cell. Assign integrity part to
  * cell.
  */
@@ -1740,6 +1743,12 @@ circuit_resume_edge_reading(circuit_t *circ, crypt_path_t *layer_hint)
                                        circ, layer_hint);
 }
 
+void
+stream_choice_seed_weak_rng(void)
+{
+  crypto_seed_weak_rng(&stream_choice_rng);
+}
+
 /** A helper function for circuit_resume_edge_reading() above.
  * The arguments are the same, except that <b>conn</b> is the head
  * of a linked list of edge streams that should each be considered.
@@ -1784,10 +1793,11 @@ circuit_resume_edge_reading_helper(edge_connection_t *first_conn,
     int num_streams = 0;
     for (conn = first_conn; conn; conn = conn->next_stream) {
       num_streams++;
-      if ((tor_weak_random() % num_streams)==0)
+      if (tor_weak_random_one_in_n(&stream_choice_rng, num_streams)) {
         chosen_stream = conn;
       /* Invariant: chosen_stream has been chosen uniformly at random from
        * among the first num_streams streams on first_conn. */
+      }
     }
   }