dns: Make TTLs fuzzy at exit relays

This change mitigates DNS-based website oracles by making the time that
a domain name is cached uncertain (+- 4 minutes of what's measurable).

Resolves TROVE-2021-009.

Fixes #40674

1 files changed, 16 insertions, 0 deletions

diff --git a/src/core/or/connection_edge.c b/src/core/or/connection_edge.c
index b96f4eb211..504980c9fd 100644
--- a/src/core/or/connection_edge.c
+++ b/src/core/or/connection_edge.c
@@ -102,6 +102,7 @@
 #include "feature/stats/predict_ports.h"
 #include "feature/stats/rephist.h"
 #include "lib/buf/buffers.h"
+#include "lib/crypt_ops/crypto_rand.h"
 #include "lib/crypt_ops/crypto_util.h"
 
 #include "core/or/cell_st.h"
@@ -498,6 +499,21 @@ clip_dns_ttl(uint32_t ttl)
     return MAX_DNS_TTL;
 }
 
+/** Given a TTL (in seconds), determine what TTL an exit relay should use by
+ * first clipping as usual and then adding some randomness which is sampled
+ * uniformly at random from [-FUZZY_DNS_TTL, FUZZY_DNS_TTL].  This facilitates
+ * fuzzy TTLs, which makes it harder to infer when a website was visited via
+ * side-channels like DNS (see "Website Fingerprinting with Website Oracles").
+ *
+ * Note that this can't underflow because FUZZY_DNS_TTL < MIN_DNS_TTL.
+ */
+uint32_t
+clip_dns_fuzzy_ttl(uint32_t ttl)
+{
+  return clip_dns_ttl(ttl) +
+    crypto_rand_uint(1 + 2*FUZZY_DNS_TTL) - FUZZY_DNS_TTL;
+}
+
 /** Send a relay end cell from stream <b>conn</b> down conn's circuit, and
  * remember that we've done so.  If this is not a client connection, set the
  * relay end cell's reason for closing as <b>reason</b>.