Clarify threat description and avoid negative uptimes.

changes/hsdir_assignment

@@ -2,6 +2,7 @@
     - Directory authorities now use data collected from rephist when
       choosing whether to assign the HSDir flag to relays, instead of
       trusting the uptime value the relay reports in its descriptor.
-      This helps prevent an attack where relatively few malaicious
-      nodes can blackhole any given hidden service. Bugfix on
-      0.2.0.10-alpha; fixes bug 2709.
+      This helps prevent an attack where a small set of nodes with
+      frequently-changing identity keys can blackhole a hidden service.
+      (Only authorities need upgrade; others will be fine once they do.)
+      Bugfix on 0.2.0.10-alpha; fixes bug 2709.

src/or/rephist.c

@@ -537,7 +537,7 @@ rep_hist_get_uptime(const char *id, time_t when)
   or_history_t *hist = get_or_history(id);
   if (!hist)
     return 0;
-  if (!hist->start_of_run)
+  if (!hist->start_of_run || when < hist->start_of_run)
     return 0;
   return when - hist->start_of_run;
 }