Proposal: Download consensus documents only when it will be trusted

[Peter Palfrader]

On Mon, 21 Apr 2008, Peter Palfrader wrote:

> On Sun, 13 Apr 2008, Nick Mathewson wrote:
> 
> > >   Servers only provide consensus documents to clients when it is known that
> > >   the client will trust it.
> 
> Here's a first go at the server side.

Split into its own function.


diff --git a/src/or/directory.c b/src/or/directory.c
index aee76a5..ac5eb04 100644
--- a/src/or/directory.c
+++ b/src/or/directory.c
@@ -57,6 +57,7 @@ static void dir_routerdesc_download_failed(smartlist_t *failed,
                                            int was_extrainfo,
                                            int was_descriptor_digests);
 static void note_request(const char *key, size_t bytes);
+static int client_likes_our_consensus(networkstatus_t *v, const char *want_url);
 
 /********* START VARIABLES **********/
 
@@ -2163,6 +2164,52 @@ directory_dump_request_log(void)
 }
 #endif
 
+/* Decide whether a client would accept the consensus we have
+ *
+ * Clients can say they only want a consensus if it's signed by more
+ * than half the authorities in <list>.  They pass this list in
+ * the url as ..consensus/<fpr>+<fpr>+<fpr>
+ * <fpr> may be an abbreviated fingerprint, i.e. only a left substring
+ * of the full authority identity digest.
+ *
+ * Returns 1 more than half of the requested authorities signed
+ * the consensus, 0 otherwise.
+ */
+int
+client_likes_our_consensus(networkstatus_t *v, const char *want_url)
+{
+  smartlist_t *want = smartlist_create();
+  smartlist_t *voterdigests = smartlist_create();
+  int need_at_least = 0;
+  int have = 0;
+
+  SMARTLIST_FOREACH(v->voters, networkstatus_voter_info_t *, vi, {
+    char *d = tor_malloc(HEX_DIGEST_LEN+1);
+    base16_encode(d, HEX_DIGEST_LEN+1, vi->identity_digest, DIGEST_LEN);
+    smartlist_add(voterdigests, d);
+  });
+
+  dir_split_resource_into_fingerprints(want_url, want, NULL, 0, 0);
+  need_at_least = smartlist_len(want)/2+1;
+  SMARTLIST_FOREACH(want, const char *, d, {
+    SMARTLIST_FOREACH(voterdigests, char *, voter, {
+      if (!strcasecmpstart(voter, d)) {
+        have++;
+        tor_free(voter);
+        SMARTLIST_DEL_CURRENT(voterdigests, voter);
+      };
+    });
+  });
+
+  SMARTLIST_FOREACH(voterdigests, char *, d, tor_free(d));
+  smartlist_free(voterdigests);
+
+  SMARTLIST_FOREACH(want, char *, d, tor_free(d));
+  smartlist_free(want);
+
+  return (have >= need_at_least);
+}
+
 /** Helper function: called when a dirserver gets a complete HTTP GET
  * request.  Look for a request for a directory or for a rendezvous
  * service descriptor.  On finding one, write a response into
@@ -2290,7 +2337,7 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
   }
 
   if (!strcmpstart(url,"/tor/status/")
-      || !strcmp(url, "/tor/status-vote/current/consensus")) {
+      || !strcmpstart(url, "/tor/status-vote/current/consensus")) {
     /* v2 or v3 network status fetch. */
     smartlist_t *dir_fps = smartlist_create();
     int is_v3 = !strcmpstart(url, "/tor/status-vote");
@@ -2311,6 +2358,14 @@ directory_handle_command_get(dir_connection_t *conn, const char *headers,
     } else {
       networkstatus_t *v = networkstatus_get_latest_consensus();
       time_t now = time(NULL);
+      #define CONSENSUS_URL_PREFIX "/tor/status-vote/current/consensus/"
+      if (!strcmpstart(url, CONSENSUS_URL_PREFIX) &&
+          !client_likes_our_consensus(v, url + strlen(CONSENSUS_URL_PREFIX))) {
+        write_http_status_line(conn, 404, "Consensus not signed by sufficient number of requested authorities");
+        smartlist_free(dir_fps);
+        goto done;
+      }
+
       smartlist_add(dir_fps, tor_memdup("\0\0\0\0\0\0\0\0\0\0"
                                         "\0\0\0\0\0\0\0\0\0\0", 20));
       request_type = compressed?"v3.z":"v3";