g10: Don't consider cross-signed keys to be in conflict.

* g10/tofu.c (cross_sigs): New function.
(ask_about_binding): If apparently conflicting keys are cross signed,
then don't mark them as conflicting.

--
Signed-off-by: Neal H. Walfield <neal@g10code.com>

If two keys are cross signed, then the same person (probably)
controlled them both.  In this case, don't raise a TOFU conflict.
This usually occurs when someone transitions to a new key.  When that
person rotates to a third key, she will typically only cross sign it
with the second key.  As such, we check this transitively to avoid
declaring a conflict between the 1st and 3rd key.

g10/tofu.c

@@ -1204,6 +1204,80 @@ format_conflict_msg_part1 (int policy, const char *conflict,
 }
 
 
+/* Return 1 if A signed B and B signed A.  */
+int
+cross_sigs (kbnode_t a, kbnode_t b)
+{
+  int i;
+
+  PKT_public_key *a_pk = a->pkt->pkt.public_key;
+  PKT_public_key *b_pk = b->pkt->pkt.public_key;
+
+  char a_keyid[33];
+  char b_keyid[33];
+
+  if (DBG_TRUST)
+    {
+      format_keyid (pk_main_keyid (a_pk),
+                    KF_DEFAULT, a_keyid, sizeof (a_keyid));
+      format_keyid (pk_main_keyid (b_pk),
+                    KF_DEFAULT, b_keyid, sizeof (b_keyid));
+    }
+
+  for (i = 0; i < 2; i ++)
+    {
+      /* See if SIGNER signed SIGNEE.  */
+
+      kbnode_t signer = i == 0 ? a : b;
+      kbnode_t signee = i == 0 ? b : a;
+
+      PKT_public_key *signer_pk = signer->pkt->pkt.public_key;
+      u32 *signer_kid = pk_main_keyid (signer_pk);
+      kbnode_t n;
+
+      /* Iterate over SIGNEE's keyblock and see if there is a valid
+         signature from SIGNER.  */
+      for (n = signee; n; n = n->next)
+        {
+          PKT_signature *sig;
+
+          if (n->pkt->pkttype != PKT_SIGNATURE)
+            continue;
+
+          sig = n->pkt->pkt.signature;
+
+          if (! (sig->sig_class == 0x10
+                 || sig->sig_class == 0x11
+                 || sig->sig_class == 0x12
+                 || sig->sig_class == 0x13))
+            /* Not a signature over a user id.  */
+            continue;
+
+          /* SIG is on SIGNEE's keyblock.  If SIG was generated by the
+             signer, then it's a match.  */
+          if (keyid_cmp (sig->keyid, signer_kid) == 0)
+            /* Match!  */
+            break;
+        }
+      if (! n)
+        /* We didn't find a signature from signer over signee.  */
+        {
+          if (DBG_TRUST)
+            log_info ("No cross sig between %s and %s\n",
+                      a_keyid, b_keyid);
+          return 0;
+        }
+    }
+
+  /* A signed B and B signed A.  */
+  if (DBG_TRUST)
+    log_info ("Cross sig between %s and %s\n",
+              a_keyid, b_keyid);
+
+  return 1;
+}
+
+
 /* Ask the user about the binding.  There are three ways we could end
  * up here:
  *
@@ -1237,7 +1311,7 @@ ask_about_binding (ctrl_t ctrl,
   strlist_t other_user_ids = NULL;
   struct signature_stats *stats = NULL;
   struct signature_stats *stats_iter = NULL;
-  char *prompt;
+  char *prompt = NULL;
   char *choices;
 
   dbs = ctrl->tofu.dbs;
@@ -1361,9 +1435,17 @@ ask_about_binding (ctrl_t ctrl,
     }
   else
     {
+      int stats_count = 0;
+      kbnode_t *kb_all;
       KEYDB_HANDLE hd;
+      int i;
       char *key = NULL;
 
+      /* Get the keyblock for each key.  */
+      for (stats_iter = stats; stats_iter; stats_iter = stats_iter->next)
+        stats_count ++;
+      kb_all = xcalloc (sizeof (kb_all[0]), stats_count);
+
       if (! stats || strcmp (stats->fingerprint, fingerprint))
         {
           /* If we have already added this key to the DB, then it will
@@ -1375,7 +1457,9 @@ ask_about_binding (ctrl_t ctrl,
 
       /* Figure out which user ids are revoked or expired.  */
       hd = keydb_new ();
-      for (stats_iter = stats; stats_iter; stats_iter = stats_iter->next)
+      for (stats_iter = stats, i = 0;
+           stats_iter;
+           stats_iter = stats_iter->next, i ++)
         {
           KEYDB_SEARCH_DESC desc;
           kbnode_t kb;
@@ -1420,6 +1504,9 @@ ask_about_binding (ctrl_t ctrl,
           merge_keys_and_selfsig (kb);
 
           log_assert (kb->pkt->pkttype == PKT_PUBLIC_KEY);
+
+          kb_all[i] = kb;
+
           pk = kb->pkt->pkt.public_key;
 
           if (pk->has_expired)
@@ -1451,7 +1538,6 @@ ask_about_binding (ctrl_t ctrl,
 
               xfree (email2);
             }
-          release_kbnode (kb);
 
           if (! found_user_id)
             log_info (_("TOFU db may be corrupted: user id (%s)"
@@ -1460,7 +1546,74 @@ ask_about_binding (ctrl_t ctrl,
         }
       keydb_release (hd);
 
-      es_fprintf (fp, _("Statistics for keys with the email address \"%s\":\n"),
+      {
+        int j;
+        struct signature_stats **stats_prevp;
+        struct signature_stats *stats_iter_next;
+        int die[stats_count];
+
+        memset (die, 0, sizeof (die));
+
+        for (i = 0; i < stats_count; i ++)
+          {
+            /* i or a key that has cross sigs with i (possible
+               indirectly)?  */
+            if (! (i == 0 || die[i]))
+              continue;
+
+            for (j = i + 1; j < stats_count; j ++)
+              if (cross_sigs (kb_all[i], kb_all[j]))
+                die[j] = 1;
+          }
+
+        /* Free the dead stat structures.  */
+        for (stats_iter = stats, stats_prevp = &stats, i = 0;
+             stats_iter;
+             stats_iter = stats_iter_next, i ++)
+          {
+            stats_iter_next = stats_iter->next;
+
+            release_kbnode (kb_all[i]);
+
+            if (die[i])
+              {
+                *stats_prevp = stats_iter_next;
+                stats_iter->next = NULL;
+                signature_stats_free (stats_iter);
+
+                bindings_with_this_email_count --;
+              }
+            else
+              {
+                stats_prevp = &stats_iter->next;
+              }
+          }
+      }
+
+      log_assert (stats);
+      log_assert (bindings_with_this_email_count >= 1);
+
+      if ((*policy == TOFU_POLICY_NONE && bindings_with_this_email_count == 1)
+          || (*policy == TOFU_POLICY_ASK && conflict))
+      if (bindings_with_this_email_count == 1)
+        {
+          /* All "conflicts" were not really conflicts.  */
+          log_assert (! stats->next);
+
+          if (DBG_TRUST)
+            log_debug ("%s: all apparent TOFU conflicts are legitimate "
+                       "(cross sigs), setting policy to auto.\n",
+                       stats_iter->fingerprint);
+
+          *policy = TOFU_POLICY_AUTO;
+          record_binding (dbs, fingerprint, email, user_id, *policy, 0);
+          *trust_level = tofu_policy_to_trust_level (*policy);
+
+          goto out;
+        }
+
+      es_fprintf (fp, _("Statistics for potentially conflicting keys"
+                        " with the email address \"%s\":\n"),
                   email);
       for (stats_iter = stats; stats_iter; stats_iter = stats_iter->next)
         {
@@ -1644,6 +1797,7 @@ ask_about_binding (ctrl_t ctrl,
         }
       xfree (response);
     }
+ out:
   tofu_resume_batch_transaction (ctrl);
 
   xfree (prompt);