hs-v3: Check the ed25519 key when picking intro point

If the intro point supports ed25519 link authentication, make sure we don't
have a zeroed key which would lead to a failure to extend to it.

We already check for an empty key if the intro point does not support it so
this makes the check on the key more consistent and symmetric.

Fixes #24002

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/bug24002

@@ -0,0 +1,5 @@
+  o Minor bugfixes (hidden service):
+    - Make sure that we have a usable ed25519 key when the intro point relay
+      does support ed25519 link authentication. We do check for an empty key
+      when the relay does not support it so this makes it nice and symmetric.
+      Fixes bug 24002; bugfix on 0.3.2.1-alpha.

src/or/hs_service.c

@@ -1564,6 +1564,10 @@ pick_intro_point(unsigned int direct_conn, smartlist_t *exclude_nodes)
    * protocol */
   if (!node_supports_ed25519_link_authentication(node)) {
     tor_assert_nonfatal(ed25519_public_key_is_zero(&info->ed_identity));
+  } else {
+    /* Make sure we *do* have an ed key if we support the link authentication.
+     * Sending an empty key would result in a failure to extend. */
+    tor_assert_nonfatal(!ed25519_public_key_is_zero(&info->ed_identity));
   }
 
   /* Create our objects and populate them with the node information. */