Stop sending unsupported ed25519 link specifiers in v3 introduce cells

Stop sending ed25519 link specifiers in v3 onion service introduce
cells, when the rendezvous point doesn't support ed25519 link
authentication.

Fixes bug 26627; bugfix on 0.3.2.4-alpha.

changes/bug26627

+  o Minor bugfixes (v3 onion services):
+    - Stop sending ed25519 link specifiers in v3 onion service introduce
+      cells, when the rendezvous point doesn't support ed25519 link
+      authentication. Fixes bug 26627; bugfix on 0.3.2.4-alpha.

src/or/hs_circuit.c

@@ -559,10 +559,14 @@ retry_service_rendezvous_point(const origin_circuit_t *circ)
   return;
 }
 
-/* Add all possible link specifiers in node to lspecs.
- * legacy ID is mandatory thus MUST be present in node. If the primary address
- * is not IPv4, log a BUG() warning, and return an empty smartlist.
- * Includes ed25519 id and IPv6 link specifiers if present in the node. */
+/* Add all possible link specifiers in node to lspecs:
+ *  - legacy ID is mandatory thus MUST be present in node;
+ *  - include ed25519 link specifier if present in the node, and the node
+ *    supports ed25519 link authentication, even if its link versions are not
+ *    compatible with us;
+ *  - include IPv4 link specifier, if the primary address is not IPv4, log a
+ *    BUG() warning, and return an empty smartlist;
+ *  - include IPv6 link specifier if present in the node. */
 static void
 get_lspecs_from_node(const node_t *node, smartlist_t *lspecs)
 {
@@ -600,8 +604,12 @@ get_lspecs_from_node(const node_t *node, smartlist_t *lspecs)
   link_specifier_set_ls_len(ls, link_specifier_getlen_un_legacy_id(ls));
   smartlist_add(lspecs, ls);
 
-  /* ed25519 ID is only included if the node has it. */
-  if (!ed25519_public_key_is_zero(&node->ed25519_id)) {
+  /* ed25519 ID is only included if the node has it, and the node declares a
+     protocol version that supports ed25519 link authentication, even if that
+     link version is not compatible with us. (We are sending the ed25519 key
+     to another tor, which may support different link versions.) */
+  if (!ed25519_public_key_is_zero(&node->ed25519_id) &&
+      node_supports_ed25519_link_authentication(node, 0)) {
     ls = link_specifier_new();
     link_specifier_set_ls_type(ls, LS_ED25519_ID);
     memcpy(link_specifier_getarray_un_ed25519_id(ls), &node->ed25519_id,