router: Keep RSA onion public key in ASN.1 format

The OpenSSL "RSA" object is currently 408 bytes compares to the ASN.1 encoding
which is 140 for a 1024 RSA key.

We save 268 bytes per descriptor (routerinfo_t) *and* microdescriptor
(microdesc_t). Scaling this to 6000 relays, and considering client usually
only have microdescriptors, we save 1.608 MB of RAM which is considerable for
mobile client.

This commit makes it that we keep the RSA onion public key (used for TAP
handshake) in ASN.1 format instead of an OpenSSL RSA object.

Changes is done in both routerinfo_t and microdesc_t.

Closes #27246

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

2 files changed, 59 insertions, 5 deletions

diff --git a/src/feature/relay/router.c b/src/feature/relay/router.c
index e4ec01af24..ad97d534c2 100644
--- a/src/feature/relay/router.c
+++ b/src/feature/relay/router.c
@@ -1464,6 +1464,8 @@ router_should_advertise_begindir(const or_options_t *options,
 static extend_info_t *
 extend_info_from_router(const routerinfo_t *r)
 {
+  crypto_pk_t *rsa_pubkey;
+  extend_info_t *info;
   tor_addr_port_t ap;
   tor_assert(r);
 
@@ -1477,10 +1479,13 @@ extend_info_from_router(const routerinfo_t *r)
     ed_id_key = NULL;
 
   router_get_prim_orport(r, &ap);
-  return extend_info_new(r->nickname, r->cache_info.identity_digest,
+  rsa_pubkey = router_get_rsa_onion_pkey(r->onion_pkey, r->onion_pkey_len);
+  info = extend_info_new(r->nickname, r->cache_info.identity_digest,
                          ed_id_key,
-                         r->onion_pkey, r->onion_curve25519_pkey,
+                         rsa_pubkey, r->onion_curve25519_pkey,
                          &ap.addr, ap.port);
+  crypto_pk_free(rsa_pubkey);
+  return info;
 }
 
 /**See if we currently believe our ORPort or DirPort to be
@@ -2313,8 +2318,10 @@ router_build_fresh_descriptor(routerinfo_t **r, extrainfo_t **e)
   ri->supports_tunnelled_dir_requests =
     directory_permits_begindir_requests(options);
   ri->cache_info.published_on = time(NULL);
-  ri->onion_pkey = crypto_pk_dup_key(get_onion_key()); /* must invoke from
-                                                        * main thread */
+  /* get_onion_key() must invoke from main thread */
+  router_set_rsa_onion_pkey(get_onion_key(), &ri->onion_pkey,
+                            &ri->onion_pkey_len);
+
   ri->onion_curve25519_pkey =
     tor_memdup(&get_current_curve25519_keypair()->pubkey,
                sizeof(curve25519_public_key_t));
@@ -2849,6 +2856,7 @@ router_dump_router_to_string(routerinfo_t *router,
 {
   char *address = NULL;
   char *onion_pkey = NULL; /* Onion key, PEM-encoded. */
+  crypto_pk_t *rsa_pubkey = NULL;
   char *identity_pkey = NULL; /* Identity key, PEM-encoded. */
   char digest[DIGEST256_LEN];
   char published[ISO_TIME_LEN+1];
@@ -2915,7 +2923,9 @@ router_dump_router_to_string(routerinfo_t *router,
   }
 
   /* PEM-encode the onion key */
-  if (crypto_pk_write_public_key_to_string(router->onion_pkey,
+  rsa_pubkey = router_get_rsa_onion_pkey(router->onion_pkey,
+                                         router->onion_pkey_len);
+  if (crypto_pk_write_public_key_to_string(rsa_pubkey,
                                            &onion_pkey,&onion_pkeylen)<0) {
     log_warn(LD_BUG,"write onion_pkey to string failed!");
     goto err;
@@ -3200,6 +3210,7 @@ router_dump_router_to_string(routerinfo_t *router,
     SMARTLIST_FOREACH(chunks, char *, cp, tor_free(cp));
     smartlist_free(chunks);
   }
+  crypto_pk_free(rsa_pubkey);
   tor_free(address);
   tor_free(family_line);
   tor_free(onion_pkey);
@@ -3827,3 +3838,42 @@ router_get_all_orports(const routerinfo_t *ri)
   fake_node.ri = (routerinfo_t *)ri;
   return node_get_all_orports(&fake_node);
 }
+
+/* From the given RSA key object, convert it to ASN-1 encoded format and set
+ * the newly allocated object in onion_pkey_out. The length of the key is set
+ * in onion_pkey_len_out. */
+void
+router_set_rsa_onion_pkey(const crypto_pk_t *pk, char **onion_pkey_out,
+                          size_t *onion_pkey_len_out)
+{
+  int len;
+  char buf[1024];
+
+  tor_assert(pk);
+  tor_assert(onion_pkey_out);
+  tor_assert(onion_pkey_len_out);
+
+  len = crypto_pk_asn1_encode(pk, buf, sizeof(buf));
+  if (BUG(len < 0)) {
+    goto done;
+  }
+
+  *onion_pkey_out = tor_memdup(buf, len);
+  *onion_pkey_len_out = len;
+
+ done:
+  return;
+}
+
+/* From an ASN-1 encoded onion pkey, return a newly allocated RSA key object.
+ * It is the caller responsability to free the returned object.
+ *
+ * Return NULL if the pkey is NULL, malformed or if the length is 0. */
+crypto_pk_t *
+router_get_rsa_onion_pkey(const char *pkey, size_t pkey_len)
+{
+  if (!pkey || pkey_len == 0) {
+    return NULL;
+  }
+  return crypto_pk_asn1_decode(pkey, pkey_len);
+}
diff --git a/src/feature/relay/router.h b/src/feature/relay/router.h
index 51ac365798..cf0d27a456 100644
--- a/src/feature/relay/router.h
+++ b/src/feature/relay/router.h
@@ -45,6 +45,10 @@ void v3_authority_check_key_expiry(void);
 int get_onion_key_lifetime(void);
 int get_onion_key_grace_period(void);
 
+crypto_pk_t *router_get_rsa_onion_pkey(const char *pkey, size_t pkey_len);
+void router_set_rsa_onion_pkey(const crypto_pk_t *pk, char **onion_pkey_out,
+                               size_t *onion_pkey_len);
+
 di_digest256_map_t *construct_ntor_key_map(void);
 void ntor_key_map_free_(di_digest256_map_t *map);
 #define ntor_key_map_free(map) \