hs-v3: Extract INTRO2 key computation to its own function.

Part of #32709

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

1 files changed, 61 insertions, 31 deletions

diff --git a/src/feature/hs/hs_cell.c b/src/feature/hs/hs_cell.c
index 52bd663200..d59ea9edb9 100644
--- a/src/feature/hs/hs_cell.c
+++ b/src/feature/hs/hs_cell.c
@@ -747,6 +747,61 @@ hs_cell_parse_intro_established(const uint8_t *payload, size_t payload_len)
   return ret;
 }
 
+/** For the encrypted INTRO2 cell in <b>encrypted_section</b>, use the crypto
+ * material in <b>data</b> to compute the right ntor keys. Also validate the
+ * INTRO2 MAC to ensure that the keys are the right ones.
+ *
+ * Return NULL on failure to either produce the key material or on MAC
+ * valication. Else a newly allocated intro keys object. */
+static hs_ntor_intro_cell_keys_t *
+get_introduce2_keys_and_verify_mac(hs_cell_introduce2_data_t *data,
+                                   const uint8_t *encrypted_section,
+                                   size_t encrypted_section_len)
+{
+  hs_ntor_intro_cell_keys_t *intro_keys = NULL;
+
+  /* Build the key material out of the key material found in the cell. */
+  intro_keys = get_introduce2_key_material(data->auth_pk, data->enc_kp,
+                                           data->subcredential,
+                                           encrypted_section,
+                                           &data->client_pk);
+  if (intro_keys == NULL) {
+    log_info(LD_REND, "Invalid INTRODUCE2 encrypted data. Unable to "
+             "compute key material");
+    goto err;
+  }
+
+  /* Validate MAC from the cell and our computed key material. The MAC field
+   * in the cell is at the end of the encrypted section. */
+  {
+    uint8_t mac[DIGEST256_LEN];
+    /* The MAC field is at the very end of the ENCRYPTED section. */
+    size_t mac_offset = encrypted_section_len - sizeof(mac);
+    /* Compute the MAC. Use the entire encoded payload with a length up to the
+     * ENCRYPTED section. */
+    compute_introduce_mac(data->payload,
+                          data->payload_len - encrypted_section_len,
+                          encrypted_section, encrypted_section_len,
+                          intro_keys->mac_key, sizeof(intro_keys->mac_key),
+                          mac, sizeof(mac));
+    if (tor_memcmp(mac, encrypted_section + mac_offset, sizeof(mac))) {
+      log_info(LD_REND, "Invalid MAC validation for INTRODUCE2 cell");
+      goto err;
+    }
+  }
+
+  goto done;
+
+ err:
+  if (intro_keys) {
+    memwipe(intro_keys, 0, sizeof(hs_ntor_intro_cell_keys_t));
+    tor_free(intro_keys);
+  }
+
+ done:
+  return intro_keys;
+}
+
 /** Parse the INTRODUCE2 cell using data which contains everything we need to
  * do so and contains the destination buffers of information we extract and
  * compute from the cell. Return 0 on success else a negative value. The
@@ -801,41 +856,16 @@ hs_cell_parse_introduce2(hs_cell_introduce2_data_t *data,
     goto done;
   }
 
-  /* Build the key material out of the key material found in the cell. */
-  intro_keys = get_introduce2_key_material(data->auth_pk, data->enc_kp,
-                                           data->subcredential,
-                                           encrypted_section,
-                                           &data->client_pk);
-  if (intro_keys == NULL) {
-    log_info(LD_REND, "Invalid INTRODUCE2 encrypted data. Unable to "
-                      "compute key material on circuit %u for service %s",
-             TO_CIRCUIT(circ)->n_circ_id,
+  /* Get the right INTRODUCE2 ntor keys and verify the cell MAC */
+  intro_keys = get_introduce2_keys_and_verify_mac(data, encrypted_section,
+                                                  encrypted_section_len);
+  if (!intro_keys) {
+    log_info(LD_REND, "Could not get valid INTRO2 keys on circuit %u "
+             "for service %s", TO_CIRCUIT(circ)->n_circ_id,
              safe_str_client(service->onion_address));
     goto done;
   }
 
-  /* Validate MAC from the cell and our computed key material. The MAC field
-   * in the cell is at the end of the encrypted section. */
-  {
-    uint8_t mac[DIGEST256_LEN];
-    /* The MAC field is at the very end of the ENCRYPTED section. */
-    size_t mac_offset = encrypted_section_len - sizeof(mac);
-    /* Compute the MAC. Use the entire encoded payload with a length up to the
-     * ENCRYPTED section. */
-    compute_introduce_mac(data->payload,
-                          data->payload_len - encrypted_section_len,
-                          encrypted_section, encrypted_section_len,
-                          intro_keys->mac_key, sizeof(intro_keys->mac_key),
-                          mac, sizeof(mac));
-    if (tor_memcmp(mac, encrypted_section + mac_offset, sizeof(mac))) {
-      log_info(LD_REND, "Invalid MAC validation for INTRODUCE2 cell on "
-                        "circuit %u for service %s",
-               TO_CIRCUIT(circ)->n_circ_id,
-               safe_str_client(service->onion_address));
-      goto done;
-    }
-  }
-
   {
     /* The ENCRYPTED_DATA section starts just after the CLIENT_PK. */
     const uint8_t *encrypted_data =