fix: Ticket #31589

- The function `decrypt_desc_layer` has a cleaner interface.
- `is_superencrypted_layer` changed from `int` -> `bool`

[ticket details](https://trac.torproject.org/projects/tor/ticket/31589)

add(changes/*): changes file
fix(src/features/hs): is_superencrypted changed from `int` -> `bool`
fix(changes/ticket31589): header
add(changes/ticket31589): subsystem(onion services) to change

changes/ticket31589

@@ -0,0 +1,2 @@
+  o Code simplification and refactoring (onion services):
+    - Interface for function `decrypt_desc_layer` cleaned up. Closes ticket 31589.

scripts/maint/practracker/exceptions.txt

@@ -244,7 +244,7 @@ problem function-size /src/feature/hs/hs_common.c:hs_get_responsible_hsdirs() 10
 problem function-size /src/feature/hs/hs_config.c:config_service_v3() 107
 problem function-size /src/feature/hs/hs_config.c:config_generic_service() 138
 problem function-size /src/feature/hs/hs_descriptor.c:desc_encode_v3() 101
-problem function-size /src/feature/hs/hs_descriptor.c:decrypt_desc_layer() 105
+problem function-size /src/feature/hs/hs_descriptor.c:decrypt_desc_layer() 111
 problem function-size /src/feature/hs/hs_descriptor.c:decode_introduction_point() 122
 problem function-size /src/feature/hs/hs_descriptor.c:desc_decode_superencrypted_v3() 107
 problem function-size /src/feature/hs/hs_descriptor.c:desc_decode_encrypted_v3() 107

src/feature/hs/hs_descriptor.c

@@ -1477,10 +1477,8 @@ decrypt_descriptor_cookie(const hs_descriptor_t *desc,
  */
 MOCK_IMPL(STATIC size_t,
 decrypt_desc_layer,(const hs_descriptor_t *desc,
-                    const uint8_t *encrypted_blob,
-                    size_t encrypted_blob_size,
                     const uint8_t *descriptor_cookie,
-                    int is_superencrypted_layer,
+                    bool is_superencrypted_layer,
                     char **decrypted_out))
 {
   uint8_t *decrypted = NULL;
@@ -1490,6 +1488,12 @@ decrypt_desc_layer,(const hs_descriptor_t *desc,
   uint8_t mac_key[DIGEST256_LEN], our_mac[DIGEST256_LEN];
   const uint8_t *salt, *encrypted, *desc_mac;
   size_t encrypted_len, result_len = 0;
+  const uint8_t *encrypted_blob = (is_superencrypted_layer)
+    ? desc->plaintext_data.superencrypted_blob
+    : desc->superencrypted_data.encrypted_blob;
+  size_t encrypted_blob_size = (is_superencrypted_layer)
+    ? desc->plaintext_data.superencrypted_blob_size
+    : desc->superencrypted_data.encrypted_blob_size;
 
   tor_assert(decrypted_out);
   tor_assert(desc);
@@ -1603,9 +1607,8 @@ desc_decrypt_superencrypted(const hs_descriptor_t *desc, char **decrypted_out)
   tor_assert(decrypted_out);
 
   superencrypted_len = decrypt_desc_layer(desc,
-                                 desc->plaintext_data.superencrypted_blob,
+                                          NULL,
-                                 desc->plaintext_data.superencrypted_blob_size,
+                                          true, &superencrypted_plaintext);
-                                 NULL, 1, &superencrypted_plaintext);
 
   if (!superencrypted_len) {
     log_warn(LD_REND, "Decrypting superencrypted desc failed.");
@@ -1654,9 +1657,9 @@ desc_decrypt_encrypted(const hs_descriptor_t *desc,
   }
 
   encrypted_len = decrypt_desc_layer(desc,
-                                 desc->superencrypted_data.encrypted_blob,
+                                     descriptor_cookie,
-                                 desc->superencrypted_data.encrypted_blob_size,
+                                     false, &encrypted_plaintext);
-                                 descriptor_cookie, 0, &encrypted_plaintext);
+
   if (!encrypted_len) {
     goto err;
   }

src/feature/hs/hs_descriptor.h

@@ -276,6 +276,7 @@ void hs_desc_authorized_client_free_(hs_desc_authorized_client_t *client);
                 hs_desc_authorized_client_free_, (client))
 
 hs_desc_authorized_client_t *hs_desc_build_fake_authorized_client(void);
+
 void hs_desc_build_authorized_client(const uint8_t *subcredential,
                                      const curve25519_public_key_t *
                                      client_auth_pk,
@@ -308,10 +309,8 @@ STATIC int desc_sig_is_valid(const char *b64_sig,
                              const char *encoded_desc, size_t encoded_len);
 
 MOCK_DECL(STATIC size_t, decrypt_desc_layer,(const hs_descriptor_t *desc,
-                                             const uint8_t *encrypted_blob,
-                                             size_t encrypted_blob_size,
                                              const uint8_t *descriptor_cookie,
-                                             int is_superencrypted_layer,
+                                             bool is_superencrypted_layer,
                                              char **decrypted_out));
 
 #endif /* defined(HS_DESCRIPTOR_PRIVATE) */

src/test/fuzz/fuzz_hsdescv3.c

@@ -35,16 +35,21 @@ mock_rsa_ed25519_crosscert_check(const uint8_t *crosscert,
 
 static size_t
 mock_decrypt_desc_layer(const hs_descriptor_t *desc,
-                        const uint8_t *encrypted_blob,
-                        size_t encrypted_blob_size,
                         const uint8_t *descriptor_cookie,
-                        int is_superencrypted_layer,
+                        bool is_superencrypted_layer,
                         char **decrypted_out)
 {
   (void)is_superencrypted_layer;
   (void)desc;
   (void)descriptor_cookie;
   const size_t overhead = HS_DESC_ENCRYPTED_SALT_LEN + DIGEST256_LEN;
+  const uint8_t *encrypted_blob = (is_superencrypted_layer)
+    ? desc->plaintext_data.superencrypted_blob
+    : desc->superencrypted_data.encrypted_blob;
+  size_t encrypted_blob_size = (is_superencrypted_layer)
+    ? desc->plaintext_data.superencrypted_blob_size
+    : desc->superencrypted_data.encrypted_blob_size;
+
   if (encrypted_blob_size < overhead)
     return 0;
   *decrypted_out = tor_memdup_nulterm(