wallet: refactor calculating ga cosigner service bip32 path

Populate the root part of the path (dependent on the wallet root key)
separately from deducing the path tail (dependent on user path).

Author: Jamie C. Driver

main/wallet.c

@@ -579,50 +579,77 @@ bool wallet_get_gaservice_fingerprint(const char* network, uint8_t* output, size
     return true;
 }
 
-// Fetch the wallet's relevant gait service path based on the user signer's path
-bool wallet_get_gaservice_path(
-    const uint32_t* path, const size_t path_len, uint32_t* ga_path, const size_t ga_path_len, size_t* written)
+static void wallet_get_gaservice_path_root(const bool subaccount_root, uint32_t* ga_path, const size_t ga_path_len)
 {
-    if (!path || path_len == 0 || !ga_path || ga_path_len != MAX_GASERVICE_PATH_LEN || !written) {
-        return false;
-    }
+    JADE_ASSERT(ga_path);
+    JADE_ASSERT(ga_path_len == GASERVICE_ROOT_PATH_LEN);
 
     const keychain_t* const keychain = keychain_get();
     JADE_ASSERT(keychain);
 
-    // We only support the following cases
-    if (path_len == 2) {
-        // 1.  1/ptr
-        if (path[0] != PATH_BRANCH || path[1] >= MAX_PATH_PTR) {
-            return false;
-        }
+    // The first element is 3 for subaccount paths, or 1 otherwise
+    ga_path[0] = subaccount_root ? 3 : 1;
 
-        // gapath: 1/<ga service path (32)>/ptr
-        ga_path[0] = path[0];
-        // skip 1-32
-        ga_path[33] = path[1];
-        *written = 34;
+    // GA service path goes in elements 1 - 32 incl.
+    JADE_STATIC_ASSERT(sizeof(keychain->gaservice_path) == (ga_path_len - 1) * sizeof(ga_path[0]));
+    memcpy(&ga_path[1], keychain->gaservice_path, sizeof(keychain->gaservice_path));
+}
+
+static bool wallet_get_gaservice_path_tail(const uint32_t* path, const size_t path_len, uint32_t* ga_path,
+    const size_t ga_path_len, size_t* written, bool* is_subaccount_path)
+{
+    JADE_ASSERT(path);
+    JADE_ASSERT(path_len);
+    JADE_ASSERT(ga_path);
+    JADE_ASSERT(ga_path_len >= MAX_GASERVICE_PATH_TAIL_LEN);
+    JADE_ASSERT(written);
+    JADE_ASSERT(is_subaccount_path);
+
+    // We only support the following cases - populate the path tail
+    if (path_len == 2 && path[0] == PATH_BRANCH && path[1] < MAX_PATH_PTR) {
+        // 1.  1/ptr
+        // service path tail: ptr
+        ga_path[0] = path[1];
+        *written = 1;
+        *is_subaccount_path = false;
+        return true;
+    }
 
-    } else if (path_len == 4) {
+    if (path_len == 4 && path[0] == SUBACT_ROOT && path[1] > SUBACT_FLOOR && path[1] < SUBACT_CEILING
+        && path[2] == PATH_BRANCH && path[3] < MAX_PATH_PTR) {
         // 2.  3'/subact'/1/ptr
-        if (path[0] != SUBACT_ROOT || path[1] <= SUBACT_FLOOR || path[1] >= SUBACT_CEILING || path[2] != PATH_BRANCH
-            || path[3] >= MAX_PATH_PTR) {
-            return false;
-        }
+        // service path tail: subact/ptr
+        ga_path[0] = unharden(path[1]);
+        ga_path[1] = path[3];
+        *written = 2;
+        *is_subaccount_path = true;
+        return true;
+    }
 
-        // gapath: 3/<ga service path (32)>/subact/ptr
-        ga_path[0] = unharden(path[0]);
-        // skip 1-32
-        ga_path[33] = unharden(path[1]);
-        ga_path[34] = path[3];
-        *written = 35;
-    } else {
+    // Path pattern not recognised
+    return false;
+}
+
+// Fetch the wallet's relevant gait service path based on the user signer's path
+bool wallet_get_gaservice_path(
+    const uint32_t* path, const size_t path_len, uint32_t* ga_path, const size_t ga_path_len, size_t* written)
+{
+    if (!path || path_len == 0 || !ga_path || ga_path_len != MAX_GASERVICE_PATH_LEN || !written) {
         return false;
     }
 
-    // GA service path goes in elements 1 - 32 incl.
-    JADE_STATIC_ASSERT(sizeof(keychain->gaservice_path) == (ga_path_len - 1) * sizeof(ga_path[0]));
-    memcpy(&ga_path[1], keychain->gaservice_path, sizeof(keychain->gaservice_path));
+    // Populate the path tail based on the user path
+    size_t tail_len = 0;
+    bool is_subaccount_path = false;
+    if (!wallet_get_gaservice_path_tail(path, path_len, &ga_path[GASERVICE_ROOT_PATH_LEN],
+            ga_path_len - GASERVICE_ROOT_PATH_LEN, &tail_len, &is_subaccount_path)) {
+        // Path pattern not recognised
+        return false;
+    }
+
+    // Populate GA service path root
+    wallet_get_gaservice_path_root(is_subaccount_path, ga_path, GASERVICE_ROOT_PATH_LEN);
+    *written = GASERVICE_ROOT_PATH_LEN + tail_len;
 
     return true;
 }

main/wallet.h

@@ -23,7 +23,9 @@ typedef struct {
 } signing_data_t;
 
 #define MAX_VARIANT_LEN 24
-#define MAX_GASERVICE_PATH_LEN 35
+#define GASERVICE_ROOT_PATH_LEN (1 + 32)
+#define MAX_GASERVICE_PATH_TAIL_LEN (1 + 1)
+#define MAX_GASERVICE_PATH_LEN (GASERVICE_ROOT_PATH_LEN + MAX_GASERVICE_PATH_TAIL_LEN)
 
 // 'm' + ( ('/' + <10 digit number>[+ ']) * n) + '\0'
 #define MAX_PATH_STR_LEN(max_path_elems) (1 + ((1 + 10 + 1) * max_path_elems) + 1)