descriptor: allow fetching the blinding key from ct descriptors

Author: jgriffiths

include/wally_descriptor.h

@@ -39,6 +39,8 @@ struct wally_descriptor;
 /*** ms-canonicalization-flags Miniscript/Descriptor canonicalization flags */
 #define WALLY_MS_CANONICAL_NO_CHECKSUM 0x01 /** Do not include a checksum */
 
+#define WALLY_MS_BLINDING_KEY_INDEX 0xffffffff /* Key index for confidential blinding key */
+
 /**
  * Parse an output descriptor or miniscript expression.
  *
@@ -196,7 +198,8 @@ WALLY_CORE_API int wally_descriptor_get_depth(
  * :param descriptor: Parsed output descriptor or miniscript expression.
  * :param value_out: Destination for the number of keys.
  *
- * .. note:: Repeated keys are counted once for each time they appear.
+ * .. note:: Repeated keys are counted once for each time they appear, and any
+ *|    blinding key within the descriptor is not included in the count.
  */
 WALLY_CORE_API int wally_descriptor_get_num_keys(
     const struct wally_descriptor *descriptor,
@@ -206,13 +209,16 @@ WALLY_CORE_API int wally_descriptor_get_num_keys(
  * Get the string representation of a key in a parsed output descriptor or miniscript expression.
  *
  * :param descriptor: Parsed output descriptor or miniscript expression.
- * :param index: The zero-based index of the key to get.
+ * :param index: The zero-based index of the key to get, or `WALLY_MS_BLINDING_KEY_INDEX`
+ *|    to fetch the descriptors blinding key representaton (if any).
  * :param output: Destination for the resulting string representation.
  *|    The string returned should be freed using `wally_free_string`.
  *
  * .. note:: Keys may be BIP32 xpub/xpriv, WIF or hex pubkeys, and may be
  *|    x-only. The caller can use `wally_descriptor_get_key_features` to
  *|    determine the type of a given key.
+ *
+ * .. note:: Raw private blinding keys are returned as hex, not WIF.
  */
 WALLY_CORE_API int wally_descriptor_get_key(
     const struct wally_descriptor *descriptor,
@@ -223,7 +229,8 @@ WALLY_CORE_API int wally_descriptor_get_key(
  * Get the features of a key in a parsed output descriptor or miniscript expression.
  *
  * :param descriptor: Parsed output descriptor or miniscript expression.
- * :param index: The zero-based index of the key to get.
+ * :param index: The zero-based index of the key to get, or `WALLY_MS_BLINDING_KEY_INDEX`
+ *|    to fetch the descriptors blinding key features (if any).
  * :param value_out: Destination for the resulting :ref:`miniscript-features`.
  */
 WALLY_CORE_API int wally_descriptor_get_key_features(

src/descriptor.c

@@ -569,6 +569,13 @@ static bool node_is_root(const ms_node *node)
     return !node->parent || node->parent->builtin;
 }
 
+#ifdef BUILD_ELEMENTS
+static bool node_is_ct(const ms_node *node)
+{
+    return !node->parent && node->kind == KIND_DESCRIPTOR_CT;
+}
+#endif
+
 static void node_free(ms_node *node)
 {
     if (node) {
@@ -3325,14 +3332,31 @@ static const ms_node *descriptor_get_key(const struct wally_descriptor *descript
 int wally_descriptor_get_key(const struct wally_descriptor *descriptor,
                              size_t index, char **output)
 {
-    const ms_node *node = descriptor_get_key(descriptor, index);
+    const ms_node *node = NULL;
+#ifdef BUILD_ELEMENTS
+    if (index == WALLY_MS_BLINDING_KEY_INDEX) {
+        if (descriptor && node_is_ct(descriptor->top_node)) {
+            node = descriptor->top_node->child;
+            if (node && node->kind == KIND_DESCRIPTOR_SLIP77)
+                node = node->child;
+        }
+    } else
+#endif
+        node = descriptor_get_key(descriptor, index);
 
     if (output)
         *output = 0;
     if (!node || !output)
         return WALLY_EINVAL;
 
+#ifdef BUILD_ELEMENTS
+    if (index == WALLY_MS_BLINDING_KEY_INDEX) {
+        if (node->kind == KIND_PRIVATE_KEY || node->kind == KIND_RAW)
+            goto return_hex;
+    }
+#endif
     if (node->kind == KIND_PUBLIC_KEY) {
+return_hex:
         return wally_hex_from_bytes((const unsigned char *)node->data,
                                     node->data_len, output);
     }
@@ -3354,7 +3378,17 @@ int wally_descriptor_get_key(const struct wally_descriptor *descriptor,
 int wally_descriptor_get_key_features(const struct wally_descriptor *descriptor,
                                       size_t index, uint32_t *value_out)
 {
-    const ms_node *node = descriptor_get_key(descriptor, index);
+    const ms_node *node = NULL;
+#ifdef BUILD_ELEMENTS
+    if (index == WALLY_MS_BLINDING_KEY_INDEX) {
+        if (descriptor && node_is_ct(descriptor->top_node)) {
+            node = descriptor->top_node->child;
+            if (node && node->kind == KIND_DESCRIPTOR_SLIP77)
+                node = node->child;
+        }
+    } else
+#endif
+        node = descriptor_get_key(descriptor, index);
 
     if (value_out)
         *value_out = 0;

src/test/test_descriptor.py

@@ -32,6 +32,8 @@
 
 NO_CHECKSUM = 0x1 # WALLY_MS_CANONICAL_NO_CHECKSUM
 
+BLINDING_KEY_INDEX = 0xffffffff
+
 def wally_map_from_dict(d):
     m = pointer(wally_map())
     assert(wally_map_init_alloc(len(d.keys()), None, m) == WALLY_OK)
@@ -307,6 +309,15 @@ def test_features_and_depth(self):
             self.assertEqual((ret, depth), (WALLY_OK, expected_depth))
             ret, num_keys = wally_descriptor_get_num_keys(d)
             self.assertEqual((ret, num_keys), (WALLY_OK, expected_keys))
+            ret, key_info = wally_descriptor_get_key(d, BLINDING_KEY_INDEX)
+            if descriptor.startswith('ct'):
+                self.assertEqual(ret, WALLY_OK)
+                expected_key_info = descriptor.split(',')[0][3:]
+                if expected_key_info.startswith('slip77'):
+                    expected_key_info = expected_key_info[7:-1]
+                self.assertEqual(key_info, expected_key_info)
+            else:
+                self.assertEqual(ret, WALLY_EINVAL)
             wally_descriptor_free(d)
             # Check the maximum depth parsing limit
             for limit, expected in [(depth-1, WALLY_EINVAL), (depth, WALLY_OK)]:

src/wasm_package/src/const.js

@@ -130,6 +130,7 @@ export const WALLY_MINISCRIPT_TAPSCRIPT = 0x01; /** Tapscript, use x-only pubkey
 export const WALLY_MINISCRIPT_UNIQUE_KEYPATHS = 0x10; /** For policy templates, ensure BIP32 derivation paths differ for identical keys */
 export const WALLY_MINOR_VER = 4;
 export const WALLY_MS_ANY_BLINDING_KEY = 0xE00; /** SLIP-77, ELIP-150 or ELIP-151 blinding key present */
+export const WALLY_MS_BLINDING_KEY_INDEX = 0xffffffff; /* Key index for confidential blinding key */
 export const WALLY_MS_CANONICAL_NO_CHECKSUM = 0x01; /** Do not include a checksum */
 export const WALLY_MS_IS_DESCRIPTOR = 0x020; /** Contains only descriptor expressions (no miniscript) */
 export const WALLY_MS_IS_ELEMENTS = 0x100; /** Contains Elements expressions or was parsed as Elements */