MuSig2 signing, rounds 1 and 2

src/commands.h

@@ -11,3 +11,7 @@ typedef enum {
  GET_MASTER_FINGERPRINT = 0x05,
  SIGN_MESSAGE = 0x10,
 } command_e;
+
+// Tags used when yielding different objects with the YIELD client command.
+#define CCMD_YIELD_MUSIG_PUBNONCE_TAG 0xffffffff
+#define CCMD_YIELD_MUSIG_PARTIALSIGNATURE_TAG 0xfffffffe

src/handler/lib/policy.c

@@ -420,7 +420,7 @@ execute_processor(policy_parser_state_t *state, policy_parser_processor_t proc,
 
 // convenience function, split from get_derived_pubkey only to improve stack usage
 // returns -1 on error, 0 if the returned key info has no wildcard (**), 1 if it has the wildcard
-__attribute__((noinline, warn_unused_result)) static int get_extended_pubkey(
+__attribute__((noinline, warn_unused_result)) int get_extended_pubkey(
  dispatcher_context_t *dispatcher_context,
  const wallet_derivation_info_t *wdi,
  int key_index,

src/handler/lib/policy.h

@@ -50,6 +50,29 @@ typedef struct {
  bool change; // whether a change address or a receive address is derived
 } wallet_derivation_info_t;
 
+/**
+ * Computes the a derived compressed pubkey for one of the key of the wallet policy,
+ * for a given change/address_index combination.
+ *
+ * This function computes the extended public key (xpub) based on the provided
+ * BIP32 derivation path. It supports both standard BIP32 derivation and
+ * the derivation of Musig (multi-signature) keys.
+ *
+ * @param[in] dispatcher_context Pointer to the dispatcher content
+ * @param[in] wdi Pointer to a `wallet_derivation_info_t` struct with the details of the
+ * necessary details of the wallet policy, and the desired change/address_index pair.
+ * @param[in] key_index Index of the pubkey in the vector of keys of the wallet policy.
+ * @param[out] out Pointer to a `serialized_extended_pubkey_t` that will contain the requested
+ * extended pubkey.
+ *
+ * @return -1 on error, 0 if the returned key info has no wildcard (**), 1 if it has the wildcard.
+ */
+__attribute__((warn_unused_result)) int get_extended_pubkey(
+ dispatcher_context_t *dispatcher_context,
+ const wallet_derivation_info_t *wdi,
+ int key_index,
+ serialized_extended_pubkey_t *out);
+
 /**
  * Computes the hash of a taptree, to be used as tweak for the internal key per BIP-0341;
  * The returned hash is the second value in the tuple returned by taproot_tree_helper in