musig: add pubkey_get to obtain a full pubkey from a keyagg_cache

Author: jonasnick

include/secp256k1_musig.h

@@ -223,6 +223,24 @@ SECP256K1_API int secp256k1_musig_pubkey_agg(
     size_t n_pubkeys
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(5);
 
+/** Obtain the aggregate public key from a keyagg_cache.
+ *
+ *  This is only useful if you need the non-xonly public key, in particular for
+ *  ordinary (non-xonly) tweaking or batch-verifying multiple key aggregations
+ *  (not implemented).
+ *
+ *  Returns: 0 if the arguments are invalid, 1 otherwise
+ *  Args:        ctx: pointer to a context object
+ *  Out:      agg_pk: the MuSig-aggregated public key.
+ *  In: keyagg_cache: pointer to a `musig_keyagg_cache` struct initialized by
+ *                    `musig_pubkey_agg`
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_pubkey_get(
+    const secp256k1_context* ctx,
+    secp256k1_pubkey *agg_pk,
+    secp256k1_musig_keyagg_cache *keyagg_cache
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
 /** Tweak an x-only public key in a given keyagg_cache by adding
  *  the generator multiplied with `tweak32` to it.
  *

src/modules/musig/keyagg_impl.h

@@ -244,6 +244,20 @@ int secp256k1_musig_pubkey_agg(const secp256k1_context* ctx, secp256k1_scratch_s
     return 1;
 }
 
+int secp256k1_musig_pubkey_get(const secp256k1_context* ctx, secp256k1_pubkey *agg_pk, secp256k1_musig_keyagg_cache *keyagg_cache) {
+    secp256k1_keyagg_cache_internal cache_i;
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(agg_pk != NULL);
+    memset(agg_pk, 0, sizeof(*agg_pk));
+    ARG_CHECK(keyagg_cache != NULL);
+
+    if(!secp256k1_keyagg_cache_load(ctx, &cache_i, keyagg_cache)) {
+        return 0;
+    }
+    secp256k1_pubkey_save(agg_pk, &cache_i.pk);
+    return 1;
+}
+
 int secp256k1_musig_pubkey_tweak_add(const secp256k1_context* ctx, secp256k1_pubkey *output_pubkey, secp256k1_musig_keyagg_cache *keyagg_cache, const unsigned char *tweak32) {
     secp256k1_keyagg_cache_internal cache_i;
     int overflow = 0;

src/modules/musig/tests_impl.h

@@ -140,6 +140,7 @@ void musig_api_tests(secp256k1_scratch_space *scratch) {
     unsigned char aggnonce_ser[66];
     unsigned char msg[32];
     secp256k1_xonly_pubkey agg_pk;
+    secp256k1_pubkey full_agg_pk;
     secp256k1_musig_keyagg_cache keyagg_cache;
     secp256k1_musig_keyagg_cache invalid_keyagg_cache;
     secp256k1_musig_session session;
@@ -243,6 +244,15 @@ void musig_api_tests(secp256k1_scratch_space *scratch) {
     CHECK(secp256k1_musig_pubkey_agg(sign, scratch, &agg_pk, &keyagg_cache, pk_ptr, 2) == 1);
     CHECK(secp256k1_musig_pubkey_agg(vrfy, scratch, &agg_pk, &keyagg_cache, pk_ptr, 2) == 1);
 
+    /* pubkey_get */
+    ecount = 0;
+    CHECK(secp256k1_musig_pubkey_get(none, &full_agg_pk, &keyagg_cache) == 1);
+    CHECK(secp256k1_musig_pubkey_get(none, NULL, &keyagg_cache) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_pubkey_get(none, &full_agg_pk, NULL) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_memcmp_var(&full_agg_pk, zeros68, sizeof(full_agg_pk)) == 0);
+
     /** Tweaking **/
     ecount = 0;
     {