musig: Add MuSig2AggregatePubkeys variant that validates the aggregate

A common pattern that MuSig2 functions will use is to aggregate the
pubkeys to get the keyagg_cache and then validate the aggregated pubkey
against a provided aggregate pubkey. A variant of MuSig2AggregatePubkeys
is added which does that.

The functionality of GetMuSig2KeyAggCache and GetCPubKeyFromMuSig2KeyAggCache
are included in MuSig2AggregatePubkeys (and used internally) so there is
no expectation that callers will need these so they are made static.

src/musig.cpp

@@ -7,7 +7,7 @@
 
 #include <secp256k1_musig.h>
 
-bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache)
+static bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache)
 {
     // Parse the pubkeys
     std::vector<secp256k1_pubkey> secp_pubkeys;
@@ -29,7 +29,7 @@ bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_k
     return true;
 }
 
-std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_cache& keyagg_cache)
+static std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_cache& keyagg_cache)
 {
     // Get the plain aggregated pubkey
     secp256k1_pubkey agg_pubkey;
@@ -44,13 +44,21 @@ std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_ca
     return CPubKey(ser_agg_pubkey, ser_agg_pubkey + ser_agg_pubkey_len);
 }
 
-std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys)
+std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache, const std::optional<CPubKey>& expected_aggregate)
 {
-    secp256k1_musig_keyagg_cache keyagg_cache;
     if (!GetMuSig2KeyAggCache(pubkeys, keyagg_cache)) {
         return std::nullopt;
     }
-    return GetCPubKeyFromMuSig2KeyAggCache(keyagg_cache);
+    std::optional<CPubKey> agg_key = GetCPubKeyFromMuSig2KeyAggCache(keyagg_cache);
+    if (!agg_key.has_value()) return std::nullopt;
+    if (expected_aggregate.has_value() && expected_aggregate != agg_key) return std::nullopt;
+    return agg_key;
+}
+
+std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys)
+{
+    secp256k1_musig_keyagg_cache keyagg_cache;
+    return MuSig2AggregatePubkeys(pubkeys, keyagg_cache, std::nullopt);
 }
 
 CExtPubKey CreateMuSig2SyntheticXpub(const CPubKey& pubkey)

src/musig.h

@@ -18,11 +18,10 @@ struct secp256k1_musig_secnonce;
 using namespace util::hex_literals;
 constexpr uint256 MUSIG_CHAINCODE{"868087ca02a6f974c4598924c36b57762d32cb45717167e300622c7167e38965"_hex_u8};
 
-//! Create a secp256k1_musig_keyagg_cache from the pubkeys in their current order. This is necessary for most MuSig2 operations
+//! Compute the full aggregate pubkey from the given participant pubkeys in their current order.
-bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache);
+//! Outputs the secp256k1_musig_keyagg_cache and validates that the computed aggregate pubkey matches an expected aggregate pubkey.
-//! Retrieve the full aggregate pubkey from the secp256k1_musig_keyagg_cache
+//! This is necessary for most MuSig2 operations.
-std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_cache& cache);
+std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache, const std::optional<CPubKey>& expected_aggregate);
-//! Compute the full aggregate pubkey from the given participant pubkeys in their current order
 std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys);
 
 //! Construct the BIP 328 synthetic xpub for a pubkey