Add MuSig2 Keyagg Cache helper functions

secp256k1 provides us secp256k1_musig_keyagg_cache objects which we are
used as part of session info and to get the aggregate pubkey. These
helper functions help us convert to/from the secp256k1 C objects into
the Bitcoin Core C++ objects.

src/CMakeLists.txt

@@ -161,6 +161,7 @@ add_library(bitcoin_common STATIC EXCLUDE_FROM_ALL
   key.cpp
   key_io.cpp
   merkleblock.cpp
+  musig.cpp
   net_permissions.cpp
   net_types.cpp
   netaddress.cpp

src/musig.cpp

@@ -0,0 +1,53 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <musig.h>
+
+#include <secp256k1_musig.h>
+
+bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache)
+{
+    // Parse the pubkeys
+    std::vector<secp256k1_pubkey> secp_pubkeys;
+    std::vector<const secp256k1_pubkey*> pubkey_ptrs;
+    for (const CPubKey& pubkey : pubkeys) {
+        if (!secp256k1_ec_pubkey_parse(secp256k1_context_static, &secp_pubkeys.emplace_back(), pubkey.data(), pubkey.size())) {
+            return false;
+        }
+    }
+    pubkey_ptrs.reserve(secp_pubkeys.size());
+    for (const secp256k1_pubkey& p : secp_pubkeys) {
+        pubkey_ptrs.push_back(&p);
+    }
+
+    // Aggregate the pubkey
+    if (!secp256k1_musig_pubkey_agg(secp256k1_context_static, nullptr, &keyagg_cache, pubkey_ptrs.data(), pubkey_ptrs.size())) {
+        return false;
+    }
+    return true;
+}
+
+std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_cache& keyagg_cache)
+{
+    // Get the plain aggregated pubkey
+    secp256k1_pubkey agg_pubkey;
+    if (!secp256k1_musig_pubkey_get(secp256k1_context_static, &agg_pubkey, &keyagg_cache)) {
+        return std::nullopt;
+    }
+
+    // Turn into CPubKey
+    unsigned char ser_agg_pubkey[CPubKey::COMPRESSED_SIZE];
+    size_t ser_agg_pubkey_len = CPubKey::COMPRESSED_SIZE;
+    secp256k1_ec_pubkey_serialize(secp256k1_context_static, ser_agg_pubkey, &ser_agg_pubkey_len, &agg_pubkey, SECP256K1_EC_COMPRESSED);
+    return CPubKey(ser_agg_pubkey, ser_agg_pubkey + ser_agg_pubkey_len);
+}
+
+std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys)
+{
+    secp256k1_musig_keyagg_cache keyagg_cache;
+    if (!GetMuSig2KeyAggCache(pubkeys, keyagg_cache)) {
+        return std::nullopt;
+    }
+    return GetCPubKeyFromMuSig2KeyAggCache(keyagg_cache);
+}

src/musig.h

@@ -0,0 +1,22 @@
+// Copyright (c) 2024-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_MUSIG_H
+#define BITCOIN_MUSIG_H
+
+#include <pubkey.h>
+
+#include <optional>
+#include <vector>
+
+struct secp256k1_musig_keyagg_cache;
+
+//! Create a secp256k1_musig_keyagg_cache from the pubkeys in their current order. This is necessary for most MuSig2 operations
+bool GetMuSig2KeyAggCache(const std::vector<CPubKey>& pubkeys, secp256k1_musig_keyagg_cache& keyagg_cache);
+//! Retrieve the full aggregate pubkey from the secp256k1_musig_keyagg_cache
+std::optional<CPubKey> GetCPubKeyFromMuSig2KeyAggCache(secp256k1_musig_keyagg_cache& cache);
+//! Compute the full aggregate pubkey from the given participant pubkeys in their current order
+std::optional<CPubKey> MuSig2AggregatePubkeys(const std::vector<CPubKey>& pubkeys);
+
+#endif // BITCOIN_MUSIG_H