bitcoin/src/musig.cpp

// 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 <support/allocators/secure.h>

#include <secp256k1_musig.h>

static 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;
}

static 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, const std::optional<CPubKey>& expected_aggregate)
{
    if (!GetMuSig2KeyAggCache(pubkeys, keyagg_cache)) {
        return std::nullopt;
    }
    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)
{
    CExtPubKey extpub;
    extpub.nDepth = 0;
    std::memset(extpub.vchFingerprint, 0, 4);
    extpub.nChild = 0;
    extpub.chaincode = MUSIG_CHAINCODE;
    extpub.pubkey = pubkey;
    return extpub;
}

class MuSig2SecNonceImpl
{
private:
    //! The actual secnonce itself
    secure_unique_ptr<secp256k1_musig_secnonce> m_nonce;

public:
    MuSig2SecNonceImpl() : m_nonce{make_secure_unique<secp256k1_musig_secnonce>()} {}

    // Delete copy constructors
    MuSig2SecNonceImpl(const MuSig2SecNonceImpl&) = delete;
    MuSig2SecNonceImpl& operator=(const MuSig2SecNonceImpl&) = delete;

    secp256k1_musig_secnonce* Get() const { return m_nonce.get(); }
    void Invalidate() { m_nonce.reset(); }
    bool IsValid() { return m_nonce != nullptr; }
};

MuSig2SecNonce::MuSig2SecNonce() : m_impl{std::make_unique<MuSig2SecNonceImpl>()} {}

MuSig2SecNonce::MuSig2SecNonce(MuSig2SecNonce&&) noexcept = default;
MuSig2SecNonce& MuSig2SecNonce::operator=(MuSig2SecNonce&&) noexcept = default;

MuSig2SecNonce::~MuSig2SecNonce() = default;

secp256k1_musig_secnonce* MuSig2SecNonce::Get() const
{
    return m_impl->Get();
}

void MuSig2SecNonce::Invalidate()
{
    return m_impl->Invalidate();
}

bool MuSig2SecNonce::IsValid()
{
    return m_impl->IsValid();
}

uint256 MuSig2SessionID(const CPubKey& script_pubkey, const CPubKey& part_pubkey, const uint256& sighash)
{
    HashWriter hasher;
    hasher << script_pubkey << part_pubkey << sighash;
    return hasher.GetSHA256();
}

std::optional<std::vector<uint8_t>> CreateMuSig2AggregateSig(const std::vector<CPubKey>& part_pubkeys, const CPubKey& aggregate_pubkey, const std::vector<std::pair<uint256, bool>>& tweaks, const uint256& sighash, const std::map<CPubKey, std::vector<uint8_t>>& pubnonces, const std::map<CPubKey, uint256>& partial_sigs)
{
    if (!part_pubkeys.size()) return std::nullopt;

    // Get the keyagg cache and aggregate pubkey
    secp256k1_musig_keyagg_cache keyagg_cache;
    if (!MuSig2AggregatePubkeys(part_pubkeys, keyagg_cache, aggregate_pubkey)) return std::nullopt;

    // Check if enough pubnonces and partial sigs
    if (pubnonces.size() != part_pubkeys.size()) return std::nullopt;
    if (partial_sigs.size() != part_pubkeys.size()) return std::nullopt;

    // Parse the pubnonces and partial sigs
    std::vector<std::tuple<secp256k1_pubkey, secp256k1_musig_pubnonce, secp256k1_musig_partial_sig>> signers_data;
    std::vector<const secp256k1_musig_pubnonce*> pubnonce_ptrs;
    std::vector<const secp256k1_musig_partial_sig*> partial_sig_ptrs;
    for (const CPubKey& part_pk : part_pubkeys) {
        const auto& pn_it = pubnonces.find(part_pk);
        if (pn_it == pubnonces.end()) return std::nullopt;
        const std::vector<uint8_t> pubnonce = pn_it->second;
        if (pubnonce.size() != MUSIG2_PUBNONCE_SIZE) return std::nullopt;
        const auto& it = partial_sigs.find(part_pk);
        if (it == partial_sigs.end()) return std::nullopt;
        const uint256& partial_sig = it->second;

        auto& [secp_pk, secp_pn, secp_ps] = signers_data.emplace_back();

        if (!secp256k1_ec_pubkey_parse(secp256k1_context_static, &secp_pk, part_pk.data(), part_pk.size())) {
            return std::nullopt;
        }

        if (!secp256k1_musig_pubnonce_parse(secp256k1_context_static, &secp_pn, pubnonce.data())) {
            return std::nullopt;
        }

        if (!secp256k1_musig_partial_sig_parse(secp256k1_context_static, &secp_ps, partial_sig.data())) {
            return std::nullopt;
        }
    }
    pubnonce_ptrs.reserve(signers_data.size());
    partial_sig_ptrs.reserve(signers_data.size());
    for (auto& [_, pn, ps] : signers_data) {
        pubnonce_ptrs.push_back(&pn);
        partial_sig_ptrs.push_back(&ps);
    }

    // Aggregate nonces
    secp256k1_musig_aggnonce aggnonce;
    if (!secp256k1_musig_nonce_agg(secp256k1_context_static, &aggnonce, pubnonce_ptrs.data(), pubnonce_ptrs.size())) {
        return std::nullopt;
    }

    // Apply tweaks
    for (const auto& [tweak, xonly] : tweaks) {
        if (xonly) {
            if (!secp256k1_musig_pubkey_xonly_tweak_add(secp256k1_context_static, nullptr, &keyagg_cache, tweak.data())) {
                return std::nullopt;
            }
        } else if (!secp256k1_musig_pubkey_ec_tweak_add(secp256k1_context_static, nullptr, &keyagg_cache, tweak.data())) {
            return std::nullopt;
        }
    }

    // Create musig_session
    secp256k1_musig_session session;
    if (!secp256k1_musig_nonce_process(secp256k1_context_static, &session, &aggnonce, sighash.data(), &keyagg_cache)) {
        return std::nullopt;
    }

    // Verify partial sigs
    for (const auto& [pk, pb, ps] : signers_data) {
        if (!secp256k1_musig_partial_sig_verify(secp256k1_context_static, &ps, &pb, &pk, &keyagg_cache, &session)) {
            return std::nullopt;
        }
    }

    // Aggregate partial sigs
    std::vector<uint8_t> sig;
    sig.resize(64);
    if (!secp256k1_musig_partial_sig_agg(secp256k1_context_static, sig.data(), &session, partial_sig_ptrs.data(), partial_sig_ptrs.size())) {
        return std::nullopt;
    }

    return sig;
}