Arbitrary M/N multisig schemes:

* support in wallet2
* support in monero-wallet-cli
* support in monero-wallet-rpc
* support in wallet api
* support in monero-gen-trusted-multisig
* unit tests for multisig wallets creation

8 files changed, 419 insertions, 99 deletions

diff --git a/src/wallet/api/wallet.cpp b/src/wallet/api/wallet.cpp
index 5827e4d1a..a7fc3dcd7 100644
--- a/src/wallet/api/wallet.cpp
+++ b/src/wallet/api/wallet.cpp
@@ -1187,6 +1187,20 @@ string WalletImpl::makeMultisig(const vector<string>& info, uint32_t threshold)
     return string();
 }
 
+std::string WalletImpl::exchangeMultisigKeys(const std::vector<std::string> &info) {
+    try {
+        clearStatus();
+        checkMultisigWalletNotReady(m_wallet);
+
+        return m_wallet->exchange_multisig_keys(epee::wipeable_string(m_password), info);
+    } catch (const exception& e) {
+        LOG_ERROR("Error on exchanging multisig keys: ") << e.what();
+        setStatusError(string(tr("Failed to make multisig: ")) + e.what());
+    }
+
+    return string();
+}
+
 bool WalletImpl::finalizeMultisig(const vector<string>& extraMultisigInfo) {
     try {
         clearStatus();
diff --git a/src/wallet/api/wallet.h b/src/wallet/api/wallet.h
index 64350cee7..8c20af347 100644
--- a/src/wallet/api/wallet.h
+++ b/src/wallet/api/wallet.h
@@ -137,6 +137,7 @@ public:
     MultisigState multisig() const override;
     std::string getMultisigInfo() const override;
     std::string makeMultisig(const std::vector<std::string>& info, uint32_t threshold) override;
+    std::string exchangeMultisigKeys(const std::vector<std::string> &info) override;
     bool finalizeMultisig(const std::vector<std::string>& extraMultisigInfo) override;
     bool exportMultisigImages(std::string& images) override;
     size_t importMultisigImages(const std::vector<std::string>& images) override;
diff --git a/src/wallet/api/wallet2_api.h b/src/wallet/api/wallet2_api.h
index e0d491705..184c0aa73 100644
--- a/src/wallet/api/wallet2_api.h
+++ b/src/wallet/api/wallet2_api.h
@@ -707,6 +707,12 @@ struct Wallet
      */
     virtual std::string makeMultisig(const std::vector<std::string>& info, uint32_t threshold) = 0;
     /**
+     * @brief exchange_multisig_keys - provides additional key exchange round for arbitrary multisig schemes (like N-1/N, M/N)
+     * @param info - base58 encoded key derivations returned by makeMultisig or exchangeMultisigKeys function call
+     * @return new info string if more rounds required or an empty string if wallet creation is done
+     */
+    virtual std::string exchangeMultisigKeys(const std::vector<std::string> &info) = 0;
+    /**
      * @brief finalizeMultisig - finalizes N - 1 / N multisig wallets creation
      * @param extraMultisigInfo - wallet participants' extra multisig info obtained with makeMultisig call
      * @return true if success
diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index 75178845a..841c5c2ab 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -121,6 +121,7 @@ using namespace cryptonote;
 #define FIRST_REFRESH_GRANULARITY     1024
 
 static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
+static const std::string MULTISIG_EXTRA_INFO_MAGIC = "MultisigxV1";
 
 namespace
 {
@@ -132,6 +133,42 @@ namespace
     dir /= ".shared-ringdb";
     return dir.string();
   }
+
+  std::string pack_multisignature_keys(const std::string& prefix, const std::vector<crypto::public_key>& keys, const crypto::secret_key& signer_secret_key)
+  {
+    std::string data;
+    crypto::public_key signer;
+    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(signer_secret_key, signer), "Failed to derive public spend key");
+    data += std::string((const char *)&signer, sizeof(crypto::public_key));
+
+    for (const auto &key: keys)
+    {
+      data += std::string((const char *)&key, sizeof(crypto::public_key));
+    }
+
+    data.resize(data.size() + sizeof(crypto::signature));
+
+    crypto::hash hash;
+    crypto::cn_fast_hash(data.data(), data.size() - sizeof(crypto::signature), hash);
+    crypto::signature &signature = *(crypto::signature*)&data[data.size() - sizeof(crypto::signature)];
+    crypto::generate_signature(hash, signer, signer_secret_key, signature);
+
+    return MULTISIG_EXTRA_INFO_MAGIC + tools::base58::encode(data);
+  }
+
+  std::vector<crypto::public_key> secret_keys_to_public_keys(const std::vector<crypto::secret_key>& keys)
+  {
+    std::vector<crypto::public_key> public_keys;
+    public_keys.reserve(keys.size());
+
+    std::transform(keys.begin(), keys.end(), std::back_inserter(public_keys), [] (const crypto::secret_key& k) -> crypto::public_key {
+      crypto::public_key p;
+      CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(k, p), "Failed to derive public spend key");
+      return p;
+    });
+
+    return public_keys;
+  }
 }
 
 namespace
@@ -758,6 +795,7 @@ wallet2::wallet2(network_type nettype, uint64_t kdf_rounds, bool unattended):
   m_callback(0),
   m_trusted_daemon(false),
   m_nettype(nettype),
+  m_multisig_rounds_passed(0),
   m_always_confirm_transfers(true),
   m_print_ring_members(false),
   m_store_tx_info(true),
@@ -2885,6 +2923,7 @@ bool wallet2::clear()
   m_address_book.clear();
   m_subaddresses.clear();
   m_subaddress_labels.clear();
+  m_multisig_rounds_passed = 0;
   return true;
 }
 
@@ -2899,6 +2938,7 @@ bool wallet2::store_keys(const std::string& keys_file_name, const epee::wipeable
 {
   std::string account_data;
   std::string multisig_signers;
+  std::string multisig_derivations;
   cryptonote::account_base account = m_account;
 
   crypto::chacha_key key;
@@ -2951,6 +2991,14 @@ bool wallet2::store_keys(const std::string& keys_file_name, const epee::wipeable
     CHECK_AND_ASSERT_MES(r, false, "failed to serialize wallet multisig signers");
     value.SetString(multisig_signers.c_str(), multisig_signers.length());
     json.AddMember("multisig_signers", value, json.GetAllocator());
+
+    r = ::serialization::dump_binary(m_multisig_derivations, multisig_derivations);
+    CHECK_AND_ASSERT_MES(r, false, "failed to serialize wallet multisig derivations");
+    value.SetString(multisig_derivations.c_str(), multisig_derivations.length());
+    json.AddMember("multisig_derivations", value, json.GetAllocator());
+
+    value2.SetUint(m_multisig_rounds_passed);
+    json.AddMember("multisig_rounds_passed", value2, json.GetAllocator());
   }
 
   value2.SetInt(m_always_confirm_transfers ? 1 :0);
@@ -3123,6 +3171,8 @@ bool wallet2::load_keys(const std::string& keys_file_name, const epee::wipeable_
     m_multisig = false;
     m_multisig_threshold = 0;
     m_multisig_signers.clear();
+    m_multisig_rounds_passed = 0;
+    m_multisig_derivations.clear();
     m_always_confirm_transfers = false;
     m_print_ring_members = false;
     m_default_mixin = 0;
@@ -3181,6 +3231,8 @@ bool wallet2::load_keys(const std::string& keys_file_name, const epee::wipeable_
     m_multisig = field_multisig;
     GET_FIELD_FROM_JSON_RETURN_ON_ERROR(json, multisig_threshold, unsigned int, Uint, m_multisig, 0);
     m_multisig_threshold = field_multisig_threshold;
+    GET_FIELD_FROM_JSON_RETURN_ON_ERROR(json, multisig_rounds_passed, unsigned int, Uint, false, 0);
+    m_multisig_rounds_passed = field_multisig_rounds_passed;
     if (m_multisig)
     {
       if (!json.HasMember("multisig_signers"))
@@ -3201,6 +3253,24 @@ bool wallet2::load_keys(const std::string& keys_file_name, const epee::wipeable_
         LOG_ERROR("Field multisig_signers found in JSON, but failed to parse");
         return false;
       }
+
+      //previous version of multisig does not have this field
+      if (json.HasMember("multisig_derivations"))
+      {
+        if (!json["multisig_derivations"].IsString())
+        {
+          LOG_ERROR("Field multisig_derivations found in JSON, but not String");
+          return false;
+        }
+        const char *field_multisig_derivations = json["multisig_derivations"].GetString();
+        std::string multisig_derivations = std::string(field_multisig_derivations, field_multisig_derivations + json["multisig_derivations"].GetStringLength());
+        r = ::serialization::parse_binary(multisig_derivations, m_multisig_derivations);
+        if (!r)
+        {
+          LOG_ERROR("Field multisig_derivations found in JSON, but failed to parse");
+          return false;
+        }
+      }
     }
     GET_FIELD_FROM_JSON_RETURN_ON_ERROR(json, always_confirm_transfers, int, Int, false, true);
     m_always_confirm_transfers = field_always_confirm_transfers;
@@ -3829,12 +3899,12 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   CHECK_AND_ASSERT_THROW_MES(!view_keys.empty(), "empty view keys");
   CHECK_AND_ASSERT_THROW_MES(view_keys.size() == spend_keys.size(), "Mismatched view/spend key sizes");
   CHECK_AND_ASSERT_THROW_MES(threshold > 1 && threshold <= spend_keys.size() + 1, "Invalid threshold");
-  CHECK_AND_ASSERT_THROW_MES(threshold == spend_keys.size() || threshold == spend_keys.size() + 1, "Unsupported threshold case");
 
   std::string extra_multisig_info;
-  crypto::hash hash;
-
-  clear();
+  std::vector<crypto::secret_key> multisig_keys;
+  rct::key spend_pkey = rct::identity();
+  rct::key spend_skey;
+  std::vector<crypto::public_key> multisig_signers;
 
   // decrypt keys
   epee::misc_utils::auto_scope_leave_caller keys_reencryptor;
@@ -3847,43 +3917,78 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
     keys_reencryptor = epee::misc_utils::create_scope_leave_handler([&, this, chacha_key]() { m_account.encrypt_keys(chacha_key); m_account.decrypt_viewkey(chacha_key); });
   }
 
-  MINFO("Creating spend key...");
-  std::vector<crypto::secret_key> multisig_keys;
-  rct::key spend_pkey, spend_skey;
+  // In common multisig scheme there are 4 types of key exchange rounds:
+  // 1. First round is exchange of view secret keys and public spend keys.
+  // 2. Middle round is exchange of derivations: Ki = b * Mj, where b - spend secret key,
+  //    M - public multisig key (in first round it equals to public spend key), K - new public multisig key.
+  // 3. Secret spend establishment round sets your secret multisig keys as follows: kl = H(Ml), where M - is *your* public multisig key,
+  //    k - secret multisig key used to sign transactions. k and M are sets of keys, of course.
+  //    And secret spend key as the sum of all participant's secret multisig keys
+  // 4. Last round establishes multisig wallet's public spend key. Participants exchange their public multisig keys
+  //    and calculate common spend public key as sum of all unique participants' public multisig keys.
+  // Note that N/N scheme has only first round. N-1/N has 2 rounds: first and last. Common M/N has all 4 rounds.
+
+  // IMPORTANT: wallet's public spend key is not equal to secret_spend_key * G!
+  // Wallet's public spend key is the sum of unique public multisig keys of all participants.
+  // secret_spend_key * G = public signer key
+
   if (threshold == spend_keys.size() + 1)
   {
+    // In N / N case we only need to do one round and calculate secret multisig keys and new secret spend key
+    MINFO("Creating spend key...");
+
+    // Calculates all multisig keys and spend key
     cryptonote::generate_multisig_N_N(get_account().get_keys(), spend_keys, multisig_keys, spend_skey, spend_pkey);
+
+    // Our signer key is b * G, where b is secret spend key.
+    multisig_signers = spend_keys;
+    multisig_signers.push_back(get_multisig_signer_public_key(get_account().get_keys().m_spend_secret_key));
   }
-  else if (threshold == spend_keys.size())
+  else
   {
-    cryptonote::generate_multisig_N1_N(get_account().get_keys(), spend_keys, multisig_keys, spend_skey, spend_pkey);
+    // We just got public spend keys of all participants and deriving multisig keys (set of Mi = b * Bi).
+    // note that derivations are public keys as DH exchange suppose it to be
+    auto derivations = cryptonote::generate_multisig_derivations(get_account().get_keys(), spend_keys);
 
-    // We need an extra step, so we package all the composite public keys
-    // we know about, and make a signed string out of them
-    std::string data;
-    crypto::public_key signer;
-    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(rct::rct2sk(spend_skey), signer), "Failed to derive public spend key");
-    data += std::string((const char *)&signer, sizeof(crypto::public_key));
+    spend_pkey = rct::identity();
+    multisig_signers = std::vector<crypto::public_key>(spend_keys.size() + 1, crypto::null_pkey);
 
-    for (const auto &msk: multisig_keys)
+    if (threshold == spend_keys.size())
     {
-      rct::key pmsk = rct::scalarmultBase(rct::sk2rct(msk));
-      data += std::string((const char *)&pmsk, sizeof(crypto::public_key));
+      // N - 1 / N case
+
+      // We need an extra step, so we package all the composite public keys
+      // we know about, and make a signed string out of them
+      MINFO("Creating spend key...");
+
+      // Calculating set of our secret multisig keys as follows: mi = H(Mi),
+      // where mi - secret multisig key, Mi - others' participants public multisig key
+      multisig_keys = cryptonote::calculate_multisig_keys(derivations);
+
+      // calculating current participant's spend secret key as sum of all secret multisig keys for current participant.
+      // IMPORTANT: participant's secret spend key is not an entire wallet's secret spend!
+      //            Entire wallet's secret spend is sum of all unique secret multisig keys
+      //            among all of participants and is not held by anyone!
+      spend_skey = rct::sk2rct(cryptonote::calculate_multisig_signer_key(multisig_keys));
+
+      // Preparing data for the last round to calculate common public spend key. The data contains public multisig keys.
+      extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, secret_keys_to_public_keys(multisig_keys), rct::rct2sk(spend_skey));
     }
+    else
+    {
+      // M / N case
+      MINFO("Preparing keys for next exchange round...");
 
-    data.resize(data.size() + sizeof(crypto::signature));
-    crypto::cn_fast_hash(data.data(), data.size() - sizeof(signature), hash);
-    crypto::signature &signature = *(crypto::signature*)&data[data.size() - sizeof(crypto::signature)];
-    crypto::generate_signature(hash, signer, rct::rct2sk(spend_skey), signature);
+      // Preparing data for middle round - packing new public multisig keys to exchage with others.
+      extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, derivations, m_account.get_keys().m_spend_secret_key);
+      spend_skey = rct::sk2rct(m_account.get_keys().m_spend_secret_key);
 
-    extra_multisig_info = std::string("MultisigxV1") + tools::base58::encode(data);
-  }
-  else
-  {
-    CHECK_AND_ASSERT_THROW_MES(false, "Unsupported threshold case");
+      // Need to store middle keys to be able to proceed in case of wallet shutdown.
+      m_multisig_derivations = derivations;
+    }
   }
 
-  // the multisig view key is shared by all, make one all can derive
+  clear();
   MINFO("Creating view key...");
   crypto::secret_key view_skey = cryptonote::generate_multisig_view_secret_key(get_account().get_keys().m_view_secret_key, view_keys);
 
@@ -3895,18 +4000,10 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   m_account_public_address = m_account.get_keys().m_account_address;
   m_watch_only = false;
   m_multisig = true;
-  m_multisig_threshold = threshold;
   m_key_device_type = hw::device::device_type::SOFTWARE;
-
-  if (threshold == spend_keys.size() + 1)
-  {
-    m_multisig_signers = spend_keys;
-    m_multisig_signers.push_back(get_multisig_signer_public_key());
-  }
-  else
-  {
-    m_multisig_signers = std::vector<crypto::public_key>(spend_keys.size() + 1, crypto::null_pkey);
-  }
+  m_multisig_threshold = threshold;
+  m_multisig_signers = multisig_signers;
+  ++m_multisig_rounds_passed;
 
   // re-encrypt keys
   keys_reencryptor = epee::misc_utils::auto_scope_leave_caller();
@@ -3921,13 +4018,147 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   return extra_multisig_info;
 }
 
-std::string wallet2::make_multisig(const epee::wipeable_string &password,
-  const std::vector<std::string> &info,
-  uint32_t threshold)
+std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &password,
+  const std::vector<std::string> &info)
+{
+  THROW_WALLET_EXCEPTION_IF(info.empty(),
+    error::wallet_internal_error, "Empty multisig info");
+
+  if (info[0].substr(0, MULTISIG_EXTRA_INFO_MAGIC.size()) != MULTISIG_EXTRA_INFO_MAGIC)
+  {
+    THROW_WALLET_EXCEPTION_IF(false,
+      error::wallet_internal_error, "Unsupported info string");
+  }
+
+  std::vector<crypto::public_key> signers;
+  std::unordered_set<crypto::public_key> pkeys;
+
+  THROW_WALLET_EXCEPTION_IF(!unpack_extra_multisig_info(info, signers, pkeys),
+    error::wallet_internal_error, "Bad extra multisig info");
+
+  return exchange_multisig_keys(password, pkeys, signers);
+}
+
+std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &password,
+  std::unordered_set<crypto::public_key> derivations,
+  std::vector<crypto::public_key> signers)
+{
+  CHECK_AND_ASSERT_THROW_MES(!derivations.empty(), "empty pkeys");
+  CHECK_AND_ASSERT_THROW_MES(!signers.empty(), "empty signers");
+
+  bool ready = false;
+  CHECK_AND_ASSERT_THROW_MES(multisig(&ready), "The wallet is not multisig");
+  CHECK_AND_ASSERT_THROW_MES(!ready, "Multisig wallet creation process has already been finished");
+
+  // keys are decrypted
+  epee::misc_utils::auto_scope_leave_caller keys_reencryptor;
+  if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
+  {
+    crypto::chacha_key chacha_key;
+    crypto::generate_chacha_key(password.data(), password.size(), chacha_key, m_kdf_rounds);
+    m_account.encrypt_viewkey(chacha_key);
+    m_account.decrypt_keys(chacha_key);
+    keys_reencryptor = epee::misc_utils::create_scope_leave_handler([&, this, chacha_key]() { m_account.encrypt_keys(chacha_key); m_account.decrypt_viewkey(chacha_key); });
+  }
+
+  if (m_multisig_rounds_passed == multisig_rounds_required(m_multisig_signers.size(), m_multisig_threshold) - 1)
+  {
+    // the last round is passed and we have to calculate spend public key
+    // add ours if not included
+    crypto::public_key local_signer = get_multisig_signer_public_key();
+
+    if (std::find(signers.begin(), signers.end(), local_signer) == signers.end())
+    {
+        signers.push_back(local_signer);
+        for (const auto &msk: get_account().get_multisig_keys())
+        {
+            derivations.insert(rct::rct2pk(rct::scalarmultBase(rct::sk2rct(msk))));
+        }
+    }
+
+    CHECK_AND_ASSERT_THROW_MES(signers.size() == m_multisig_signers.size(), "Bad signers size");
+
+    // Summing all of unique public multisig keys to calculate common public spend key
+    crypto::public_key spend_public_key = cryptonote::generate_multisig_M_N_spend_public_key(std::vector<crypto::public_key>(derivations.begin(), derivations.end()));
+    m_account_public_address.m_spend_public_key = spend_public_key;
+    m_account.finalize_multisig(spend_public_key);
+
+    m_multisig_signers = signers;
+    std::sort(m_multisig_signers.begin(), m_multisig_signers.end(), [](const crypto::public_key &e0, const crypto::public_key &e1){ return memcmp(&e0, &e1, sizeof(e0)); });
+
+    ++m_multisig_rounds_passed;
+    m_multisig_derivations.clear();
+
+    // keys are encrypted again
+    keys_reencryptor = epee::misc_utils::auto_scope_leave_caller();
+
+    if (!m_wallet_file.empty())
+    {
+      bool r = store_keys(m_keys_file, password, false);
+      THROW_WALLET_EXCEPTION_IF(!r, error::file_save_error, m_keys_file);
+
+      if (boost::filesystem::exists(m_wallet_file + ".address.txt"))
+      {
+        r = file_io_utils::save_string_to_file(m_wallet_file + ".address.txt", m_account.get_public_address_str(m_nettype));
+        if(!r) MERROR("String with address text not saved");
+      }
+    }
+
+    m_subaddresses.clear();
+    m_subaddress_labels.clear();
+    add_subaddress_account(tr("Primary account"));
+
+    if (!m_wallet_file.empty())
+      store();
+
+    return {};
+  }
+
+  // Below are either middle or secret spend key establishment rounds
+
+  for (const auto& key: m_multisig_derivations)
+    derivations.erase(key);
+
+  // Deriving multisig keys (set of Mi = b * Bi) according to DH from other participants' multisig keys.
+  auto new_derivations = cryptonote::generate_multisig_derivations(get_account().get_keys(), std::vector<crypto::public_key>(derivations.begin(), derivations.end()));
+
+  std::string extra_multisig_info;
+  if (m_multisig_rounds_passed == multisig_rounds_required(m_multisig_signers.size(), m_multisig_threshold) - 2) // next round is last
+  {
+    // Next round is last therefore we are performing secret spend establishment round as described above.
+    MINFO("Creating spend key...");
+
+    // Calculating our secret multisig keys by hashing our public multisig keys.
+    auto multisig_keys = cryptonote::calculate_multisig_keys(std::vector<crypto::public_key>(new_derivations.begin(), new_derivations.end()));
+    // And summing it to get personal secret spend key
+    crypto::secret_key spend_skey = cryptonote::calculate_multisig_signer_key(multisig_keys);
+
+    m_account.make_multisig(m_account.get_keys().m_view_secret_key, spend_skey, rct::rct2pk(rct::identity()), multisig_keys);
+
+    // Packing public multisig keys to exchange with others and calculate common public spend key in the last round
+    extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, secret_keys_to_public_keys(multisig_keys), spend_skey);
+  }
+  else
+  {
+    // This is just middle round
+    MINFO("Preparing keys for next exchange round...");
+    extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, new_derivations, m_account.get_keys().m_spend_secret_key);
+    m_multisig_derivations = new_derivations;
+  }
+
+  ++m_multisig_rounds_passed;
+
+  create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
+  return extra_multisig_info;
+}
+
+void wallet2::unpack_multisig_info(const std::vector<std::string>& info,
+  std::vector<crypto::public_key> &public_keys,
+  std::vector<crypto::secret_key> &secret_keys) const
 {
   // parse all multisig info
-  std::vector<crypto::secret_key> secret_keys(info.size());
-  std::vector<crypto::public_key> public_keys(info.size());
+  public_keys.resize(info.size());
+  secret_keys.resize(info.size());
   for (size_t i = 0; i < info.size(); ++i)
   {
     THROW_WALLET_EXCEPTION_IF(!verify_multisig_info(info[i], secret_keys[i], public_keys[i]),
@@ -3971,75 +4202,51 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
           "Found local spend public key, but not local view secret key - something very weird");
     }
   }
+}
 
+std::string wallet2::make_multisig(const epee::wipeable_string &password,
+  const std::vector<std::string> &info,
+  uint32_t threshold)
+{
+  std::vector<crypto::secret_key> secret_keys(info.size());
+  std::vector<crypto::public_key> public_keys(info.size());
+  unpack_multisig_info(info, public_keys, secret_keys);
   return make_multisig(password, secret_keys, public_keys, threshold);
 }
 
 bool wallet2::finalize_multisig(const epee::wipeable_string &password, std::unordered_set<crypto::public_key> pkeys, std::vector<crypto::public_key> signers)
 {
-  CHECK_AND_ASSERT_THROW_MES(!pkeys.empty(), "empty pkeys");
-
-  // keys are decrypted
-  epee::misc_utils::auto_scope_leave_caller keys_reencryptor;
-  if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
-  {
-    crypto::chacha_key chacha_key;
-    crypto::generate_chacha_key(password.data(), password.size(), chacha_key, m_kdf_rounds);
-    m_account.encrypt_viewkey(chacha_key);
-    m_account.decrypt_keys(chacha_key);
-    keys_reencryptor = epee::misc_utils::create_scope_leave_handler([&, this, chacha_key]() { m_account.encrypt_keys(chacha_key); m_account.decrypt_viewkey(chacha_key); });
-  }
+  exchange_multisig_keys(password, pkeys, signers);
+  return true;
+}
 
-  // add ours if not included
-  crypto::public_key local_signer;
-  CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(get_account().get_keys().m_spend_secret_key, local_signer),
-      "Failed to derive public spend key");
-  if (std::find(signers.begin(), signers.end(), local_signer) == signers.end())
+bool wallet2::unpack_extra_multisig_info(const std::vector<std::string>& info,
+  std::vector<crypto::public_key> &signers,
+  std::unordered_set<crypto::public_key> &pkeys) const
+{
+  // parse all multisig info
+  signers.resize(info.size(), crypto::null_pkey);
+  for (size_t i = 0; i < info.size(); ++i)
   {
-    signers.push_back(local_signer);
-    for (const auto &msk: get_account().get_multisig_keys())
-    {
-      pkeys.insert(rct::rct2pk(rct::scalarmultBase(rct::sk2rct(msk))));
-    }
+      if (!verify_extra_multisig_info(info[i], pkeys, signers[i]))
+      {
+          return false;
+      }
   }
 
-  CHECK_AND_ASSERT_THROW_MES(signers.size() == m_multisig_signers.size(), "Bad signers size");
-
-  crypto::public_key spend_public_key = cryptonote::generate_multisig_N1_N_spend_public_key(std::vector<crypto::public_key>(pkeys.begin(), pkeys.end()));
-  m_account_public_address.m_spend_public_key = spend_public_key;
-  m_account.finalize_multisig(spend_public_key);
-
-  m_multisig_signers = signers;
-  std::sort(m_multisig_signers.begin(), m_multisig_signers.end(), [](const crypto::public_key &e0, const crypto::public_key &e1){ return memcmp(&e0, &e1, sizeof(e0)); });
-
-  // keys are encrypted again
-  keys_reencryptor = epee::misc_utils::auto_scope_leave_caller();
-
-  create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
-
-  m_subaddresses.clear();
-  m_subaddress_labels.clear();
-  add_subaddress_account(tr("Primary account"));
-
-  if (!m_wallet_file.empty())
-    store();
-
   return true;
 }
 
 bool wallet2::finalize_multisig(const epee::wipeable_string &password, const std::vector<std::string> &info)
 {
-  // parse all multisig info
   std::unordered_set<crypto::public_key> public_keys;
-  std::vector<crypto::public_key> signers(info.size(), crypto::null_pkey);
-  for (size_t i = 0; i < info.size(); ++i)
+  std::vector<crypto::public_key> signers;
+  if (!unpack_extra_multisig_info(info, signers, public_keys))
   {
-    if (!verify_extra_multisig_info(info[i], public_keys, signers[i]))
-    {
-      MERROR("Bad multisig info");
-      return false;
-    }
+    MERROR("Bad multisig info");
+    return false;
   }
+
   return finalize_multisig(password, public_keys, signers);
 }
 
@@ -4102,14 +4309,13 @@ bool wallet2::verify_multisig_info(const std::string &data, crypto::secret_key &
 
 bool wallet2::verify_extra_multisig_info(const std::string &data, std::unordered_set<crypto::public_key> &pkeys, crypto::public_key &signer)
 {
-  const size_t header_len = strlen("MultisigxV1");
-  if (data.size() < header_len || data.substr(0, header_len) != "MultisigxV1")
+  if (data.size() < MULTISIG_EXTRA_INFO_MAGIC.size() || data.substr(0, MULTISIG_EXTRA_INFO_MAGIC.size()) != MULTISIG_EXTRA_INFO_MAGIC)
   {
     MERROR("Multisig info header check error");
     return false;
   }
   std::string decoded;
-  if (!tools::base58::decode(data.substr(header_len), decoded))
+  if (!tools::base58::decode(data.substr(MULTISIG_EXTRA_INFO_MAGIC.size()), decoded))
   {
     MERROR("Multisig info decoding error");
     return false;
diff --git a/src/wallet/wallet2.h b/src/wallet/wallet2.h
index acbc09c36..440ac709b 100644
--- a/src/wallet/wallet2.h
+++ b/src/wallet/wallet2.h
@@ -573,6 +573,14 @@ namespace tools
       const std::vector<crypto::secret_key> &view_keys,
       const std::vector<crypto::public_key> &spend_keys,
       uint32_t threshold);
+    std::string exchange_multisig_keys(const epee::wipeable_string &password,
+      const std::vector<std::string> &info);
+    /*!
+     * \brief Any but first round of keys exchange
+     */
+    std::string exchange_multisig_keys(const epee::wipeable_string &password,
+      std::unordered_set<crypto::public_key> pkeys,
+      std::vector<crypto::public_key> signers);
     /*!
      * \brief Finalizes creation of a multisig wallet
      */
@@ -1245,6 +1253,12 @@ namespace tools
     bool get_rct_distribution(uint64_t &start_height, std::vector<uint64_t> &distribution);
 
     uint64_t get_segregation_fork_height() const;
+    void unpack_multisig_info(const std::vector<std::string>& info,
+      std::vector<crypto::public_key> &public_keys,
+      std::vector<crypto::secret_key> &secret_keys) const;
+    bool unpack_extra_multisig_info(const std::vector<std::string>& info,
+      std::vector<crypto::public_key> &signers,
+      std::unordered_set<crypto::public_key> &pkeys) const;
 
     void cache_tx_data(const cryptonote::transaction& tx, const crypto::hash &txid, tx_cache_data &tx_cache_data) const;
 
@@ -1295,6 +1309,9 @@ namespace tools
     bool m_multisig; /*!< if > 1 spend secret key will not match spend public key */
     uint32_t m_multisig_threshold;
     std::vector<crypto::public_key> m_multisig_signers;
+    //in case of general M/N multisig wallet we should perform N - M + 1 key exchange rounds and remember how many rounds are passed.
+    uint32_t m_multisig_rounds_passed;
+    std::vector<crypto::public_key> m_multisig_derivations;
     bool m_always_confirm_transfers;
     bool m_print_ring_members;
     bool m_store_tx_info; /*!< request txkey to be returned in RPC, and store in the wallet cache file */
diff --git a/src/wallet/wallet_rpc_server.cpp b/src/wallet/wallet_rpc_server.cpp
index e6eb64d12..b8379448d 100644
--- a/src/wallet/wallet_rpc_server.cpp
+++ b/src/wallet/wallet_rpc_server.cpp
@@ -3119,7 +3119,7 @@ namespace tools
       return false;
     }
 
-    if (req.multisig_info.size() < threshold - 1)
+    if (req.multisig_info.size() < 1 || req.multisig_info.size() > total)
     {
       er.code = WALLET_RPC_ERROR_CODE_THRESHOLD_NOT_REACHED;
       er.message = "Needs multisig info from more participants";
@@ -3146,6 +3146,55 @@ namespace tools
     return true;
   }
   //------------------------------------------------------------------------------------------------------------------------------
+  bool wallet_rpc_server::on_exchange_multisig_keys(const wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS::request& req, wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS::response& res, epee::json_rpc::error& er)
+  {
+    if (!m_wallet) return not_open(er);
+    if (m_restricted)
+    {
+      er.code = WALLET_RPC_ERROR_CODE_DENIED;
+      er.message = "Command unavailable in restricted mode.";
+      return false;
+    }
+    bool ready;
+    uint32_t threshold, total;
+    if (!m_wallet->multisig(&ready, &threshold, &total))
+    {
+      er.code = WALLET_RPC_ERROR_CODE_NOT_MULTISIG;
+      er.message = "This wallet is not multisig";
+      return false;
+    }
+
+    if (ready)
+    {
+      er.code = WALLET_RPC_ERROR_CODE_ALREADY_MULTISIG;
+      er.message = "This wallet is multisig, and already finalized";
+      return false;
+    }
+
+    if (req.multisig_info.size() < 1 || req.multisig_info.size() > total)
+    {
+      er.code = WALLET_RPC_ERROR_CODE_THRESHOLD_NOT_REACHED;
+      er.message = "Needs multisig info from more participants";
+      return false;
+    }
+
+    try
+    {
+      res.multisig_info = m_wallet->exchange_multisig_keys(req.password, req.multisig_info);
+      if (res.multisig_info.empty())
+      {
+        res.address = m_wallet->get_account().get_public_address_str(m_wallet->nettype());
+      }
+    }
+    catch (const std::exception &e)
+    {
+      er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
+      er.message = std::string("Error calling exchange_multisig_info: ") + e.what();
+      return false;
+    }
+    return true;
+  }
+  //------------------------------------------------------------------------------------------------------------------------------
   bool wallet_rpc_server::on_sign_multisig(const wallet_rpc::COMMAND_RPC_SIGN_MULTISIG::request& req, wallet_rpc::COMMAND_RPC_SIGN_MULTISIG::response& res, epee::json_rpc::error& er)
   {
     if (!m_wallet) return not_open(er);
diff --git a/src/wallet/wallet_rpc_server.h b/src/wallet/wallet_rpc_server.h
index ab7917a78..ab896aa3b 100644
--- a/src/wallet/wallet_rpc_server.h
+++ b/src/wallet/wallet_rpc_server.h
@@ -141,6 +141,7 @@ namespace tools
         MAP_JON_RPC_WE("export_multisig_info", on_export_multisig,  wallet_rpc::COMMAND_RPC_EXPORT_MULTISIG)
         MAP_JON_RPC_WE("import_multisig_info", on_import_multisig,  wallet_rpc::COMMAND_RPC_IMPORT_MULTISIG)
         MAP_JON_RPC_WE("finalize_multisig",  on_finalize_multisig,  wallet_rpc::COMMAND_RPC_FINALIZE_MULTISIG)
+        MAP_JON_RPC_WE("exchange_multisig_keys",  on_exchange_multisig_keys,  wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS)
         MAP_JON_RPC_WE("sign_multisig",      on_sign_multisig,      wallet_rpc::COMMAND_RPC_SIGN_MULTISIG)
         MAP_JON_RPC_WE("submit_multisig",    on_submit_multisig,    wallet_rpc::COMMAND_RPC_SUBMIT_MULTISIG)
         MAP_JON_RPC_WE("get_version",        on_get_version,        wallet_rpc::COMMAND_RPC_GET_VERSION)
@@ -218,6 +219,7 @@ namespace tools
       bool on_export_multisig(const wallet_rpc::COMMAND_RPC_EXPORT_MULTISIG::request& req, wallet_rpc::COMMAND_RPC_EXPORT_MULTISIG::response& res, epee::json_rpc::error& er);
       bool on_import_multisig(const wallet_rpc::COMMAND_RPC_IMPORT_MULTISIG::request& req, wallet_rpc::COMMAND_RPC_IMPORT_MULTISIG::response& res, epee::json_rpc::error& er);
       bool on_finalize_multisig(const wallet_rpc::COMMAND_RPC_FINALIZE_MULTISIG::request& req, wallet_rpc::COMMAND_RPC_FINALIZE_MULTISIG::response& res, epee::json_rpc::error& er);
+      bool on_exchange_multisig_keys(const wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS::request& req, wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS::response& res, epee::json_rpc::error& er);
       bool on_sign_multisig(const wallet_rpc::COMMAND_RPC_SIGN_MULTISIG::request& req, wallet_rpc::COMMAND_RPC_SIGN_MULTISIG::response& res, epee::json_rpc::error& er);
       bool on_submit_multisig(const wallet_rpc::COMMAND_RPC_SUBMIT_MULTISIG::request& req, wallet_rpc::COMMAND_RPC_SUBMIT_MULTISIG::response& res, epee::json_rpc::error& er);
       bool on_get_version(const wallet_rpc::COMMAND_RPC_GET_VERSION::request& req, wallet_rpc::COMMAND_RPC_GET_VERSION::response& res, epee::json_rpc::error& er);
diff --git a/src/wallet/wallet_rpc_server_commands_defs.h b/src/wallet/wallet_rpc_server_commands_defs.h
index 2168e0f71..c769929f5 100644
--- a/src/wallet/wallet_rpc_server_commands_defs.h
+++ b/src/wallet/wallet_rpc_server_commands_defs.h
@@ -1990,6 +1990,31 @@ namespace wallet_rpc
     };
   };
 
+  struct COMMAND_RPC_EXCHANGE_MULTISIG_KEYS
+  {
+    struct request
+    {
+      std::string password;
+      std::vector<std::string> multisig_info;
+
+      BEGIN_KV_SERIALIZE_MAP()
+        KV_SERIALIZE(password)
+        KV_SERIALIZE(multisig_info)
+      END_KV_SERIALIZE_MAP()
+    };
+
+    struct response
+    {
+      std::string address;
+      std::string multisig_info;
+
+      BEGIN_KV_SERIALIZE_MAP()
+        KV_SERIALIZE(address)
+        KV_SERIALIZE(multisig_info)
+      END_KV_SERIALIZE_MAP()
+    };
+  };
+
   struct COMMAND_RPC_SIGN_MULTISIG
   {
     struct request