add key exchange round booster to multisig_account

src/multisig/multisig_account.h

@@ -90,7 +90,8 @@ namespace multisig
     * 
     * - prepares a kex msg for the first round of multisig key construction.
     *    - the local account's kex msgs are signed with the base_privkey
-    *    - the first kex msg transmits the local base_common_privkey to other participants, for creating the group's common_privkey
+    *    - the first kex msg transmits the local base_common_privkey to other participants, for creating the group's
+    *      common_privkey
     */
     multisig_account(const crypto::secret_key &base_privkey,
       const crypto::secret_key &base_common_privkey);
@@ -177,24 +178,48 @@ namespace multisig
     *      - If force updating with maliciously-crafted messages, the resulting account will be invalid (either unable
     *        to complete signatures, or a 'hostage' to the malicious signer [i.e. can't sign without his participation]).
     */
-    void kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
-      const bool force_update_use_with_caution = false);
+    void kex_update(const std::vector<multisig_kex_msg> &expanded_msgs, const bool force_update_use_with_caution = false);
+    /**
+    * brief: get_multisig_kex_round_booster - Create a multisig kex msg for the kex round that follows the kex round this
+    *    account is currently working on, in order to 'boost' another participant's kex setup.
+    *    - A booster message is for the round after the in-progress round because get_next_kex_round_msg() provides access
+    *      to the in-progress round's message.
+    *    - Useful for 'jumpstarting' the following kex round when you don't have messages from all other signers to complete
+    *      the current round.
+    *    - Sanitizes input messages and produces a new kex msg for round 'num_completed_rounds + 2'.
+    *
+    *    - For example, in 2-of-3 escrowed purchasing, the [vendor, arbitrator] pair can boost the second round
+    *      of key exchange by calling this function with the 'round 1' messages of each other.
+    *      Then the [buyer] can use the resulting boost messages, in combination with [vender, arbitrator] round 1 messages,
+    *      to complete the address in one step. In other words, call initialize_kex() on the round 1 messages,
+    *      then call kex_update() on the round 2 booster messages to finish the multisig key.
+    * 
+    *    - Note: The 'threshold' and 'num_signers' are inputs here in case kex has not been initialized yet.
+    * param: threshold - threshold for multisig (M in M-of-N)
+    * param: num_signers - number of participants in multisig (N)
+    * param: expanded_msgs - set of multisig kex messages to process
+    * return: multisig kex message for next round
+    */
+    multisig_kex_msg get_multisig_kex_round_booster(const std::uint32_t threshold,
+      const std::uint32_t num_signers,
+      const std::vector<multisig_kex_msg> &expanded_msgs) const;
 
   private:
     // implementation of kex_update() (non-transactional)
     void kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs, const bool incomplete_signer_set);
     /**
-    * brief: initialize_kex_update - Helper for kex_update_impl()
-    *    - Collect the local signer's shared keys to ignore in incoming messages, build the aggregate ancillary key
-    *      if appropriate.
+    * brief: get_kex_exclude_pubkeys - collect the local signer's shared keys to ignore in incoming messages
+    * return: keys held by the local account corresponding to the 'in-progress round'
+    *    - If 'in-progress round' is the final round, these are the local account's shares of the final aggregate key.
+    */
+    std::vector<crypto::public_key> get_kex_exclude_pubkeys() const;
+    /**
+    * brief: initialize_kex_update - initialize the multisig account for the first kex round
     * param: expanded_msgs - set of multisig kex messages to process
     * param: kex_rounds_required - number of rounds required for kex (not including post-kex verification round)
-    * outparam: exclude_pubkeys_out - keys held by the local account corresponding to round 'current_round'
-    *    - If 'current_round' is the final round, these are the local account's shares of the final aggregate key.
     */
     void initialize_kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
-      const std::uint32_t kex_rounds_required,
-      std::vector<crypto::public_key> &exclude_pubkeys_out);
+      const std::uint32_t kex_rounds_required);
     /**
     * brief: finalize_kex_update - Helper for kex_update_impl()
     * param: kex_rounds_required - number of rounds required for kex (not including post-kex verification round)

src/multisig/multisig_account_kex_impl.cpp

@@ -567,7 +567,7 @@ namespace multisig
 
     // note: do NOT remove the local signer from the pubkey origins map, since the post-kex round can be force-updated with
     //       just the local signer's post-kex message (if the local signer were removed, then the post-kex message's pubkeys
-    //       would be completely lost)
+    //       would be completely deleted)
 
     // evaluate pubkeys collected
 
@@ -608,7 +608,7 @@ namespace multisig
   * INTERNAL
   *
   * brief: multisig_kex_process_round_msgs - Process kex messages for the active kex round.
-  *    - A wrapper around evaluate_multisig_kex_round_msgs() -> multisig_kex_make_next_msg().
+  *    - A wrapper around evaluate_multisig_kex_round_msgs() -> multisig_kex_make_round_keys().
   *      - In other words, evaluate the input messages and try to make a message for the next round.
   *    - Note: Must be called on the final round's msgs to evaluate the final key components
   *            recommended by other participants.
@@ -623,7 +623,7 @@ namespace multisig
   * outparam: keys_to_origins_map_out - map between round keys and identity keys
   *    - If in the final round, these are key shares recommended by other signers for the final aggregate key.
   *    - Otherwise, these are the local account's DH derivations for the next round.
-  *      - See multisig_kex_make_next_msg() for an explanation.
+  *      - See multisig_kex_make_round_keys() for an explanation.
   * return: multisig kex message for next round, or empty message if 'current_round' is the final round
   */
   //----------------------------------------------------------------------------------------------------------------------
@@ -684,59 +684,67 @@ namespace multisig
   //----------------------------------------------------------------------------------------------------------------------
   // multisig_account: INTERNAL
   //----------------------------------------------------------------------------------------------------------------------
-  void multisig_account::initialize_kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
-    const std::uint32_t kex_rounds_required,
-    std::vector<crypto::public_key> &exclude_pubkeys_out)
+  std::vector<crypto::public_key> multisig_account::get_kex_exclude_pubkeys() const
   {
+    // exclude all keys the local account recommends
+    std::vector<crypto::public_key> exclude_pubkeys;
+
     if (m_kex_rounds_complete == 0)
     {
-      // the first round of kex msgs will contain each participant's base pubkeys and ancillary privkeys
+      // in the first round, only the local pubkey is recommended by the local signer
+      exclude_pubkeys.emplace_back(m_base_pubkey);
+    }
+    else
+    {
+      // in other rounds, kex msgs will contain participants' shared keys, so ignore shared keys the account helped
+      //     create for this round
+      for (const auto &shared_key_with_origins : m_kex_keys_to_origins_map)
+        exclude_pubkeys.emplace_back(shared_key_with_origins.first);
+    }
 
-      // collect participants' base common privkey shares
-      // note: duplicate privkeys are acceptable, and duplicates due to duplicate signers
-      //       will be blocked by duplicate-signer errors after this function is called
-      std::vector<crypto::secret_key> participant_base_common_privkeys;
-      participant_base_common_privkeys.reserve(expanded_msgs.size() + 1);
+    return exclude_pubkeys;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::initialize_kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
+    const std::uint32_t kex_rounds_required)
+  {
+    // initialization is only needed during the first round
+    if (m_kex_rounds_complete > 0)
+      return;
 
-      // add local ancillary base privkey
-      participant_base_common_privkeys.emplace_back(m_base_common_privkey);
+    // the first round of kex msgs will contain each participant's base pubkeys and ancillary privkeys, so we prepare
+    //    them here
 
-      // add other signers' base common privkeys
-      for (const auto &expanded_msg : expanded_msgs)
-      {
-        if (expanded_msg.get_signing_pubkey() != m_base_pubkey)
-        {
-          participant_base_common_privkeys.emplace_back(expanded_msg.get_msg_privkey());
-        }
-      }
+    // collect participants' base common privkey shares
+    // note: duplicate privkeys are acceptable, and duplicates due to duplicate signers
+    //       will be blocked by duplicate-signer errors after this function is called
+    std::vector<crypto::secret_key> participant_base_common_privkeys;
+    participant_base_common_privkeys.reserve(expanded_msgs.size() + 1);
 
-      // make common privkey
-      make_multisig_common_privkey(std::move(participant_base_common_privkeys), m_common_privkey);
+    // add local ancillary base privkey
+    participant_base_common_privkeys.emplace_back(m_base_common_privkey);
 
-      // set common pubkey
-      CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(m_common_privkey, m_common_pubkey),
-        "Failed to derive public key");
+    // add other signers' base common privkeys
+    for (const multisig_kex_msg &expanded_msg : expanded_msgs)
+    {
+      if (expanded_msg.get_signing_pubkey() != m_base_pubkey)
+        participant_base_common_privkeys.emplace_back(expanded_msg.get_msg_privkey());
+    }
 
-      // if N-of-N, then the base privkey will be used directly to make the account's share of the final key
-      if (kex_rounds_required == 1)
-      {
-        m_multisig_privkeys.clear();
-        m_multisig_privkeys.emplace_back(m_base_privkey);
-      }
+    // make common privkey
+    make_multisig_common_privkey(std::move(participant_base_common_privkeys), m_common_privkey);
 
-      // exclude all keys the local account recommends
-      // - in the first round, only the local pubkey is recommended by the local signer
-      exclude_pubkeys_out.emplace_back(m_base_pubkey);
-    }
-    else
-    {
-      // in other rounds, kex msgs will contain participants' shared keys
+    // set common pubkey
+    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(m_common_privkey, m_common_pubkey),
+      "Failed to derive public key");
 
-      // ignore shared keys the account helped create for this round
-      for (const auto &shared_key_with_origins : m_kex_keys_to_origins_map)
-      {
-        exclude_pubkeys_out.emplace_back(shared_key_with_origins.first);
-      }
+    // if N-of-N, then the base privkey will be used directly to make the account's share of the final key
+    if (kex_rounds_required == 1)
+    {
+      m_multisig_privkeys.clear();
+      m_multisig_privkeys.emplace_back(m_base_privkey);
     }
   }
   //----------------------------------------------------------------------------------------------------------------------
@@ -771,9 +779,7 @@ namespace multisig
       result_keys.reserve(result_keys_to_origins_map.size());
 
       for (const auto &result_key_and_origins : result_keys_to_origins_map)
-      {
         result_keys.emplace_back(result_key_and_origins.first);
-      }
 
       // compute final aggregate key, update local multisig privkeys with aggregation coefficients applied
       m_multisig_pubkey = generate_multisig_aggregate_key(std::move(result_keys), m_multisig_privkeys);
@@ -811,7 +817,8 @@ namespace multisig
         // derived pubkey = multisig_key * G
         crypto::public_key_memsafe derived_pubkey;
         m_multisig_privkeys.push_back(
-          calculate_multisig_keypair_from_derivation(derivation_and_origins.first, derived_pubkey));
+            calculate_multisig_keypair_from_derivation(derivation_and_origins.first, derived_pubkey)
+          );
 
         // save the account's kex key mappings for this round [derived pubkey : other signers who will have the same key]
         m_kex_keys_to_origins_map[derived_pubkey] = std::move(derivation_and_origins.second);
@@ -863,8 +870,7 @@ namespace multisig
       "Multisig kex has already completed all required rounds (including post-kex verification).");
 
     // initialize account update
-    std::vector<crypto::public_key> exclude_pubkeys;
-    initialize_kex_update(expanded_msgs, kex_rounds_required, exclude_pubkeys);
+    this->initialize_kex_update(expanded_msgs, kex_rounds_required);
 
     // process messages into a [pubkey : {origins}] map
     multisig_keyset_map_memsafe_t result_keys_to_origins_map;
@@ -875,12 +881,75 @@ namespace multisig
       m_threshold,
       m_signers,
       expanded_msgs,
-      exclude_pubkeys,
+      this->get_kex_exclude_pubkeys(),
       incomplete_signer_set,
       result_keys_to_origins_map);
 
     // finish account update
-    finalize_kex_update(kex_rounds_required, std::move(result_keys_to_origins_map));
+    this->finalize_kex_update(kex_rounds_required, std::move(result_keys_to_origins_map));
+  }
+  //-----------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //-----------------------------------------------------------------
+  multisig_kex_msg multisig_account::get_multisig_kex_round_booster(const std::uint32_t threshold,
+    const std::uint32_t num_signers,
+    const std::vector<multisig_kex_msg> &expanded_msgs) const
+  {
+    // the messages passed in should be required for the next kex round of this account (the round it is currently
+    //   working on)
+    const std::uint32_t expected_msgs_round{m_kex_rounds_complete + 1};
+    const std::uint32_t kex_rounds_required{multisig_kex_rounds_required(num_signers, threshold)};
+
+    CHECK_AND_ASSERT_THROW_MES(num_signers > 1, "Must be at least one other multisig signer.");
+    CHECK_AND_ASSERT_THROW_MES(num_signers <= config::MULTISIG_MAX_SIGNERS, "Too many multisig signers specified.");
+    CHECK_AND_ASSERT_THROW_MES(expected_msgs_round < kex_rounds_required,
+      "Multisig kex booster: this account has already completed all intermediate kex rounds so it can't make a kex "
+      "booster (there is no round available to boost).");
+    CHECK_AND_ASSERT_THROW_MES(expanded_msgs.size() > 0, "At least one input kex message expected.");
+
+    // sanitize pubkeys from input msgs
+    multisig_keyset_map_memsafe_t pubkey_origins_map;
+    const std::uint32_t msgs_round{
+        multisig_kex_msgs_sanitize_pubkeys(expanded_msgs, this->get_kex_exclude_pubkeys(), pubkey_origins_map)
+      };
+    CHECK_AND_ASSERT_THROW_MES(msgs_round == expected_msgs_round, "Kex messages were not for expected round.");
+
+    // remove the local signer from sanitized messages
+    remove_key_from_mapped_sets(m_base_pubkey, pubkey_origins_map);
+
+    // make DH derivations for booster message
+    multisig_keyset_map_memsafe_t derivation_to_origins_map;
+    multisig_kex_make_round_keys(m_base_privkey, std::move(pubkey_origins_map), derivation_to_origins_map);
+
+    // collect keys for booster message
+    std::vector<crypto::public_key> next_msg_keys;
+    next_msg_keys.reserve(derivation_to_origins_map.size());
+
+    if (msgs_round + 1 == kex_rounds_required)
+    {
+      // final kex round: send DH derivation pubkeys in the message
+      for (const auto &derivation_and_origins : derivation_to_origins_map)
+      {
+        // multisig_privkey = H(derivation)
+        // derived pubkey = multisig_key * G
+        crypto::public_key_memsafe derived_pubkey;
+        calculate_multisig_keypair_from_derivation(derivation_and_origins.first, derived_pubkey);
+
+        // save keys that should be recommended to other signers
+        // - The keys multisig_key*G are sent to other participants in the message, so they can be used to produce the final
+        //   multisig key via generate_multisig_spend_public_key().
+        next_msg_keys.push_back(derived_pubkey);
+      }
+    }
+    else //(msgs_round + 1 < kex_rounds_required)
+    {
+      // intermediate kex round: send DH derivations directly in the message
+      for (const auto &derivation_and_origins : derivation_to_origins_map)
+        next_msg_keys.push_back(derivation_and_origins.first);
+    }
+
+    // produce a kex message for the round after the round this account is currently working on
+    return multisig_kex_msg{msgs_round + 1, m_base_privkey, std::move(next_msg_keys)}.get_msg();
   }
   //----------------------------------------------------------------------------------------------------------------------
 } //namespace multisig

src/wallet/api/wallet.cpp

@@ -1338,6 +1338,21 @@ std::string WalletImpl::exchangeMultisigKeys(const std::vector<std::string> &inf
     return string();
 }
 
+std::string WalletImpl::getMultisigKeyExchangeBooster(const std::vector<std::string> &info,
+    const std::uint32_t threshold,
+    const std::uint32_t num_signers) {
+    try {
+        clearStatus();
+
+        return m_wallet->get_multisig_key_exchange_booster(epee::wipeable_string(m_password), info, threshold, num_signers);
+    } catch (const exception& e) {
+        LOG_ERROR("Error on boosting multisig key exchange: " << e.what());
+        setStatusError(string(tr("Failed to boost multisig key exchange: ")) + e.what());
+    }
+
+    return string();
+}
+
 bool WalletImpl::exportMultisigImages(string& images) {
     try {
         clearStatus();

src/wallet/api/wallet.h

@@ -148,6 +148,7 @@ public:
     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, const bool force_update_use_with_caution = false) override;
+    std::string getMultisigKeyExchangeBooster(const std::vector<std::string> &info, const uint32_t threshold, const uint32_t num_signers) override;
     bool exportMultisigImages(std::string& images) override;
     size_t importMultisigImages(const std::vector<std::string>& images) override;
     bool hasMultisigPartialKeyImages() const override;

src/wallet/api/wallet2_api.h

@@ -801,6 +801,15 @@ struct Wallet
      * @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, const bool force_update_use_with_caution) = 0;
+    /**
+     * @brief getMultisigKeyExchangeBooster - obtain partial information for the key exchange round after the in-progress round,
+     *                                        to speed up another signer's key exchange process
+     * @param info - base58 encoded key derivations returned by makeMultisig or exchangeMultisigKeys function call
+     * @param threshold - number of required signers to make valid transaction. Must be <= number of participants.
+     * @param num_signers - total number of multisig participants.
+     * @return new info string if more rounds required or exception if no more rounds (i.e. no rounds to boost)
+     */
+    virtual std::string getMultisigKeyExchangeBooster(const std::vector<std::string> &info, const uint32_t threshold, const uint32_t num_signers) = 0;
     /**
      * @brief exportMultisigImages - exports transfers' key images
      * @param images - output paramter for hex encoded array of images

src/wallet/wallet2.cpp

@@ -5156,32 +5156,75 @@ void wallet2::restore(const std::string& wallet_, const epee::wipeable_string& p
   }
 }
 //----------------------------------------------------------------------------------------------------
-std::string wallet2::make_multisig(const epee::wipeable_string &password,
-  const std::vector<std::string> &initial_kex_msgs,
-  const std::uint32_t threshold)
+epee::misc_utils::auto_scope_leave_caller wallet2::decrypt_account_for_multisig(const epee::wipeable_string &password)
 {
   // decrypt account keys
+  // note: this conditional's clauses are old and undocumented
   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);
+    this->decrypt_keys(chacha_key);
     keys_reencryptor = epee::misc_utils::create_scope_leave_handler(
-        [&, this, chacha_key]()
+        [this, chacha_key]()
         {
-          m_account.encrypt_keys(chacha_key);
-          m_account.decrypt_viewkey(chacha_key);
+          this->encrypt_keys(chacha_key);
         }
       );
   }
 
-  // create multisig account
-  multisig::multisig_account multisig_account{
-      multisig::get_multisig_blinded_secret_key(get_account().get_keys().m_spend_secret_key),
-      multisig::get_multisig_blinded_secret_key(get_account().get_keys().m_view_secret_key)
+  return keys_reencryptor;
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::get_uninitialized_multisig_account(multisig::multisig_account &account_out) const
+{
+  // create uninitialized multisig account
+  account_out = multisig::multisig_account{
+      // k_base = H(normal private spend key)
+      multisig::get_multisig_blinded_secret_key(this->get_account().get_keys().m_spend_secret_key),
+      // k_view = H(normal private view key)
+      multisig::get_multisig_blinded_secret_key(this->get_account().get_keys().m_view_secret_key)
+    };
+}
+//----------------------------------------------------------------------------------------------------
+void wallet2::get_reconstructed_multisig_account(multisig::multisig_account &account_out) const
+{
+  CHECK_AND_ASSERT_THROW_MES(this->multisig(), "The wallet is not multisig, so the multisig account couldn't be reconstructed");
+
+  // reconstruct multisig account
+  crypto::public_key common_pubkey;
+  crypto::secret_key_to_public_key(this->get_account().get_keys().m_view_secret_key, common_pubkey);
+
+  multisig::multisig_keyset_map_memsafe_t kex_origins_map;
+  for (const auto &derivation : m_multisig_derivations)
+    kex_origins_map[derivation];
+
+  account_out = multisig::multisig_account{
+      m_multisig_threshold,
+      m_multisig_signers,
+      this->get_account().get_keys().m_spend_secret_key,
+      this->get_account().get_keys().m_view_secret_key,
+      this->get_account().get_keys().m_multisig_keys,
+      this->get_account().get_keys().m_view_secret_key,
+      m_account_public_address.m_spend_public_key,
+      common_pubkey,
+      m_multisig_rounds_passed,
+      std::move(kex_origins_map),
+      ""
     };
+}
+//----------------------------------------------------------------------------------------------------
+std::string wallet2::make_multisig(const epee::wipeable_string &password,
+  const std::vector<std::string> &initial_kex_msgs,
+  const std::uint32_t threshold)
+{
+  // decrypt account keys
+  epee::misc_utils::auto_scope_leave_caller keys_reencryptor{this->decrypt_account_for_multisig(password)};
+
+  // create multisig account
+  multisig::multisig_account multisig_account;
+  this->get_uninitialized_multisig_account(multisig_account);
 
   // open initial kex messages, validate them, extract signers
   std::vector<multisig::multisig_kex_msg> expanded_msgs;
@@ -5189,7 +5232,7 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   expanded_msgs.reserve(initial_kex_msgs.size());
   signers.reserve(initial_kex_msgs.size() + 1);
 
-  for (const auto &msg : initial_kex_msgs)
+  for (const std::string &msg : initial_kex_msgs)
   {
     expanded_msgs.emplace_back(msg);
 
@@ -5198,17 +5241,19 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
     CHECK_AND_ASSERT_THROW_MES(expanded_msgs.back().get_round() == 1,
       "Trying to make multisig with message that has invalid multisig kex round (should be '1').");
 
-    // 2. duplicate signers not allowed
+    // 2. duplicate signers not allowed (the number of signers is implied by the number of initial kex messages passed
+    //    in, so we can't just ignore duplicates here)
     CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), expanded_msgs.back().get_signing_pubkey()) == signers.end(),
       "Duplicate signers not allowed when converting a wallet to multisig.");
 
-    // add signer (skip self for now)
-    if (expanded_msgs.back().get_signing_pubkey() != multisig_account.get_base_pubkey())
-      signers.push_back(expanded_msgs.back().get_signing_pubkey());
+    // add signer
+    signers.push_back(expanded_msgs.back().get_signing_pubkey());
   }
 
-  // add self to signers
-  signers.push_back(multisig_account.get_base_pubkey());
+  // expect that self is in the input list (this guarantees that the input list size always equals the number of intended
+  //   signers for the account [when combined with duplicate checking])
+  CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), multisig_account.get_base_pubkey()) != signers.end(),
+    "The local account's signer key was not found in initial multisig kex messages when converting a wallet to multisig.");
 
   // intialize key exchange
   multisig_account.initialize_kex(threshold, signers, expanded_msgs);
@@ -5219,12 +5264,13 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   {
     // Save the original i.e. non-multisig keys so the MMS can continue to use them to encrypt and decrypt messages
     // (making a wallet multisig overwrites those keys, see account_base::make_multisig)
-    m_original_address = get_account().get_keys().m_account_address;
-    m_original_view_secret_key = get_account().get_keys().m_view_secret_key;
+    m_original_address = this->get_account().get_keys().m_account_address;
+    m_original_view_secret_key = this->get_account().get_keys().m_view_secret_key;
     m_original_keys_available = true;
   }
 
-  clear();
+  // clear wallet caches
+  this->clear();
 
   // account base
   MINFO("Creating multisig address...");
@@ -5234,14 +5280,14 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
     multisig_account.get_multisig_privkeys()),
       "Failed to create multisig wallet account due to bad keys");
 
-  init_type(hw::device::device_type::SOFTWARE);
+  this->init_type(hw::device::device_type::SOFTWARE);
   m_original_keys_available = true;
   m_multisig = true;
   m_multisig_threshold = threshold;
   m_multisig_signers = signers;
   m_multisig_rounds_passed = 1;
 
-  // derivations stored (should be empty in last round)
+  // derivations stored (note: should be empty in last kex round)
   m_multisig_derivations.clear();
   m_multisig_derivations.reserve(multisig_account.get_kex_keys_to_origins_map().size());
 
@@ -5255,12 +5301,12 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   keys_reencryptor = epee::misc_utils::auto_scope_leave_caller();
 
   if (!m_wallet_file.empty())
-    create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
+    this->create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
 
-  setup_new_blockchain();
+  this->setup_new_blockchain();
 
   if (!m_wallet_file.empty())
-    store();
+    this->store();
 
   return multisig_account.get_next_kex_round_msg();
 }
@@ -5270,44 +5316,14 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
   const bool force_update_use_with_caution /*= false*/)
 {
   bool ready{false};
-  CHECK_AND_ASSERT_THROW_MES(multisig(&ready), "The wallet is not multisig");
+  CHECK_AND_ASSERT_THROW_MES(this->multisig(&ready), "The wallet is not multisig");
 
   // decrypt account keys
-  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);
-        }
-      );
-  }
+  epee::misc_utils::auto_scope_leave_caller keys_reencryptor{this->decrypt_account_for_multisig(password)};
 
   // reconstruct multisig account
-  multisig::multisig_keyset_map_memsafe_t kex_origins_map;
-
-  for (const auto &derivation : m_multisig_derivations)
-    kex_origins_map[derivation];
-
-  multisig::multisig_account multisig_account{
-      m_multisig_threshold,
-      m_multisig_signers,
-      get_account().get_keys().m_spend_secret_key,
-      crypto::null_skey,  //base common privkey: not used
-      get_account().get_keys().m_multisig_keys,
-      get_account().get_keys().m_view_secret_key,
-      m_account_public_address.m_spend_public_key,
-      m_account_public_address.m_view_public_key,
-      m_multisig_rounds_passed,
-      std::move(kex_origins_map),
-      ""
-    };
+  multisig::multisig_account multisig_account;
+  this->get_reconstructed_multisig_account(multisig_account);
 
   // KLUDGE: early return if there are no kex messages and main kex is complete (will return the post-kex verification round
   //         message) (it's a kludge because this behavior would be more appropriate for a standalone wallet method)
@@ -5323,7 +5339,7 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
   std::vector<multisig::multisig_kex_msg> expanded_msgs;
   expanded_msgs.reserve(kex_messages.size());
 
-  for (const auto &msg : kex_messages)
+  for (const std::string &msg : kex_messages)
     expanded_msgs.emplace_back(msg);
 
   // update multisig kex
@@ -5359,27 +5375,27 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
 
     if (!m_wallet_file.empty())
     {
-      bool r = store_keys(m_keys_file, password, false);
+      bool r = this->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 = save_to_file(m_wallet_file + ".address.txt", m_account.get_public_address_str(m_nettype), true);
+        r = this->save_to_file(m_wallet_file + ".address.txt", m_account.get_public_address_str(m_nettype), true);
         if(!r) MERROR("String with address text not saved");
       }
     }
 
     m_subaddresses.clear();
     m_subaddress_labels.clear();
-    add_subaddress_account(tr("Primary account"));
+    this->add_subaddress_account(tr("Primary account"));
 
     if (!m_wallet_file.empty())
-      store();
+      this->store();
   }
 
   // wallet/file relationship
   if (!m_wallet_file.empty())
-    create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
+    this->create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
 
   return multisig_account.get_next_kex_round_msg();
 }
@@ -5387,16 +5403,65 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
 std::string wallet2::get_multisig_first_kex_msg() const
 {
   // create multisig account
-  multisig::multisig_account multisig_account{
-      // k_base = H(normal private spend key)
-      multisig::get_multisig_blinded_secret_key(get_account().get_keys().m_spend_secret_key),
-      // k_view = H(normal private view key)
-      multisig::get_multisig_blinded_secret_key(get_account().get_keys().m_view_secret_key)
-    };
+  multisig::multisig_account multisig_account;
+  this->get_uninitialized_multisig_account(multisig_account);
 
   return multisig_account.get_next_kex_round_msg();
 }
 //----------------------------------------------------------------------------------------------------
+std::string wallet2::get_multisig_key_exchange_booster(const epee::wipeable_string &password,
+  const std::vector<std::string> &kex_messages,
+  const std::uint32_t threshold,
+  const std::uint32_t num_signers)
+{
+  CHECK_AND_ASSERT_THROW_MES(kex_messages.size() > 0, "No key exchange messages passed in.");
+
+  // decrypt account keys
+  epee::misc_utils::auto_scope_leave_caller keys_reencryptor{this->decrypt_account_for_multisig(password)};
+
+  // prepare multisig account
+  multisig::multisig_account multisig_account;
+
+  bool ready{false};
+  bool is_multisig{this->multisig(&ready)};
+
+  CHECK_AND_ASSERT_THROW_MES(!ready, "Multisig wallet creation process has already been finished.");
+
+  if (is_multisig)
+  {
+    // case: this wallet is in the middle of multisig key exchange
+    // - boost the round that comes after the in-progress round
+
+    CHECK_AND_ASSERT_THROW_MES(threshold == m_multisig_threshold,
+      "Expected threshold does not match multisig wallet setting.");
+    CHECK_AND_ASSERT_THROW_MES(num_signers == m_multisig_signers.size(),
+      "Expected number of signers does not match multisig wallet setting.");
+
+    // reconstruct multisig account
+    this->get_reconstructed_multisig_account(multisig_account);
+  }
+  else
+  {
+    // case: make_multisig() has not been called
+    // DANGER: If 'num_signers - threshold > 1', but this wallet's future multisig settings
+    //         will be 'num_signers - threshold == 1', then the booster message WILL leak the
+    //         future multisig wallet's private keys in this case where the wallet2 multisig wallet is uninitialized.
+
+    this->get_uninitialized_multisig_account(multisig_account);
+  }
+
+  // open kex messages
+  std::vector<multisig::multisig_kex_msg> expanded_msgs;
+  expanded_msgs.reserve(kex_messages.size());
+
+  for (const std::string &msg : kex_messages)
+    expanded_msgs.emplace_back(msg);
+
+  // get kex booster message
+  // note: booster does not change wallet state other than decrypting/reencrypting account keys
+  return multisig_account.get_multisig_kex_round_booster(threshold, num_signers, expanded_msgs).get_msg();
+}
+//----------------------------------------------------------------------------------------------------
 bool wallet2::multisig(bool *ready, uint32_t *threshold, uint32_t *total) const
 {
   if (!m_multisig)

src/wallet/wallet2.h

@@ -85,6 +85,7 @@
 
 class Serialization_portability_wallet_Test;
 class wallet_accessor_test;
+namespace multisig { class multisig_account; }
 
 namespace tools
 {
@@ -866,6 +867,23 @@ private:
      */
     void restore(const std::string& wallet_, const epee::wipeable_string& password, const std::string &device_name, bool create_address_file = false);
 
+  private:
+    /*!
+     * \brief Decrypts the account keys
+     * \return an RAII reencryptor for the account keys
+     */
+    epee::misc_utils::auto_scope_leave_caller decrypt_account_for_multisig(const epee::wipeable_string &password);
+    /*!
+     * \brief Creates an uninitialized multisig account
+     * \outparam: the uninitialized multisig account
+     */
+    void get_uninitialized_multisig_account(multisig::multisig_account &account_out) const;
+    /*!
+     * \brief Reconstructs a multisig account from wallet2 state
+     * \outparam: the reconstructed multisig account
+     */
+    void get_reconstructed_multisig_account(multisig::multisig_account &account_out) const;
+  public:
     /*!
      * \brief Creates a multisig wallet
      * \return empty if done, non empty if we need to send another string
@@ -887,6 +905,13 @@ private:
      * \return string to send to other participants
      */
     std::string get_multisig_first_kex_msg() const;
+    /*!
+     * \brief Use multisig kex messages for an in-progress kex round to 'boost' the following round for another group member
+     */
+    std::string get_multisig_key_exchange_booster(const epee::wipeable_string &password,
+      const std::vector<std::string> &kex_messages,
+      const std::uint32_t threshold,
+      const std::uint32_t num_signers);
     /*!
      * Export multisig info
      * This will generate and remember new k values

src/wallet/wallet_rpc_server.cpp

@@ -4178,6 +4178,48 @@ namespace tools
     return true;
   }
   //------------------------------------------------------------------------------------------------------------------------------
+  bool wallet_rpc_server::on_get_multisig_key_exchange_booster(const wallet_rpc::COMMAND_RPC_GET_MULTISIG_KEY_EXCHANGE_BOOSTER::request& req, wallet_rpc::COMMAND_RPC_GET_MULTISIG_KEY_EXCHANGE_BOOSTER::response& res, epee::json_rpc::error& er, const connection_context *ctx)
+  {
+    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;
+    m_wallet->multisig(&ready);
+
+    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() == 0)
+    {
+      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->get_multisig_key_exchange_booster(req.password,
+        req.multisig_info,
+        req.threshold,
+        req.num_signers);
+    }
+    catch (const std::exception &e)
+    {
+      er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
+      er.message = std::string("Error calling exchange_multisig_info_booster: ") + 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, const connection_context *ctx)
   {
     if (!m_wallet) return not_open(er);

src/wallet/wallet_rpc_server.h

@@ -152,6 +152,7 @@ namespace tools
         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("get_multisig_key_exchange_booster",  on_get_multisig_key_exchange_booster,  wallet_rpc::COMMAND_RPC_GET_MULTISIG_KEY_EXCHANGE_BOOSTER)
         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("validate_address",   on_validate_address,   wallet_rpc::COMMAND_RPC_VALIDATE_ADDRESS)
@@ -243,6 +244,7 @@ namespace tools
       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, const connection_context *ctx = NULL);
       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, const connection_context *ctx = NULL);
       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, const connection_context *ctx = NULL);
+      bool on_get_multisig_key_exchange_booster(const wallet_rpc::COMMAND_RPC_GET_MULTISIG_KEY_EXCHANGE_BOOSTER::request& req, wallet_rpc::COMMAND_RPC_GET_MULTISIG_KEY_EXCHANGE_BOOSTER::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);
       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, const connection_context *ctx = NULL);
       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, const connection_context *ctx = NULL);
       bool on_validate_address(const wallet_rpc::COMMAND_RPC_VALIDATE_ADDRESS::request& req, wallet_rpc::COMMAND_RPC_VALIDATE_ADDRESS::response& res, epee::json_rpc::error& er, const connection_context *ctx = NULL);

src/wallet/wallet_rpc_server_commands_defs.h

@@ -2558,6 +2558,35 @@ namespace wallet_rpc
     typedef epee::misc_utils::struct_init<response_t> response;
   };
 
+  struct COMMAND_RPC_GET_MULTISIG_KEY_EXCHANGE_BOOSTER
+  {
+    struct request_t
+    {
+      std::string password;
+      std::vector<std::string> multisig_info;
+      uint32_t threshold;
+      uint32_t num_signers;
+
+      BEGIN_KV_SERIALIZE_MAP()
+        KV_SERIALIZE(password)
+        KV_SERIALIZE(multisig_info)
+        KV_SERIALIZE(threshold)
+        KV_SERIALIZE(num_signers)
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<request_t> request;
+
+    struct response_t
+    {
+      std::string multisig_info;
+
+      BEGIN_KV_SERIALIZE_MAP()
+        KV_SERIALIZE(multisig_info)
+      END_KV_SERIALIZE_MAP()
+    };
+    typedef epee::misc_utils::struct_init<response_t> response;
+  };
+
   struct COMMAND_RPC_SIGN_MULTISIG
   {
     struct request_t

tests/unit_tests/multisig.cpp

@@ -149,6 +149,7 @@ static void check_results(const std::vector<std::string> &intermediate_infos,
   std::unordered_set<crypto::secret_key> unique_privkeys;
   rct::key composite_pubkey = rct::identity();
 
+  ASSERT_TRUE(wallets.size() > 0);
   wallets[0].decrypt_keys("");
   crypto::public_key spend_pubkey = wallets[0].get_account().get_keys().m_account_address.m_spend_public_key;
   crypto::secret_key view_privkey = wallets[0].get_account().get_keys().m_view_secret_key;
@@ -156,32 +157,49 @@ static void check_results(const std::vector<std::string> &intermediate_infos,
   EXPECT_TRUE(crypto::secret_key_to_public_key(view_privkey, view_pubkey));
   wallets[0].encrypt_keys("");
 
-  for (size_t i = 0; i < wallets.size(); ++i)
+  // at the end of multisig kex, all wallets should emit a post-kex message with the same two pubkeys
+  std::vector<crypto::public_key> post_kex_msg_pubkeys;
+  ASSERT_TRUE(intermediate_infos.size() == wallets.size());
+  for (const std::string &intermediate_info : intermediate_infos)
+  {
+    multisig::multisig_kex_msg post_kex_msg;
+    EXPECT_TRUE(!intermediate_info.empty());
+    EXPECT_NO_THROW(post_kex_msg = intermediate_info);
+
+    if (post_kex_msg_pubkeys.size() != 0)
+      EXPECT_TRUE(post_kex_msg_pubkeys == post_kex_msg.get_msg_pubkeys());  //assumes sorting is always the same
+    else
+      post_kex_msg_pubkeys = post_kex_msg.get_msg_pubkeys();
+
+    EXPECT_TRUE(post_kex_msg_pubkeys.size() == 2);
+  }
+
+  // the post-kex pubkeys should equal the account's public view and spend keys
+  EXPECT_TRUE(std::find(post_kex_msg_pubkeys.begin(), post_kex_msg_pubkeys.end(), spend_pubkey) != post_kex_msg_pubkeys.end());
+  EXPECT_TRUE(std::find(post_kex_msg_pubkeys.begin(), post_kex_msg_pubkeys.end(), view_pubkey) != post_kex_msg_pubkeys.end());
+
+  // each wallet should have the same state (private view key, public spend key), and the public spend key should be
+  //   reproducible from the private spend keys found in each account
+  bool ready;
+  uint32_t threshold, total;
+
+  for (tools::wallet2 &wallet : wallets)
   {
-    EXPECT_TRUE(!intermediate_infos[i].empty());
-    bool ready;
-    uint32_t threshold, total;
-    EXPECT_TRUE(wallets[i].multisig(&ready, &threshold, &total));
+    wallet.decrypt_keys("");
+    EXPECT_TRUE(wallet.multisig(&ready, &threshold, &total));
     EXPECT_TRUE(ready);
     EXPECT_TRUE(threshold == M);
     EXPECT_TRUE(total == wallets.size());
 
-    wallets[i].decrypt_keys("");
-
-    if (i != 0)
-    {
-      // "equals" is transitive relation so we need only to compare first wallet's address to each others' addresses.
-      // no need to compare 0's address with itself.
-      EXPECT_TRUE(wallets[0].get_account().get_public_address_str(cryptonote::TESTNET) ==
-        wallets[i].get_account().get_public_address_str(cryptonote::TESTNET));
-      
-      EXPECT_EQ(spend_pubkey, wallets[i].get_account().get_keys().m_account_address.m_spend_public_key);
-      EXPECT_EQ(view_privkey, wallets[i].get_account().get_keys().m_view_secret_key);
-      EXPECT_EQ(view_pubkey, wallets[i].get_account().get_keys().m_account_address.m_view_public_key);
-    }
+    EXPECT_TRUE(wallets[0].get_account().get_public_address_str(cryptonote::TESTNET) ==
+      wallet.get_account().get_public_address_str(cryptonote::TESTNET));
+    
+    EXPECT_EQ(spend_pubkey, wallet.get_account().get_keys().m_account_address.m_spend_public_key);
+    EXPECT_EQ(view_privkey, wallet.get_account().get_keys().m_view_secret_key);
+    EXPECT_EQ(view_pubkey, wallet.get_account().get_keys().m_account_address.m_view_public_key);
 
     // sum together unique multisig keys
-    for (const auto &privkey : wallets[i].get_account().get_keys().m_multisig_keys)
+    for (const auto &privkey : wallet.get_account().get_keys().m_multisig_keys)
     {
       EXPECT_NE(privkey, crypto::null_skey);
 
@@ -189,17 +207,17 @@ static void check_results(const std::vector<std::string> &intermediate_infos,
       {
         unique_privkeys.insert(privkey);
         crypto::public_key pubkey;
-        crypto::secret_key_to_public_key(privkey, pubkey);
+        EXPECT_TRUE(crypto::secret_key_to_public_key(privkey, pubkey));
         EXPECT_NE(privkey, crypto::null_skey);
         EXPECT_NE(pubkey, crypto::null_pkey);
         EXPECT_NE(pubkey, rct::rct2pk(rct::identity()));
         rct::addKeys(composite_pubkey, composite_pubkey, rct::pk2rct(pubkey));
       }
     }
-    wallets[i].encrypt_keys("");
+    wallet.encrypt_keys("");
   }
 
-  // final key via sums should equal the wallets' public spend key
+  // final key via sum of privkeys should equal the wallets' public spend key
   wallets[0].decrypt_keys("");
   EXPECT_EQ(wallets[0].get_account().get_keys().m_account_address.m_spend_public_key, rct::rct2pk(composite_pubkey));
   wallets[0].encrypt_keys("");
@@ -260,6 +278,104 @@ static void make_wallets(const unsigned int M, const unsigned int N, const bool
   check_results(intermediate_infos, wallets, M);
 }
 
+static void make_wallets_boosting(std::vector<tools::wallet2>& wallets, unsigned int M)
+{
+  ASSERT_TRUE(wallets.size() > 1 && wallets.size() <= KEYS_COUNT);
+  ASSERT_TRUE(M <= wallets.size());
+  std::uint32_t kex_rounds_required = multisig::multisig_kex_rounds_required(wallets.size(), M);
+  std::uint32_t rounds_required = multisig::multisig_setup_rounds_required(wallets.size(), M);
+  std::uint32_t rounds_complete{0};
+
+  // initialize wallets, get first round multisig kex msgs
+  std::vector<std::string> initial_infos(wallets.size());
+
+  for (size_t i = 0; i < wallets.size(); ++i)
+  {
+    make_wallet(i, wallets[i]);
+
+    wallets[i].decrypt_keys("");
+    initial_infos[i] = wallets[i].get_multisig_first_kex_msg();
+    wallets[i].encrypt_keys("");
+  }
+
+  // wallets should not be multisig yet
+  for (const auto &wallet: wallets)
+  {
+    ASSERT_FALSE(wallet.multisig());
+  }
+
+  // get round 2 booster messages for wallet0 (if appropriate)
+  auto initial_infos_truncated = initial_infos;
+  initial_infos_truncated.erase(initial_infos_truncated.begin());
+
+  std::vector<std::string> wallet0_booster_infos;
+  wallet0_booster_infos.reserve(wallets.size() - 1);
+
+  if (rounds_complete + 1 < kex_rounds_required)
+  {
+    for (size_t i = 1; i < wallets.size(); ++i)
+    {
+      wallet0_booster_infos.push_back(
+          wallets[i].get_multisig_key_exchange_booster("", initial_infos_truncated, M, wallets.size())
+        );
+    }
+  }
+
+  // make wallets multisig
+  std::vector<std::string> intermediate_infos(wallets.size());
+
+  for (size_t i = 0; i < wallets.size(); ++i)
+    intermediate_infos[i] = wallets[i].make_multisig("", initial_infos, M);
+
+  ++rounds_complete;
+
+  // perform all kex rounds
+  // boost wallet0 each round, so wallet0 is always 1 round ahead
+  std::string wallet0_intermediate_info;
+  std::vector<std::string> new_infos(intermediate_infos.size());
+  bool ready;
+  wallets[0].multisig(&ready);
+  while (!ready)
+  {
+    // use booster infos to update wallet0 'early'
+    if (rounds_complete < kex_rounds_required)
+      new_infos[0] = wallets[0].exchange_multisig_keys("", wallet0_booster_infos);
+    else
+    {
+      // force update the post-kex round with wallet0's post-kex message since wallet0 is 'ahead' of the other wallets
+      wallet0_booster_infos = {wallets[0].exchange_multisig_keys("", {})};
+      new_infos[0] = wallets[0].exchange_multisig_keys("", wallet0_booster_infos, true);
+    }
+
+    // get wallet0 booster infos for next round
+    if (rounds_complete + 1 < kex_rounds_required)
+    {
+      // remove wallet0 info for this round (so boosters have incomplete kex message set)
+      auto intermediate_infos_truncated = intermediate_infos;
+      intermediate_infos_truncated.erase(intermediate_infos_truncated.begin());
+
+      // obtain booster messages from all other wallets
+      for (size_t i = 1; i < wallets.size(); ++i)
+      {
+        wallet0_booster_infos[i-1] =
+          wallets[i].get_multisig_key_exchange_booster("", intermediate_infos_truncated, M, wallets.size());
+      }
+    }
+
+    // update other wallets
+    for (size_t i = 1; i < wallets.size(); ++i)
+        new_infos[i] = wallets[i].exchange_multisig_keys("", intermediate_infos);
+
+    intermediate_infos = new_infos;
+    ++rounds_complete;
+    wallets[0].multisig(&ready);
+  }
+
+  EXPECT_EQ(rounds_required, rounds_complete);
+
+  check_results(intermediate_infos, wallets, M);
+}
+
 TEST(multisig, make_1_2)
 {
   make_wallets(1, 2, false);
@@ -296,6 +412,12 @@ TEST(multisig, make_2_4)
   make_wallets(2, 4, true);
 }
 
+TEST(multisig, make_2_4_boosting)
+{
+  std::vector<tools::wallet2> wallets(4);
+  make_wallets_boosting(wallets, 2);
+}
+
 TEST(multisig, multisig_kex_msg)
 {
   using namespace multisig;

utils/python-rpc/framework/wallet.py

@@ -537,6 +537,20 @@ class Wallet(object):
         }
         return self.rpc.send_json_rpc_request(exchange_multisig_keys)
 
+    def get_multisig_key_exchange_booster(self, multisig_info, threshold, num_signers, password = ''):
+        exchange_multisig_keys = {
+            'method': 'get_multisig_key_exchange_booster',
+            'params' : {
+                'multisig_info': multisig_info,
+                'threshold': threshold,
+                'num_signers': num_signers,
+                'password': password,
+            },
+            'jsonrpc': '2.0', 
+            'id': '0'
+        }
+        return self.rpc.send_json_rpc_request(get_multisig_key_exchange_booster)
+
     def export_multisig_info(self):
         export_multisig_info = {
             'method': 'export_multisig_info',