add an option to force-update multisig key exchange under some circumstances

15 files changed, 244 insertions, 115 deletions

diff --git a/src/gen_multisig/gen_multisig.cpp b/src/gen_multisig/gen_multisig.cpp
index f13e74b0f..eedd1511d 100644
--- a/src/gen_multisig/gen_multisig.cpp
+++ b/src/gen_multisig/gen_multisig.cpp
@@ -50,7 +50,6 @@
 using namespace std;
 using namespace epee;
 using namespace cryptonote;
-using boost::lexical_cast;
 namespace po = boost::program_options;
 
 #undef MONERO_DEFAULT_LOG_CATEGORY
@@ -84,6 +83,9 @@ static bool generate_multisig(uint32_t threshold, uint32_t total, const std::str
 
   try
   {
+    if (total == 0)
+      throw std::runtime_error("Signer group of size 0 is not allowed.");
+
     // create M wallets first
     std::vector<boost::shared_ptr<tools::wallet2>> wallets(total);
     for (size_t n = 0; n < total; ++n)
@@ -118,13 +120,17 @@ static bool generate_multisig(uint32_t threshold, uint32_t total, const std::str
       ss << "  " << name << std::endl;
     }
 
-    //exchange keys unless exchange_multisig_keys returns no extra info
-    while (!kex_msgs_intermediate[0].empty())
+    // exchange keys until the wallets are done
+    bool ready{false};
+    wallets[0]->multisig(&ready);
+    while (!ready)
     {
       for (size_t n = 0; n < total; ++n)
       {
           kex_msgs_intermediate[n] = wallets[n]->exchange_multisig_keys(pwd_container->password(), kex_msgs_intermediate);
       }
+
+      wallets[0]->multisig(&ready);
     }
 
     std::string address = wallets[0]->get_account().get_public_address_str(wallets[0]->nettype());
diff --git a/src/multisig/multisig_account.cpp b/src/multisig/multisig_account.cpp
index 9bdcf2dbc..971970277 100644
--- a/src/multisig/multisig_account.cpp
+++ b/src/multisig/multisig_account.cpp
@@ -175,19 +175,20 @@ namespace multisig
     // only mutate account if update succeeds
     multisig_account temp_account{*this};
     temp_account.set_multisig_config(threshold, std::move(signers));
-    temp_account.kex_update_impl(expanded_msgs_rnd1);
+    temp_account.kex_update_impl(expanded_msgs_rnd1, false);
     *this = std::move(temp_account);
   }
   //----------------------------------------------------------------------------------------------------------------------
   // multisig_account: EXTERNAL
   //----------------------------------------------------------------------------------------------------------------------
-  void multisig_account::kex_update(const std::vector<multisig_kex_msg> &expanded_msgs)
+  void multisig_account::kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
+    const bool force_update_use_with_caution /*= false*/)
   {
     CHECK_AND_ASSERT_THROW_MES(account_is_active(), "multisig account: tried to update kex, but kex isn't initialized yet.");
     CHECK_AND_ASSERT_THROW_MES(!multisig_is_ready(), "multisig account: tried to update kex, but kex is already complete.");
 
     multisig_account temp_account{*this};
-    temp_account.kex_update_impl(expanded_msgs);
+    temp_account.kex_update_impl(expanded_msgs, force_update_use_with_caution);
     *this = std::move(temp_account);
   }
   //----------------------------------------------------------------------------------------------------------------------
diff --git a/src/multisig/multisig_account.h b/src/multisig/multisig_account.h
index 7b372bbff..794230ffc 100644
--- a/src/multisig/multisig_account.h
+++ b/src/multisig/multisig_account.h
@@ -169,12 +169,20 @@ namespace multisig
     *    - The main interface for multisig key exchange, this handles all the work of processing input messages,
     *      creating new messages for new rounds, and finalizing the multisig shared public key when kex is complete.
     * param: expanded_msgs - kex messages corresponding to the account's 'in progress' round
+    * param: force_update_use_with_caution - try to force the account to update with messages from an incomplete signer set.
+    *    - If this is the post-kex verification round, only require one input message.
+    *      - Force updating here should only be done if we can safely assume an honest signer subgroup of size 'threshold'
+    *        will complete the account.
+    *    - If this is an intermediate round, only require messages from 'num signers - 1 - (round - 1)' other signers.
+    *      - 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);
+    void kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
+      const bool force_update_use_with_caution = false);
 
   private:
     // implementation of kex_update() (non-transactional)
-    void kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs);
+    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
diff --git a/src/multisig/multisig_account_kex_impl.cpp b/src/multisig/multisig_account_kex_impl.cpp
index be9ed9cb2..141b20ead 100644
--- a/src/multisig/multisig_account_kex_impl.cpp
+++ b/src/multisig/multisig_account_kex_impl.cpp
@@ -181,7 +181,8 @@ namespace multisig
   *    Key aggregation via aggregation coefficients prevents key cancellation attacks.
   *    See: https://www.getmonero.org/resources/research-lab/pubs/MRL-0009.pdf
   * param: final_keys - address components (public keys) obtained from other participants (not shared with local)
-  * param: privkeys_inout - private keys of address components known by local; each key will be multiplied by an aggregation coefficient (return by reference)
+  * param: privkeys_inout - private keys of address components known by local; each key will be multiplied by an aggregation
+  *                         coefficient (return by reference)
   * return: final multisig public spend key for the account
   */
   //----------------------------------------------------------------------------------------------------------------------
@@ -199,7 +200,8 @@ namespace multisig
     for (std::size_t multisig_keys_index{0}; multisig_keys_index < privkeys_inout.size(); ++multisig_keys_index)
     {
       crypto::public_key pubkey;
-      CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(privkeys_inout[multisig_keys_index], pubkey), "Failed to derive public key");
+      CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(privkeys_inout[multisig_keys_index], pubkey),
+        "Failed to derive public key");
 
       own_keys_mapping[pubkey] = multisig_keys_index;
 
@@ -307,8 +309,7 @@ namespace multisig
   * INTERNAL
   *
   * brief: multisig_kex_msgs_sanitize_pubkeys - Sanitize multisig kex messages.
-  *    - Removes duplicates from msg pubkeys, ignores pubkeys equal to the local account's signing key,
-  *      ignores messages signed by the local account, ignores keys found in input 'exclusion set',
+  *    - Removes duplicates from msg pubkeys, ignores keys found in input 'exclusion set',
   *      constructs map of pubkey:origins.
   *    - Requires that all input msgs have the same round number.
   *
@@ -316,15 +317,13 @@ namespace multisig
   *
   *    - If the messages' round numbers are all '1', then only the message signing pubkey is considered
   *      'recommended'. Furthermore, the 'exclusion set' is ignored.
-  * param: own_pubkey - local account's signing key (key used to sign multisig messages)
   * param: expanded_msgs - set of multisig kex messages to process
   * param: exclude_pubkeys - pubkeys to exclude from output set
   * outparam: sanitized_pubkeys_out - processed pubkeys obtained from msgs, mapped to their origins
   * return: round number shared by all input msgs
   */
   //----------------------------------------------------------------------------------------------------------------------
-  static std::uint32_t multisig_kex_msgs_sanitize_pubkeys(const crypto::public_key &own_pubkey,
-    const std::vector<multisig_kex_msg> &expanded_msgs,
+  static std::uint32_t multisig_kex_msgs_sanitize_pubkeys(const std::vector<multisig_kex_msg> &expanded_msgs,
     const std::vector<crypto::public_key> &exclude_pubkeys,
     multisig_keyset_map_memsafe_t &sanitized_pubkeys_out)
   {
@@ -339,10 +338,6 @@ namespace multisig
     // - origins = all the signing pubkeys that recommended a given msg pubkey
     for (const auto &expanded_msg : expanded_msgs)
     {
-      // ignore messages from self
-      if (expanded_msg.get_signing_pubkey() == own_pubkey)
-        continue;
-
       // in round 1, only the signing pubkey is treated as a msg pubkey
       if (round == 1)
       {
@@ -355,10 +350,6 @@ namespace multisig
         // copy all pubkeys from message into list
         for (const auto &pubkey : expanded_msg.get_msg_pubkeys())
         {
-          // ignore own pubkey
-          if (pubkey == own_pubkey)
-            continue;
-
           // ignore pubkeys in 'ignore' set
           if (std::find(exclude_pubkeys.begin(), exclude_pubkeys.end(), pubkey) != exclude_pubkeys.end())
             continue;
@@ -375,6 +366,31 @@ namespace multisig
   /**
   * INTERNAL
   *
+  * brief: remove_key_from_mapped_sets - Remove a specified key from the mapped sets in a multisig keyset map.
+  * param: key_to_remove - specified key to remove
+  * inoutparam: keyset_inout - keyset to update
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static void remove_key_from_mapped_sets(const crypto::public_key &key_to_remove,
+    multisig_keyset_map_memsafe_t &keyset_inout)
+  {
+    // remove specified key from each mapped set
+    for (auto keyset_it = keyset_inout.begin(); keyset_it != keyset_inout.end();)
+    {
+      // remove specified key from this set
+      keyset_it->second.erase(key_to_remove);
+
+      // remove empty keyset positions or increment iterator
+      if (keyset_it->second.size() == 0)
+        keyset_it = keyset_inout.erase(keyset_it);
+      else
+        ++keyset_it;
+    }
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  *
   * brief: evaluate_multisig_kex_round_msgs - Evaluate pubkeys from a kex round in order to prepare for the next round.
   *    - Sanitizes input msgs.
   *    - Require uniqueness in: 'exclude_pubkeys'.
@@ -392,6 +408,8 @@ namespace multisig
   * param: signers - expected participants in multisig kex
   * param: expanded_msgs - set of multisig kex messages to process
   * param: exclude_pubkeys - derivations held by the local account corresponding to round 'expected_round'
+  * param: incomplete_signer_set - only require the minimum number of signers to complete this round
+  *                                minimum = num_signers - (round num - 1)   (including local signer)
   * return: fully sanitized and validated pubkey:origins map for building the account's next kex round message
   */
   //----------------------------------------------------------------------------------------------------------------------
@@ -400,7 +418,8 @@ namespace multisig
     const std::uint32_t expected_round,
     const std::vector<crypto::public_key> &signers,
     const std::vector<multisig_kex_msg> &expanded_msgs,
-    const std::vector<crypto::public_key> &exclude_pubkeys)
+    const std::vector<crypto::public_key> &exclude_pubkeys,
+    const bool incomplete_signer_set)
   {
     // exclude_pubkeys should all be unique
     for (auto it = exclude_pubkeys.begin(); it != exclude_pubkeys.end(); ++it)
@@ -410,21 +429,31 @@ namespace multisig
     }
 
     // sanitize input messages
-    multisig_keyset_map_memsafe_t pubkey_origins_map;
-    const std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(base_pubkey, expanded_msgs, exclude_pubkeys, pubkey_origins_map);
+    multisig_keyset_map_memsafe_t pubkey_origins_map;  //map: [pubkey : [origins]]
+    const std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(expanded_msgs, exclude_pubkeys, pubkey_origins_map);
     CHECK_AND_ASSERT_THROW_MES(round == expected_round,
       "Kex messages were for round [" << round << "], but expected round is [" << expected_round << "]");
 
+    // remove the local signer from each origins set in the sanitized pubkey map
+    // note: intermediate kex rounds only need keys from other signers to make progress (keys from self are useless)
+    remove_key_from_mapped_sets(base_pubkey, pubkey_origins_map);
+
     // evaluate pubkeys collected
-    std::unordered_map<crypto::public_key, std::unordered_set<crypto::public_key>> origin_pubkeys_map;
+    std::unordered_map<crypto::public_key, std::unordered_set<crypto::public_key>> origin_pubkeys_map;  //map: [origin: [pubkeys]]
 
     // 1. each pubkey should be recommended by a precise number of signers
+    const std::size_t num_recommendations_per_pubkey_required{
+        incomplete_signer_set
+        ? 1
+        : round
+      };
+
     for (const auto &pubkey_and_origins : pubkey_origins_map)
     {
       // expected amount = round_num
       // With each successive round, pubkeys are shared by incrementally larger groups,
       //  starting at 1 in round 1 (i.e. the local multisig key to start kex with).
-      CHECK_AND_ASSERT_THROW_MES(pubkey_and_origins.second.size() == round,
+      CHECK_AND_ASSERT_THROW_MES(pubkey_and_origins.second.size() >= num_recommendations_per_pubkey_required,
         "A pubkey recommended by multisig kex messages had an unexpected number of recommendations.");
 
       // map (sanitized) pubkeys back to origins
@@ -433,8 +462,18 @@ namespace multisig
     }
 
     // 2. the number of unique signers recommending pubkeys should equal the number of signers passed in (minus the local signer)
-    CHECK_AND_ASSERT_THROW_MES(origin_pubkeys_map.size() == signers.size() - 1,
-      "Number of unique other signers does not equal number of other signers that recommended pubkeys.");
+    // - if an incomplete set is allowed, then we need at least one signer to represent each subgroup in this round that
+    //   doesn't include the local signer
+    const std::size_t num_signers_required{
+        incomplete_signer_set
+        ? signers.size() - 1 - (round - 1)
+        : signers.size() - 1
+      };
+
+    CHECK_AND_ASSERT_THROW_MES(origin_pubkeys_map.size() >= num_signers_required,
+      "Number of unique other signers recommending pubkeys does not equal number of required other signers "
+      "(kex round: " << round << ", num signers found: " << origin_pubkeys_map.size() << ", num signers required: " <<
+      num_signers_required << ").");
 
     // 3. each origin should recommend a precise number of pubkeys
 
@@ -461,19 +500,20 @@ namespace multisig
 
     // other signers: (N - 2) choose (msg_round_num - 1)
       // - Each signer recommends keys they share with other signers.
-      // - In each round, a signer shares a key with 'round num - 1' other signers.
-      // - Since 'origins pubkey map' excludes keys shared with the local account,
-      //   only keys shared with participants 'other than local and self' will be in the map (e.g. N - 2 signers).
-      // - So other signers will recommend (N - 2) choose (msg_round_num - 1) pubkeys (after removing keys shared with local).
-      // - Each origin should have a shared key with each group of size 'round - 1'.
-      // Note: Keys shared with local are ignored to facilitate kex round boosting, where one or more signers may
+      // - In each round, every group of size 'round num' will have a key. From a single signer's perspective,
+      //   they will share a key with every group of size 'round num - 1' of other signers.
+      // - Since 'origins pubkey map' excludes keys shared with the local account, only keys shared with participants
+      //   'other than local and self' will be in the map (e.g. N - 2 signers).
+      // - Other signers will recommend (N - 2) choose (msg_round_num - 1) pubkeys (after removing keys shared with local).
+      // Note: Keys shared with local are filtered out to facilitate kex round boosting, where one or more signers may
       //       have boosted the local signer (implying they didn't have access to the local signer's previous round msg).
     const std::uint32_t expected_recommendations_others = n_choose_k_f(signers.size() - 2, round - 1);
 
     // local: (N - 1) choose (msg_round_num - 1)
     const std::uint32_t expected_recommendations_self = n_choose_k_f(signers.size() - 1, round - 1);
 
-    // note: expected_recommendations_others would be 0 in the last round of 1-of-N, but we return early for that case
+    // note: expected_recommendations_others would be 0 in the last round of 1-of-N, but we don't call this function for
+    //       that case
     CHECK_AND_ASSERT_THROW_MES(expected_recommendations_self > 0 && expected_recommendations_others > 0,
       "Bad num signers or round num (possibly numerical limits exceeded).");
 
@@ -485,7 +525,7 @@ namespace multisig
     for (const auto &origin_and_pubkeys : origin_pubkeys_map)
     {
       CHECK_AND_ASSERT_THROW_MES(origin_and_pubkeys.second.size() == expected_recommendations_others,
-        "A pubkey recommended by multisig kex messages had an unexpected number of recommendations.");
+        "A multisig signer recommended an unexpected number of pubkeys.");
 
       // 2 (continued). only expected signers should be recommending keys
       CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), origin_and_pubkeys.first) != signers.end(),
@@ -507,6 +547,7 @@ namespace multisig
   * param: expected_round - expected kex round of input messages
   * param: signers - expected participants in multisig kex
   * param: expanded_msgs - set of multisig kex messages to process
+  * param: incomplete_signer_set - only require the minimum amount of messages to complete this round (1 message)
   * return: sanitized and validated pubkey:origins map
   */
   //----------------------------------------------------------------------------------------------------------------------
@@ -514,15 +555,20 @@ namespace multisig
     const crypto::public_key &base_pubkey,
     const std::uint32_t expected_round,
     const std::vector<crypto::public_key> &signers,
-    const std::vector<multisig_kex_msg> &expanded_msgs)
+    const std::vector<multisig_kex_msg> &expanded_msgs,
+    const bool incomplete_signer_set)
   {
     // sanitize input messages
     const std::vector<crypto::public_key> dummy;
-    multisig_keyset_map_memsafe_t pubkey_origins_map;
-    const std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(base_pubkey, expanded_msgs, dummy, pubkey_origins_map);
+    multisig_keyset_map_memsafe_t pubkey_origins_map;  //map: [pubkey : [origins]]
+    const std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(expanded_msgs, dummy, pubkey_origins_map);
     CHECK_AND_ASSERT_THROW_MES(round == expected_round,
       "Kex messages were for round [" << round << "], but expected round is [" << expected_round << "]");
 
+    // 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)
+
     // evaluate pubkeys collected
 
     // 1) there should only be two pubkeys
@@ -533,17 +579,26 @@ namespace multisig
     CHECK_AND_ASSERT_THROW_MES(pubkey_origins_map.begin()->second == (++(pubkey_origins_map.begin()))->second,
       "Multisig post-kex round messages from other signers did not all recommend the same pubkey pair.");
 
-    // 3) all signers should be present in the recommendation list
+    // 3) all signers should be present in the recommendation list (unless an incomplete list is permitted)
     auto origins = pubkey_origins_map.begin()->second;
-    origins.insert(base_pubkey);  //add self
+    origins.insert(base_pubkey);  //add self if missing
 
-    CHECK_AND_ASSERT_THROW_MES(origins.size() == signers.size(),
-      "Multisig post-kex round message origins don't line up with multisig signer set.");
+    const std::size_t num_signers_required{
+        incomplete_signer_set
+        ? 1
+        : signers.size()
+      };
+
+    CHECK_AND_ASSERT_THROW_MES(origins.size() >= num_signers_required,
+      "Multisig post-kex round message origins don't line up with multisig signer set "
+      "(num signers found: " << origins.size() << ", num signers required: " << num_signers_required << ").");
 
-    for (const crypto::public_key &signer : signers)
+    for (const crypto::public_key &origin : origins)
     {
-      CHECK_AND_ASSERT_THROW_MES(origins.find(signer) != origins.end(),
-        "Could not find an expected signer in multisig post-kex round messages (all signers expected).");
+      // note: if num_signers_required == signers.size(), then this test will ensure all signers are present in 'origins',
+      //       which contains only unique pubkeys
+      CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), origin) != signers.end(),
+        "An unknown origin recommended a multisig post-kex verification messsage.");
     }
 
     return pubkey_origins_map;
@@ -564,6 +619,7 @@ namespace multisig
   * param: expanded_msgs - set of multisig kex messages to process
   * param: exclude_pubkeys - 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.
+  * param: incomplete_signer_set - allow messages from an incomplete signer set
   * 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.
@@ -578,6 +634,7 @@ namespace multisig
     const std::vector<crypto::public_key> &signers,
     const std::vector<multisig_kex_msg> &expanded_msgs,
     const std::vector<crypto::public_key> &exclude_pubkeys,
+    const bool incomplete_signer_set,
     multisig_keyset_map_memsafe_t &keys_to_origins_map_out)
   {
     check_multisig_config(current_round, threshold, signers.size());
@@ -598,7 +655,8 @@ namespace multisig
         current_round,
         signers,
         expanded_msgs,
-        exclude_pubkeys);
+        exclude_pubkeys,
+        incomplete_signer_set);
     }
     else //(current_round == kex_rounds_required + 1)
     {
@@ -606,7 +664,8 @@ namespace multisig
       evaluated_pubkeys = evaluate_multisig_post_kex_round_msgs(base_pubkey,
         current_round,
         signers,
-        expanded_msgs);
+        expanded_msgs,
+        incomplete_signer_set);
     }
 
     // prepare keys-to-origins map for updating the multisig account
@@ -693,9 +752,9 @@ namespace multisig
     {
       // post-kex verification round: check that the multisig pubkey and common pubkey were recommended by other signers
       CHECK_AND_ASSERT_THROW_MES(result_keys_to_origins_map.count(m_multisig_pubkey) > 0,
-        "Multisig post-kex round: expected multisig pubkey wasn't found in other signers' messages.");
+        "Multisig post-kex round: expected multisig pubkey wasn't found in input messages.");
       CHECK_AND_ASSERT_THROW_MES(result_keys_to_origins_map.count(m_common_pubkey) > 0,
-        "Multisig post-kex round: expected common pubkey wasn't found in other signers' messages.");
+        "Multisig post-kex round: expected common pubkey wasn't found in input messages.");
 
       // save keys that should be recommended to other signers
       // - for convenience, re-recommend the post-kex verification message once an account is complete
@@ -790,7 +849,8 @@ namespace multisig
   //----------------------------------------------------------------------------------------------------------------------
   // multisig_account: INTERNAL
   //----------------------------------------------------------------------------------------------------------------------
-  void multisig_account::kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs)
+  void multisig_account::kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs,
+    const bool incomplete_signer_set)
   {
     // check messages are for the expected kex round
     check_messages_round(expanded_msgs, m_kex_rounds_complete + 1);
@@ -816,6 +876,7 @@ namespace multisig
       m_signers,
       expanded_msgs,
       exclude_pubkeys,
+      incomplete_signer_set,
       result_keys_to_origins_map);
 
     // finish account update
diff --git a/src/simplewallet/simplewallet.cpp b/src/simplewallet/simplewallet.cpp
index a8f4e5a07..db5be4dc2 100644
--- a/src/simplewallet/simplewallet.cpp
+++ b/src/simplewallet/simplewallet.cpp
@@ -244,7 +244,7 @@ namespace
   const char* USAGE_IMPORT_OUTPUTS("import_outputs <filename>");
   const char* USAGE_SHOW_TRANSFER("show_transfer <txid>");
   const char* USAGE_MAKE_MULTISIG("make_multisig <threshold> <string1> [<string>...]");
-  const char* USAGE_EXCHANGE_MULTISIG_KEYS("exchange_multisig_keys <string> [<string>...]");
+  const char* USAGE_EXCHANGE_MULTISIG_KEYS("exchange_multisig_keys [force-update-use-with-caution] <string> [<string>...]");
   const char* USAGE_EXPORT_MULTISIG_INFO("export_multisig_info <filename>");
   const char* USAGE_IMPORT_MULTISIG_INFO("import_multisig_info <filename> [<filename>...]");
   const char* USAGE_SIGN_MULTISIG("sign_multisig <filename>");
@@ -1139,11 +1139,22 @@ bool simple_wallet::make_multisig_main(const std::vector<std::string> &args, boo
 bool simple_wallet::exchange_multisig_keys(const std::vector<std::string> &args)
 {
   CHECK_MULTISIG_ENABLED();
-  exchange_multisig_keys_main(args, false);
+  bool force_update_use_with_caution = false;
+
+  auto local_args = args;
+  if (args.size() >= 1 && local_args[0] == "force-update-use-with-caution")
+  {
+    force_update_use_with_caution = true;
+    local_args.erase(local_args.begin());
+  }
+
+  exchange_multisig_keys_main(local_args, force_update_use_with_caution, false);
   return true;
 }
 
-bool simple_wallet::exchange_multisig_keys_main(const std::vector<std::string> &args, bool called_by_mms) {
+bool simple_wallet::exchange_multisig_keys_main(const std::vector<std::string> &args,
+  const bool force_update_use_with_caution,
+  const bool called_by_mms) {
     CHECK_MULTISIG_ENABLED();
     bool ready;
     if (m_wallet->key_on_device())
@@ -1169,15 +1180,9 @@ bool simple_wallet::exchange_multisig_keys_main(const std::vector<std::string> &
       return false;
     }
 
-    if (args.size() < 1)
-    {
-      PRINT_USAGE(USAGE_EXCHANGE_MULTISIG_KEYS);
-      return false;
-    }
-
     try
     {
-      std::string multisig_extra_info = m_wallet->exchange_multisig_keys(orig_pwd_container->password(), args);
+      std::string multisig_extra_info = m_wallet->exchange_multisig_keys(orig_pwd_container->password(), args, force_update_use_with_caution);
       bool ready;
       m_wallet->multisig(&ready);
       if (!ready)
@@ -11157,7 +11162,8 @@ void simple_wallet::mms_next(const std::vector<std::string> &args)
         mms::message m = ms.get_message_by_id(data.message_ids[i]);
         sig_args[i] = m.content;
       }
-      command_successful = exchange_multisig_keys_main(sig_args, true);
+      // todo: update mms to enable 'key exchange force updating'
+      command_successful = exchange_multisig_keys_main(sig_args, false, true);
       break;
     }
 
diff --git a/src/simplewallet/simplewallet.h b/src/simplewallet/simplewallet.h
index 6a9fa149d..1b67d9754 100644
--- a/src/simplewallet/simplewallet.h
+++ b/src/simplewallet/simplewallet.h
@@ -235,7 +235,7 @@ namespace cryptonote
     bool make_multisig(const std::vector<std::string>& args);
     bool make_multisig_main(const std::vector<std::string>& args, bool called_by_mms);
     bool exchange_multisig_keys(const std::vector<std::string> &args);
-    bool exchange_multisig_keys_main(const std::vector<std::string> &args, bool called_by_mms);
+    bool exchange_multisig_keys_main(const std::vector<std::string> &args, const bool force_update_use_with_caution, const bool called_by_mms);
     bool export_multisig(const std::vector<std::string>& args);
     bool export_multisig_main(const std::vector<std::string>& args, bool called_by_mms);
     bool import_multisig(const std::vector<std::string>& args);
diff --git a/src/wallet/api/wallet.cpp b/src/wallet/api/wallet.cpp
index 1ee2e20b6..3327c2821 100644
--- a/src/wallet/api/wallet.cpp
+++ b/src/wallet/api/wallet.cpp
@@ -1396,12 +1396,12 @@ string WalletImpl::makeMultisig(const vector<string>& info, const uint32_t thres
     return string();
 }
 
-std::string WalletImpl::exchangeMultisigKeys(const std::vector<std::string> &info) {
+std::string WalletImpl::exchangeMultisigKeys(const std::vector<std::string> &info, const bool force_update_use_with_caution /*= false*/) {
     try {
         clearStatus();
         checkMultisigWalletNotReady(m_wallet);
 
-        return m_wallet->exchange_multisig_keys(epee::wipeable_string(m_password), info);
+        return m_wallet->exchange_multisig_keys(epee::wipeable_string(m_password), info, force_update_use_with_caution);
     } catch (const exception& e) {
         LOG_ERROR("Error on exchanging multisig keys: " << e.what());
         setStatusError(string(tr("Failed to exchange multisig keys: ")) + e.what());
diff --git a/src/wallet/api/wallet.h b/src/wallet/api/wallet.h
index 018b2a0ed..ec2d7e9b3 100644
--- a/src/wallet/api/wallet.h
+++ b/src/wallet/api/wallet.h
@@ -146,7 +146,7 @@ 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;
+    std::string exchangeMultisigKeys(const std::vector<std::string> &info, const bool force_update_use_with_caution = false) override;
     bool exportMultisigImages(std::string& images) override;
     size_t importMultisigImages(const std::vector<std::string>& images) override;
     bool hasMultisigPartialKeyImages() const override;
diff --git a/src/wallet/api/wallet2_api.h b/src/wallet/api/wallet2_api.h
index b67bce60c..0ae84adb9 100644
--- a/src/wallet/api/wallet2_api.h
+++ b/src/wallet/api/wallet2_api.h
@@ -796,9 +796,10 @@ struct Wallet
     /**
      * @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
+     * @param force_update_use_with_caution - force multisig account to update even if not all signers contribute round messages
      * @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;
+    virtual std::string exchangeMultisigKeys(const std::vector<std::string> &info, const bool force_update_use_with_caution) = 0;
     /**
      * @brief exportMultisigImages - exports transfers' key images
      * @param images - output paramter for hex encoded array of images
diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index 195763949..61b34102b 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -5075,12 +5075,11 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
 }
 //----------------------------------------------------------------------------------------------------
 std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &password,
-  const std::vector<std::string> &kex_messages)
+  const std::vector<std::string> &kex_messages,
+  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(!ready, "Multisig wallet creation process has already been finished");
-  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;
@@ -5099,13 +5098,6 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
       );
   }
 
-  // open kex messages
-  std::vector<multisig::multisig_kex_msg> expanded_msgs;
-  expanded_msgs.reserve(kex_messages.size());
-
-  for (const auto &msg : kex_messages)
-    expanded_msgs.emplace_back(msg);
-
   // reconstruct multisig account
   multisig::multisig_keyset_map_memsafe_t kex_origins_map;
 
@@ -5126,8 +5118,25 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
       ""
     };
 
+  // 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)
+  if (kex_messages.size() == 0)
+  {
+    CHECK_AND_ASSERT_THROW_MES(multisig_account.main_kex_rounds_done(),
+      "Exchange multisig keys: there are no kex messages but the main kex rounds are not done.");
+
+    return multisig_account.get_next_kex_round_msg();
+  }
+
+  // open kex messages
+  std::vector<multisig::multisig_kex_msg> expanded_msgs;
+  expanded_msgs.reserve(kex_messages.size());
+
+  for (const auto &msg : kex_messages)
+    expanded_msgs.emplace_back(msg);
+
   // update multisig kex
-  multisig_account.kex_update(expanded_msgs);
+  multisig_account.kex_update(expanded_msgs, force_update_use_with_caution);
 
   // update wallet state
 
diff --git a/src/wallet/wallet2.h b/src/wallet/wallet2.h
index 16e898ad8..81472c200 100644
--- a/src/wallet/wallet2.h
+++ b/src/wallet/wallet2.h
@@ -862,7 +862,8 @@ private:
      * to other participants
      */
     std::string exchange_multisig_keys(const epee::wipeable_string &password,
-      const std::vector<std::string> &kex_messages);
+      const std::vector<std::string> &kex_messages,
+      const bool force_update_use_with_caution = false);
     /*!
      * \brief Get initial message to start multisig key exchange (before 'make_multisig()' is called)
      * \return string to send to other participants
diff --git a/src/wallet/wallet_rpc_server.cpp b/src/wallet/wallet_rpc_server.cpp
index 7ec5fc7a1..4cb837d38 100644
--- a/src/wallet/wallet_rpc_server.cpp
+++ b/src/wallet/wallet_rpc_server.cpp
@@ -4146,13 +4146,6 @@ namespace tools
       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;
-    }
     CHECK_MULTISIG_ENABLED();
 
     if (req.multisig_info.size() + 1 < total)
@@ -4164,7 +4157,7 @@ namespace tools
 
     try
     {
-      res.multisig_info = m_wallet->exchange_multisig_keys(req.password, req.multisig_info);
+      res.multisig_info = m_wallet->exchange_multisig_keys(req.password, req.multisig_info, req.force_update_use_with_caution);
       m_wallet->multisig(&ready);
       if (ready)
       {
diff --git a/src/wallet/wallet_rpc_server_commands_defs.h b/src/wallet/wallet_rpc_server_commands_defs.h
index ecfc8e673..61b7cf5cd 100644
--- a/src/wallet/wallet_rpc_server_commands_defs.h
+++ b/src/wallet/wallet_rpc_server_commands_defs.h
@@ -47,7 +47,7 @@
 // advance which version they will stop working with
 // Don't go over 32767 for any of these
 #define WALLET_RPC_VERSION_MAJOR 1
-#define WALLET_RPC_VERSION_MINOR 25
+#define WALLET_RPC_VERSION_MINOR 26
 #define MAKE_WALLET_RPC_VERSION(major,minor) (((major)<<16)|(minor))
 #define WALLET_RPC_VERSION MAKE_WALLET_RPC_VERSION(WALLET_RPC_VERSION_MAJOR, WALLET_RPC_VERSION_MINOR)
 namespace tools
@@ -2531,10 +2531,12 @@ namespace wallet_rpc
     {
       std::string password;
       std::vector<std::string> multisig_info;
+      bool force_update_use_with_caution;
 
       BEGIN_KV_SERIALIZE_MAP()
         KV_SERIALIZE(password)
         KV_SERIALIZE(multisig_info)
+        KV_SERIALIZE_OPT(force_update_use_with_caution, false)
       END_KV_SERIALIZE_MAP()
     };
     typedef epee::misc_utils::struct_init<request_t> request;
diff --git a/tests/unit_tests/multisig.cpp b/tests/unit_tests/multisig.cpp
index 5ddd78955..7015781b4 100644
--- a/tests/unit_tests/multisig.cpp
+++ b/tests/unit_tests/multisig.cpp
@@ -96,8 +96,46 @@ static std::vector<std::string> exchange_round(std::vector<tools::wallet2>& wall
   new_infos.reserve(infos.size());
 
   for (size_t i = 0; i < wallets.size(); ++i)
+    new_infos.push_back(wallets[i].exchange_multisig_keys("", infos));
+
+  return new_infos;
+}
+
+static std::vector<std::string> exchange_round_force_update(std::vector<tools::wallet2>& wallets,
+  const std::vector<std::string>& infos,
+  const std::size_t round_in_progress)
+{
+  EXPECT_TRUE(wallets.size() == infos.size());
+  std::vector<std::string> new_infos;
+  std::vector<std::string> temp_force_update_infos;
+  new_infos.reserve(infos.size());
+
+  // when force-updating, we only need at most 'num_signers - 1 - (round - 1)' messages from other signers
+  size_t num_other_messages_required{wallets.size() - 1 - (round_in_progress - 1)};
+  if (num_other_messages_required > wallets.size())
+    num_other_messages_required = 0;  //overflow case for post-kex verification round of 1-of-N
+
+  for (size_t i = 0; i < wallets.size(); ++i)
   {
-      new_infos.push_back(wallets[i].exchange_multisig_keys("", infos));
+    temp_force_update_infos.clear();
+    temp_force_update_infos.reserve(num_other_messages_required + 1);
+    temp_force_update_infos.push_back(infos[i]);  //always include the local signer's message for this round
+
+    size_t infos_collected{0};
+    for (size_t wallet_index = 0; wallet_index < wallets.size(); ++wallet_index)
+    {
+      // skip the local signer's message
+      if (wallet_index == i)
+        continue;
+
+      temp_force_update_infos.push_back(infos[wallet_index]);
+      ++infos_collected;
+
+      if (infos_collected == num_other_messages_required)
+        break;
+    }
+
+    new_infos.push_back(wallets[i].exchange_multisig_keys("", temp_force_update_infos, true));
   }
 
   return new_infos;
@@ -105,7 +143,7 @@ static std::vector<std::string> exchange_round(std::vector<tools::wallet2>& wall
 
 static void check_results(const std::vector<std::string> &intermediate_infos,
   std::vector<tools::wallet2>& wallets,
-  std::uint32_t M)
+  const std::uint32_t M)
 {
   // check results
   std::unordered_set<crypto::secret_key> unique_privkeys;
@@ -167,8 +205,9 @@ static void check_results(const std::vector<std::string> &intermediate_infos,
   wallets[0].encrypt_keys("");
 }
 
-static void make_wallets(std::vector<tools::wallet2>& wallets, unsigned int M)
+static void make_wallets(const unsigned int M, const unsigned int N, const bool force_update)
 {
+  std::vector<tools::wallet2> wallets(N);
   ASSERT_TRUE(wallets.size() > 1 && wallets.size() <= KEYS_COUNT);
   ASSERT_TRUE(M <= wallets.size());
   std::uint32_t total_rounds_required = multisig::multisig_kex_rounds_required(wallets.size(), M) + 1;
@@ -207,9 +246,12 @@ static void make_wallets(std::vector<tools::wallet2>& wallets, unsigned int M)
   wallets[0].multisig(&ready);
   while (!ready)
   {
-    intermediate_infos = exchange_round(wallets, intermediate_infos);
-    wallets[0].multisig(&ready);
+    if (force_update)
+      intermediate_infos = exchange_round_force_update(wallets, intermediate_infos, rounds_complete + 1);
+    else
+      intermediate_infos = exchange_round(wallets, intermediate_infos);
 
+    wallets[0].multisig(&ready);
     ++rounds_complete;
   }
 
@@ -220,38 +262,38 @@ static void make_wallets(std::vector<tools::wallet2>& wallets, unsigned int M)
 
 TEST(multisig, make_1_2)
 {
-  std::vector<tools::wallet2> wallets(2);
-  make_wallets(wallets, 1);
+  make_wallets(1, 2, false);
+  make_wallets(1, 2, true);
 }
 
 TEST(multisig, make_1_3)
 {
-  std::vector<tools::wallet2> wallets(3);
-  make_wallets(wallets, 1);
+  make_wallets(1, 3, false);
+  make_wallets(1, 3, true);
 }
 
 TEST(multisig, make_2_2)
 {
-  std::vector<tools::wallet2> wallets(2);
-  make_wallets(wallets, 2);
+  make_wallets(2, 2, false);
+  make_wallets(2, 2, true);
 }
 
 TEST(multisig, make_3_3)
 {
-  std::vector<tools::wallet2> wallets(3);
-  make_wallets(wallets, 3);
+  make_wallets(3, 3, false);
+  make_wallets(3, 3, true);
 }
 
 TEST(multisig, make_2_3)
 {
-  std::vector<tools::wallet2> wallets(3);
-  make_wallets(wallets, 2);
+  make_wallets(2, 3, false);
+  make_wallets(2, 3, true);
 }
 
 TEST(multisig, make_2_4)
 {
-  std::vector<tools::wallet2> wallets(4);
-  make_wallets(wallets, 2);
+  make_wallets(2, 4, false);
+  make_wallets(2, 4, true);
 }
 
 TEST(multisig, multisig_kex_msg)
@@ -272,9 +314,7 @@ TEST(multisig, multisig_kex_msg)
     signing_skey = rct::rct2sk(rct::skGen());
   }
 
-  crypto::secret_key ancillary_skey = rct::rct2sk(rct::skGen());
-  while (ancillary_skey == crypto::null_skey)
-    ancillary_skey = rct::rct2sk(rct::skGen());
+  const crypto::secret_key ancillary_skey{rct::rct2sk(rct::skGen())};
 
   // misc. edge cases
   EXPECT_NO_THROW((multisig_kex_msg{}));
@@ -312,8 +352,8 @@ TEST(multisig, multisig_kex_msg)
   // test that keys can be recovered if stored in a message and the message's reverse
 
   // round 1
-  multisig_kex_msg msg_rnd1{1, signing_skey, std::vector<crypto::public_key>{pubkey1}, ancillary_skey};
-  multisig_kex_msg msg_rnd1_reverse{msg_rnd1.get_msg()};
+  const multisig_kex_msg msg_rnd1{1, signing_skey, std::vector<crypto::public_key>{pubkey1}, ancillary_skey};
+  const multisig_kex_msg msg_rnd1_reverse{msg_rnd1.get_msg()};
   EXPECT_EQ(msg_rnd1.get_round(), 1);
   EXPECT_EQ(msg_rnd1.get_round(), msg_rnd1_reverse.get_round());
   EXPECT_EQ(msg_rnd1.get_signing_pubkey(), signing_pubkey);
@@ -324,8 +364,8 @@ TEST(multisig, multisig_kex_msg)
   EXPECT_EQ(msg_rnd1.get_msg_privkey(), msg_rnd1_reverse.get_msg_privkey());
 
   // round 2
-  multisig_kex_msg msg_rnd2{2, signing_skey, std::vector<crypto::public_key>{pubkey1, pubkey2}, ancillary_skey};
-  multisig_kex_msg msg_rnd2_reverse{msg_rnd2.get_msg()};
+  const multisig_kex_msg msg_rnd2{2, signing_skey, std::vector<crypto::public_key>{pubkey1, pubkey2}, ancillary_skey};
+  const multisig_kex_msg msg_rnd2_reverse{msg_rnd2.get_msg()};
   EXPECT_EQ(msg_rnd2.get_round(), 2);
   EXPECT_EQ(msg_rnd2.get_round(), msg_rnd2_reverse.get_round());
   EXPECT_EQ(msg_rnd2.get_signing_pubkey(), signing_pubkey);
diff --git a/utils/python-rpc/framework/wallet.py b/utils/python-rpc/framework/wallet.py
index 01e937627..062166e6b 100644
--- a/utils/python-rpc/framework/wallet.py
+++ b/utils/python-rpc/framework/wallet.py
@@ -524,12 +524,13 @@ class Wallet(object):
         }
         return self.rpc.send_json_rpc_request(finalize_multisig)
 
-    def exchange_multisig_keys(self, multisig_info, password = ''):
+    def exchange_multisig_keys(self, multisig_info, password = '', force_update_use_with_caution = False):
         exchange_multisig_keys = {
             'method': 'exchange_multisig_keys',
             'params' : {
                 'multisig_info': multisig_info,
                 'password': password,
+                'force_update_use_with_caution': force_update_use_with_caution,
             },
             'jsonrpc': '2.0', 
             'id': '0'