Merge pull request #7877

e08abaa multisig key exchange update and refactor (koe)

25 files changed, 1945 insertions, 822 deletions

diff --git a/src/crypto/crypto.h b/src/crypto/crypto.h
index 7ddc0150f..599ae4f91 100644
--- a/src/crypto/crypto.h
+++ b/src/crypto/crypto.h
@@ -64,6 +64,11 @@ namespace crypto {
     friend class crypto_ops;
   };
 
+  POD_CLASS public_key_memsafe : epee::mlocked<tools::scrubbed<public_key>> {
+    public_key_memsafe() = default;
+    public_key_memsafe(const public_key &original) { memcpy(this->data, original.data, 32); }
+  };
+
   using secret_key = epee::mlocked<tools::scrubbed<ec_scalar>>;
 
   POD_CLASS public_keyV {
@@ -100,7 +105,7 @@ namespace crypto {
   void random32_unbiased(unsigned char *bytes);
 
   static_assert(sizeof(ec_point) == 32 && sizeof(ec_scalar) == 32 &&
-    sizeof(public_key) == 32 && sizeof(secret_key) == 32 &&
+    sizeof(public_key) == 32 && sizeof(public_key_memsafe) == 32 && sizeof(secret_key) == 32 &&
     sizeof(key_derivation) == 32 && sizeof(key_image) == 32 &&
     sizeof(signature) == 64, "Invalid structure size");
 
@@ -310,9 +315,13 @@ namespace crypto {
 
   const extern crypto::public_key null_pkey;
   const extern crypto::secret_key null_skey;
+
+  inline bool operator<(const public_key &p1, const public_key &p2) { return memcmp(&p1, &p2, sizeof(public_key)) < 0; }
+  inline bool operator>(const public_key &p1, const public_key &p2) { return p2 < p1; }
 }
 
 CRYPTO_MAKE_HASHABLE(public_key)
 CRYPTO_MAKE_HASHABLE_CONSTANT_TIME(secret_key)
+CRYPTO_MAKE_HASHABLE_CONSTANT_TIME(public_key_memsafe)
 CRYPTO_MAKE_HASHABLE(key_image)
 CRYPTO_MAKE_COMPARABLE(signature)
diff --git a/src/cryptonote_basic/account.cpp b/src/cryptonote_basic/account.cpp
index 36ff41684..9927351a9 100644
--- a/src/cryptonote_basic/account.cpp
+++ b/src/cryptonote_basic/account.cpp
@@ -253,11 +253,6 @@ DISABLE_VS_WARNINGS(4244 4345)
     return crypto::secret_key_to_public_key(view_secret_key, m_keys.m_account_address.m_view_public_key);
   }
   //-----------------------------------------------------------------
-  void account_base::finalize_multisig(const crypto::public_key &spend_public_key)
-  {
-    m_keys.m_account_address.m_spend_public_key = spend_public_key;
-  }
-  //-----------------------------------------------------------------
   const account_keys& account_base::get_keys() const
   {
     return m_keys;
diff --git a/src/cryptonote_basic/account.h b/src/cryptonote_basic/account.h
index 5288b9b04..96b024c3c 100644
--- a/src/cryptonote_basic/account.h
+++ b/src/cryptonote_basic/account.h
@@ -82,7 +82,6 @@ namespace cryptonote
     void create_from_keys(const cryptonote::account_public_address& address, const crypto::secret_key& spendkey, const crypto::secret_key& viewkey);
     void create_from_viewkey(const cryptonote::account_public_address& address, const crypto::secret_key& viewkey);
     bool make_multisig(const crypto::secret_key &view_secret_key, const crypto::secret_key &spend_secret_key, const crypto::public_key &spend_public_key, const std::vector<crypto::secret_key> &multisig_keys);
-    void finalize_multisig(const crypto::public_key &spend_public_key);
     const account_keys& get_keys() const;
     std::string get_public_address_str(network_type nettype) const;
     std::string get_public_integrated_address_str(const crypto::hash8 &payment_id, network_type nettype) const;
diff --git a/src/cryptonote_basic/cryptonote_format_utils.cpp b/src/cryptonote_basic/cryptonote_format_utils.cpp
index 17adcdc35..835f59d69 100644
--- a/src/cryptonote_basic/cryptonote_format_utils.cpp
+++ b/src/cryptonote_basic/cryptonote_format_utils.cpp
@@ -306,7 +306,26 @@ namespace cryptonote
     {
       // derive secret key with subaddress - step 1: original CN derivation
       crypto::secret_key scalar_step1;
-      hwdev.derive_secret_key(recv_derivation, real_output_index, ack.m_spend_secret_key, scalar_step1); // computes Hs(a*R || idx) + b
+      crypto::secret_key spend_skey = crypto::null_skey;
+
+      if (ack.m_multisig_keys.empty())
+      {
+        // if not multisig, use normal spend skey
+        spend_skey = ack.m_spend_secret_key;
+      }
+      else
+      {
+        // if multisig, use sum of multisig privkeys (local account's share of aggregate spend key)
+        for (const auto &multisig_key : ack.m_multisig_keys)
+        {
+          sc_add((unsigned char*)spend_skey.data,
+            (const unsigned char*)multisig_key.data,
+            (const unsigned char*)spend_skey.data);
+        }
+      }
+
+      // computes Hs(a*R || idx) + b
+      hwdev.derive_secret_key(recv_derivation, real_output_index, spend_skey, scalar_step1);
 
       // step 2: add Hs(a || index_major || index_minor)
       crypto::secret_key subaddr_sk;
diff --git a/src/cryptonote_config.h b/src/cryptonote_config.h
index 915835d1b..b738960a3 100644
--- a/src/cryptonote_config.h
+++ b/src/cryptonote_config.h
@@ -229,6 +229,7 @@ namespace config
   const unsigned char HASH_KEY_RPC_PAYMENT_NONCE = 0x58;
   const unsigned char HASH_KEY_MEMORY = 'k';
   const unsigned char HASH_KEY_MULTISIG[] = {'M', 'u', 'l', 't' , 'i', 's', 'i', 'g', 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 };
+  const unsigned char HASH_KEY_MULTISIG_KEY_AGGREGATION[] = "Multisig_key_agg";
   const unsigned char HASH_KEY_TXPROOF_V2[] = "TXPROOF_V2";
   const unsigned char HASH_KEY_CLSAG_ROUND[] = "CLSAG_round";
   const unsigned char HASH_KEY_CLSAG_AGG_0[] = "CLSAG_agg_0";
@@ -236,6 +237,9 @@ namespace config
   const char HASH_KEY_MESSAGE_SIGNING[] = "MoneroMessageSignature";
   const unsigned char HASH_KEY_MM_SLOT = 'm';
 
+  // Multisig
+  const uint32_t MULTISIG_MAX_SIGNERS{16};
+
   namespace testnet
   {
     uint64_t const CRYPTONOTE_PUBLIC_ADDRESS_BASE58_PREFIX = 53;
diff --git a/src/cryptonote_core/cryptonote_tx_utils.cpp b/src/cryptonote_core/cryptonote_tx_utils.cpp
index f6061b803..a50ebb550 100644
--- a/src/cryptonote_core/cryptonote_tx_utils.cpp
+++ b/src/cryptonote_core/cryptonote_tx_utils.cpp
@@ -43,7 +43,6 @@ using namespace epee;
 #include "crypto/crypto.h"
 #include "crypto/hash.h"
 #include "ringct/rctSigs.h"
-#include "multisig/multisig.h"
 
 using namespace crypto;
 
diff --git a/src/gen_multisig/gen_multisig.cpp b/src/gen_multisig/gen_multisig.cpp
index 4aa21b149..87de3c351 100644
--- a/src/gen_multisig/gen_multisig.cpp
+++ b/src/gen_multisig/gen_multisig.cpp
@@ -95,55 +95,35 @@ static bool generate_multisig(uint32_t threshold, uint32_t total, const std::str
     }
 
     // gather the keys
-    std::vector<crypto::secret_key> sk(total);
-    std::vector<crypto::public_key> pk(total);
+    std::vector<std::string> first_round_msgs;
+    first_round_msgs.reserve(total);
     for (size_t n = 0; n < total; ++n)
     {
       wallets[n]->decrypt_keys(pwd_container->password());
-      if (!tools::wallet2::verify_multisig_info(wallets[n]->get_multisig_info(), sk[n], pk[n]))
-      {
-        tools::fail_msg_writer() << genms::tr("Failed to verify multisig info");
-        return false;
-      }
+
+      first_round_msgs.emplace_back(wallets[n]->get_multisig_first_kex_msg());
+
       wallets[n]->encrypt_keys(pwd_container->password());
     }
 
     // make the wallets multisig
-    std::vector<std::string> extra_info(total);
+    std::vector<std::string> kex_msgs_intermediate(total);
     std::stringstream ss;
     for (size_t n = 0; n < total; ++n)
     {
       std::string name = basename + "-" + std::to_string(n + 1);
-      std::vector<crypto::secret_key> skn;
-      std::vector<crypto::public_key> pkn;
-      for (size_t k = 0; k < total; ++k)
-      {
-        if (k != n)
-        {
-          skn.push_back(sk[k]);
-          pkn.push_back(pk[k]);
-        }
-      }
-      extra_info[n] = wallets[n]->make_multisig(pwd_container->password(), skn, pkn, threshold);
+
+      kex_msgs_intermediate[n] = wallets[n]->make_multisig(pwd_container->password(), first_round_msgs, threshold);
+
       ss << "  " << name << std::endl;
     }
 
     //exchange keys unless exchange_multisig_keys returns no extra info
-    while (!extra_info[0].empty())
+    while (!kex_msgs_intermediate[0].empty())
     {
-      std::unordered_set<crypto::public_key> pkeys;
-      std::vector<crypto::public_key> signers(total);
-      for (size_t n = 0; n < total; ++n)
-      {
-        if (!tools::wallet2::verify_extra_multisig_info(extra_info[n], pkeys, signers[n]))
-        {
-          tools::fail_msg_writer() << genms::tr("Error verifying multisig extra info");
-          return false;
-        }
-      }
       for (size_t n = 0; n < total; ++n)
       {
-          extra_info[n] = wallets[n]->exchange_multisig_keys(pwd_container->password(), pkeys, signers);
+          kex_msgs_intermediate[n] = wallets[n]->exchange_multisig_keys(pwd_container->password(), kex_msgs_intermediate);
       }
     }
 
diff --git a/src/multisig/CMakeLists.txt b/src/multisig/CMakeLists.txt
index eaa2c6f71..14099e64a 100644
--- a/src/multisig/CMakeLists.txt
+++ b/src/multisig/CMakeLists.txt
@@ -27,12 +27,17 @@
 # THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 set(multisig_sources
-  multisig.cpp)
+  multisig.cpp
+  multisig_account.cpp
+  multisig_account_kex_impl.cpp
+  multisig_kex_msg.cpp)
 
 set(multisig_headers)
 
 set(multisig_private_headers
-  multisig.h)
+  multisig.h
+  multisig_account.h
+  multisig_kex_msg.h)
 
 monero_private_headers(multisig
   ${multisig_private_headers})
diff --git a/src/multisig/multisig.cpp b/src/multisig/multisig.cpp
index 272de73b2..85c45bc31 100644
--- a/src/multisig/multisig.cpp
+++ b/src/multisig/multisig.cpp
@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2020, The Monero Project
+// Copyright (c) 2017-2021, The Monero Project
 // 
 // All rights reserved.
 // 
@@ -26,29 +26,34 @@
 // STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
 // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#include <unordered_set>
-#include "include_base_utils.h"
 #include "crypto/crypto.h"
-#include "ringct/rctOps.h"
 #include "cryptonote_basic/account.h"
 #include "cryptonote_basic/cryptonote_format_utils.h"
-#include "multisig.h"
 #include "cryptonote_config.h"
+#include "include_base_utils.h"
+#include "multisig.h"
+#include "ringct/rctOps.h"
+
+#include <algorithm>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
 
 #undef MONERO_DEFAULT_LOG_CATEGORY
 #define MONERO_DEFAULT_LOG_CATEGORY "multisig"
 
-using namespace std;
-
-namespace cryptonote
+namespace multisig
 {
-  //-----------------------------------------------------------------
+  //----------------------------------------------------------------------------------------------------------------------
   crypto::secret_key get_multisig_blinded_secret_key(const crypto::secret_key &key)
   {
+    CHECK_AND_ASSERT_THROW_MES(key != crypto::null_skey, "Unexpected null secret key (danger!).");
+
     rct::key multisig_salt;
     static_assert(sizeof(rct::key) == sizeof(config::HASH_KEY_MULTISIG), "Hash domain separator is an unexpected size");
     memcpy(multisig_salt.bytes, config::HASH_KEY_MULTISIG, sizeof(rct::key));
 
+    // private key = H(key, domain-sep)
     rct::keyV data;
     data.reserve(2);
     data.push_back(rct::sk2rct(key));
@@ -57,134 +62,79 @@ namespace cryptonote
     memwipe(&data[0], sizeof(rct::key));
     return result;
   }
-  //-----------------------------------------------------------------
-  void generate_multisig_N_N(const account_keys &keys, const std::vector<crypto::public_key> &spend_keys, std::vector<crypto::secret_key> &multisig_keys, rct::key &spend_skey, rct::key &spend_pkey)
-  {
-    // the multisig spend public key is the sum of all spend public keys
-    multisig_keys.clear();
-    const crypto::secret_key spend_secret_key = get_multisig_blinded_secret_key(keys.m_spend_secret_key);
-    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(spend_secret_key, (crypto::public_key&)spend_pkey), "Failed to derive public key");
-    for (const auto &k: spend_keys)
-      rct::addKeys(spend_pkey, spend_pkey, rct::pk2rct(k));
-    multisig_keys.push_back(spend_secret_key);
-    spend_skey = rct::sk2rct(spend_secret_key);
-  }
-  //-----------------------------------------------------------------
-  void generate_multisig_N1_N(const account_keys &keys, const std::vector<crypto::public_key> &spend_keys, std::vector<crypto::secret_key> &multisig_keys, rct::key &spend_skey, rct::key &spend_pkey)
-  {
-    multisig_keys.clear();
-    spend_pkey = rct::identity();
-    spend_skey = rct::zero();
-
-    // create all our composite private keys
-    crypto::secret_key blinded_skey = get_multisig_blinded_secret_key(keys.m_spend_secret_key);
-    for (const auto &k: spend_keys)
-    {
-      rct::key sk = rct::scalarmultKey(rct::pk2rct(k), rct::sk2rct(blinded_skey));
-      crypto::secret_key msk = get_multisig_blinded_secret_key(rct::rct2sk(sk));
-      memwipe(&sk, sizeof(sk));
-      multisig_keys.push_back(msk);
-      sc_add(spend_skey.bytes, spend_skey.bytes, (const unsigned char*)msk.data);
-    }
-  }
-  //-----------------------------------------------------------------
-  std::vector<crypto::public_key> generate_multisig_derivations(const account_keys &keys, const std::vector<crypto::public_key> &derivations)
-  {
-    std::vector<crypto::public_key> multisig_keys;
-    crypto::secret_key blinded_skey = get_multisig_blinded_secret_key(keys.m_spend_secret_key);
-    for (const auto &k: derivations)
-    {
-      rct::key d = rct::scalarmultKey(rct::pk2rct(k), rct::sk2rct(blinded_skey));
-      multisig_keys.push_back(rct::rct2pk(d));
-    }
-
-    return multisig_keys;
-  }
-  //-----------------------------------------------------------------
-  crypto::secret_key calculate_multisig_signer_key(const std::vector<crypto::secret_key>& multisig_keys)
-  {
-    rct::key secret_key = rct::zero();
-    for (const auto &k: multisig_keys)
-    {
-      sc_add(secret_key.bytes, secret_key.bytes, (const unsigned char*)k.data);
-    }
-
-    return rct::rct2sk(secret_key);
-  }
-  //-----------------------------------------------------------------
-  std::vector<crypto::secret_key> calculate_multisig_keys(const std::vector<crypto::public_key>& derivations)
-  {
-    std::vector<crypto::secret_key> multisig_keys;
-    multisig_keys.reserve(derivations.size());
-
-    for (const auto &k: derivations)
-    {
-      multisig_keys.emplace_back(get_multisig_blinded_secret_key(rct::rct2sk(rct::pk2rct(k))));
-    }
-
-    return multisig_keys;
-  }
-  //-----------------------------------------------------------------
-  crypto::secret_key generate_multisig_view_secret_key(const crypto::secret_key &skey, const std::vector<crypto::secret_key> &skeys)
-  {
-    crypto::secret_key view_skey = get_multisig_blinded_secret_key(skey);
-    for (const auto &k: skeys)
-      sc_add((unsigned char*)&view_skey, rct::sk2rct(view_skey).bytes, rct::sk2rct(k).bytes);
-    return view_skey;
-  }
-  //-----------------------------------------------------------------
-  crypto::public_key generate_multisig_M_N_spend_public_key(const std::vector<crypto::public_key> &pkeys)
-  {
-    rct::key spend_public_key = rct::identity();
-    for (const auto &pk: pkeys)
-    {
-      rct::addKeys(spend_public_key, spend_public_key, rct::pk2rct(pk));
-    }
-    return rct::rct2pk(spend_public_key);
-  }
-  //-----------------------------------------------------------------
-  bool generate_multisig_key_image(const account_keys &keys, size_t multisig_key_index, const crypto::public_key& out_key, crypto::key_image& ki)
+  //----------------------------------------------------------------------------------------------------------------------
+  bool generate_multisig_key_image(const cryptonote::account_keys &keys,
+    std::size_t multisig_key_index,
+    const crypto::public_key& out_key,
+    crypto::key_image& ki)
   {
     if (multisig_key_index >= keys.m_multisig_keys.size())
       return false;
     crypto::generate_key_image(out_key, keys.m_multisig_keys[multisig_key_index], ki);
     return true;
   }
-  //-----------------------------------------------------------------
-  void generate_multisig_LR(const crypto::public_key pkey, const crypto::secret_key &k, crypto::public_key &L, crypto::public_key &R)
+  //----------------------------------------------------------------------------------------------------------------------
+  void generate_multisig_LR(const crypto::public_key pkey,
+    const crypto::secret_key &k,
+    crypto::public_key &L,
+    crypto::public_key &R)
   {
     rct::scalarmultBase((rct::key&)L, rct::sk2rct(k));
     crypto::generate_key_image(pkey, k, (crypto::key_image&)R);
   }
-  //-----------------------------------------------------------------
-  bool generate_multisig_composite_key_image(const account_keys &keys, const std::unordered_map<crypto::public_key, subaddress_index>& subaddresses, const crypto::public_key& out_key, const crypto::public_key &tx_public_key, const std::vector<crypto::public_key>& additional_tx_public_keys, size_t real_output_index, const std::vector<crypto::key_image> &pkis, crypto::key_image &ki)
+  //----------------------------------------------------------------------------------------------------------------------
+  bool generate_multisig_composite_key_image(const cryptonote::account_keys &keys,
+    const std::unordered_map<crypto::public_key, cryptonote::subaddress_index> &subaddresses,
+    const crypto::public_key &out_key,
+    const crypto::public_key &tx_public_key,
+    const std::vector<crypto::public_key> &additional_tx_public_keys,
+    std::size_t real_output_index,
+    const std::vector<crypto::key_image> &pkis,
+    crypto::key_image &ki)
   {
+    // create a multisig partial key image
+    // KI_partial = ([view key component] + [subaddress component] + [multisig privkeys]) * Hp(output one-time address)
+    // - the 'multisig priv keys' here are those held by the local account
+    // - later, we add in the components held by other participants
     cryptonote::keypair in_ephemeral;
     if (!cryptonote::generate_key_image_helper(keys, subaddresses, out_key, tx_public_key, additional_tx_public_keys, real_output_index, in_ephemeral, ki, keys.get_device()))
       return false;
     std::unordered_set<crypto::key_image> used;
-    for (size_t m = 0; m < keys.m_multisig_keys.size(); ++m)
+
+    // create a key image component for each of the local account's multisig private keys
+    for (std::size_t m = 0; m < keys.m_multisig_keys.size(); ++m)
     {
       crypto::key_image pki;
-      bool r = cryptonote::generate_multisig_key_image(keys, m, out_key, pki);
+      // pki = keys.m_multisig_keys[m] * Hp(out_key)
+      // pki = key image component
+      // out_key = one-time address of an output owned by the multisig group
+      bool r = generate_multisig_key_image(keys, m, out_key, pki);
       if (!r)
         return false;
+
+      // this KI component is 'used' because it was included in the partial key image 'ki' above
       used.insert(pki);
     }
+
+    // add the KI components from other participants to the partial KI
+    // if they not included yet
     for (const auto &pki: pkis)
     {
       if (used.find(pki) == used.end())
       {
+        // ignore components that have already been 'used'
         used.insert(pki);
+
+        // KI_partial = KI_partial + KI_component[...]
         rct::addKeys((rct::key&)ki, rct::ki2rct(ki), rct::ki2rct(pki));
       }
     }
+
+    // at the end, 'ki' will hold the true key image for our output if inputs were sufficient
+    // - if 'pkis' (the other participants' KI components) is missing some components
+    //   then 'ki' will not be complete
+
     return true;
   }
-  //-----------------------------------------------------------------
-  uint32_t multisig_rounds_required(uint32_t participants, uint32_t threshold)
-  {
-    CHECK_AND_ASSERT_THROW_MES(participants >= threshold, "participants must be greater or equal than threshold");
-    return participants - threshold + 1;
-  }
-}
+  //----------------------------------------------------------------------------------------------------------------------
+} //namespace multisig
diff --git a/src/multisig/multisig.h b/src/multisig/multisig.h
index eab32187c..e041ea670 100644
--- a/src/multisig/multisig.h
+++ b/src/multisig/multisig.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2020, The Monero Project
+// Copyright (c) 2017-2021, The Monero Project
 // 
 // All rights reserved.
 // 
@@ -28,44 +28,42 @@
 
 #pragma once
 
-#include <vector>
-#include <unordered_map>
 #include "crypto/crypto.h"
 #include "cryptonote_basic/cryptonote_format_utils.h"
 #include "ringct/rctTypes.h"
 
-namespace cryptonote
-{
-  struct account_keys;
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
 
-  crypto::secret_key get_multisig_blinded_secret_key(const crypto::secret_key &key);
-  void generate_multisig_N_N(const account_keys &keys, const std::vector<crypto::public_key> &spend_keys, std::vector<crypto::secret_key> &multisig_keys, rct::key &spend_skey, rct::key &spend_pkey);
-  void generate_multisig_N1_N(const account_keys &keys, const std::vector<crypto::public_key> &spend_keys, std::vector<crypto::secret_key> &multisig_keys, rct::key &spend_skey, rct::key &spend_pkey);
-  /**
-   * @brief generate_multisig_derivations performs common DH key derivation.
-   *    Each middle round in M/N scheme is DH exchange of public multisig keys of other participants multiplied by secret spend key of current participant.
-   *    this functions does the following: new multisig key = secret spend * public multisig key
-   * @param keys - current wallet's keys
-   * @param derivations - public multisig keys of other participants
-   * @return new public multisig keys derived from previous round. This data needs to be exchange with other participants
-   */
-  std::vector<crypto::public_key> generate_multisig_derivations(const account_keys &keys, const std::vector<crypto::public_key> &derivations);
-  crypto::secret_key calculate_multisig_signer_key(const std::vector<crypto::secret_key>& derivations);
-  /**
-   * @brief calculate_multisig_keys. Calculates secret multisig keys from others' participants ones as follows: mi = H(Mi)
-   * @param derivations - others' participants public multisig keys.
-   * @return vector of current wallet's multisig secret keys
-   */
-  std::vector<crypto::secret_key> calculate_multisig_keys(const std::vector<crypto::public_key>& derivations);
-  crypto::secret_key generate_multisig_view_secret_key(const crypto::secret_key &skey, const std::vector<crypto::secret_key> &skeys);
+namespace cryptonote { struct account_keys; }
+
+namespace multisig
+{
   /**
-   * @brief generate_multisig_M_N_spend_public_key calculates multisig wallet's spend public key by summing all of public multisig keys
-   * @param pkeys unique public multisig keys
-   * @return multisig wallet's spend public key
-   */
-  crypto::public_key generate_multisig_M_N_spend_public_key(const std::vector<crypto::public_key> &pkeys);
-  bool generate_multisig_key_image(const account_keys &keys, size_t multisig_key_index, const crypto::public_key& out_key, crypto::key_image& ki);
-  void generate_multisig_LR(const crypto::public_key pkey, const crypto::secret_key &k, crypto::public_key &L, crypto::public_key &R);
-  bool generate_multisig_composite_key_image(const account_keys &keys, const std::unordered_map<crypto::public_key, cryptonote::subaddress_index>& subaddresses, const crypto::public_key& out_key, const crypto::public_key &tx_public_key, const std::vector<crypto::public_key>& additional_tx_public_keys, size_t real_output_index, const std::vector<crypto::key_image> &pkis, crypto::key_image &ki);
-  uint32_t multisig_rounds_required(uint32_t participants, uint32_t threshold);
-}
+  * @brief get_multisig_blinded_secret_key - converts an input private key into a blinded multisig private key
+  *    Use 1a: converts account private spend key into multisig private key, which is used for key exchange and message signing
+  *    Use 1b: converts account private view key into ancillary private key share, for the composite multisig private view key
+  *    Use 2: converts DH shared secrets (curve points) into private keys, which are intermediate private keys in multisig key exchange
+  * @param key - private key to transform
+  * @return transformed private key
+  */
+  crypto::secret_key get_multisig_blinded_secret_key(const crypto::secret_key &key);
+
+  bool generate_multisig_key_image(const cryptonote::account_keys &keys,
+    std::size_t multisig_key_index,
+    const crypto::public_key& out_key,
+    crypto::key_image& ki);
+  void generate_multisig_LR(const crypto::public_key pkey,
+    const crypto::secret_key &k,
+    crypto::public_key &L,
+    crypto::public_key &R);
+  bool generate_multisig_composite_key_image(const cryptonote::account_keys &keys,
+    const std::unordered_map<crypto::public_key, cryptonote::subaddress_index> &subaddresses,
+    const crypto::public_key &out_key,
+    const crypto::public_key &tx_public_key,
+    const std::vector<crypto::public_key> &additional_tx_public_keys,
+    std::size_t real_output_index,
+    const std::vector<crypto::key_image> &pkis,
+    crypto::key_image &ki);
+} //namespace multisig
diff --git a/src/multisig/multisig_account.cpp b/src/multisig/multisig_account.cpp
new file mode 100644
index 000000000..b7298c4b6
--- /dev/null
+++ b/src/multisig/multisig_account.cpp
@@ -0,0 +1,184 @@
+// Copyright (c) 2021, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "multisig_account.h"
+
+#include "crypto/crypto.h"
+#include "cryptonote_config.h"
+#include "include_base_utils.h"
+#include "multisig.h"
+#include "multisig_kex_msg.h"
+#include "ringct/rctOps.h"
+#include "ringct/rctTypes.h"
+
+#include <cstdint>
+#include <utility>
+#include <vector>
+
+
+#undef MONERO_DEFAULT_LOG_CATEGORY
+#define MONERO_DEFAULT_LOG_CATEGORY "multisig"
+
+namespace multisig
+{
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  multisig_account::multisig_account(const crypto::secret_key &base_privkey,
+    const crypto::secret_key &base_common_privkey) :
+      m_base_privkey{base_privkey},
+      m_base_common_privkey{base_common_privkey},
+      m_multisig_pubkey{rct::rct2pk(rct::identity())},
+      m_common_pubkey{rct::rct2pk(rct::identity())},
+      m_kex_rounds_complete{0},
+      m_next_round_kex_message{multisig_kex_msg{1, base_privkey, std::vector<crypto::public_key>{}, base_common_privkey}.get_msg()}
+  {
+    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(m_base_privkey, m_base_pubkey),
+      "Failed to derive public key");
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  multisig_account::multisig_account(const std::uint32_t threshold,
+    std::vector<crypto::public_key> signers,
+    const crypto::secret_key &base_privkey,
+    const crypto::secret_key &base_common_privkey,
+    std::vector<crypto::secret_key> multisig_privkeys,
+    const crypto::secret_key &common_privkey,
+    const crypto::public_key &multisig_pubkey,
+    const crypto::public_key &common_pubkey,
+    const std::uint32_t kex_rounds_complete,
+    kex_origins_map_t kex_origins_map,
+    std::string next_round_kex_message) :
+      m_base_privkey{base_privkey},
+      m_base_common_privkey{base_common_privkey},
+      m_multisig_privkeys{std::move(multisig_privkeys)},
+      m_common_privkey{common_privkey},
+      m_multisig_pubkey{multisig_pubkey},
+      m_common_pubkey{common_pubkey},
+      m_kex_rounds_complete{kex_rounds_complete},
+      m_kex_keys_to_origins_map{std::move(kex_origins_map)},
+      m_next_round_kex_message{std::move(next_round_kex_message)}
+  {
+    CHECK_AND_ASSERT_THROW_MES(kex_rounds_complete > 0, "multisig account: can't reconstruct account if its kex wasn't initialized");
+    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(m_base_privkey, m_base_pubkey),
+      "Failed to derive public key");
+    set_multisig_config(threshold, std::move(signers));
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  bool multisig_account::account_is_active() const
+  {
+    return m_kex_rounds_complete > 0;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  bool multisig_account::multisig_is_ready() const
+  {
+    if (account_is_active())
+      return multisig_kex_rounds_required(m_signers.size(), m_threshold) == m_kex_rounds_complete;
+    else
+      return false;
+  }
+    //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::set_multisig_config(const std::size_t threshold, std::vector<crypto::public_key> signers)
+  {
+    // validate
+    CHECK_AND_ASSERT_THROW_MES(threshold > 0 && threshold <= signers.size(), "multisig account: tried to set invalid threshold.");
+    CHECK_AND_ASSERT_THROW_MES(signers.size() >= 2 && signers.size() <= config::MULTISIG_MAX_SIGNERS,
+      "multisig account: tried to set invalid number of signers.");
+
+    for (auto signer_it = signers.begin(); signer_it != signers.end(); ++signer_it)
+    {
+      // signers should all be unique
+      CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signer_it, *signer_it) == signer_it,
+        "multisig account: tried to set signers, but found a duplicate signer unexpectedly.");
+
+      // signer pubkeys must be in main subgroup, and not identity
+      CHECK_AND_ASSERT_THROW_MES(rct::isInMainSubgroup(rct::pk2rct(*signer_it)) && !(*signer_it == rct::rct2pk(rct::identity())),
+        "multisig account: tried to set signers, but a signer pubkey is invalid.");
+    }
+
+    // own pubkey should be in signers list
+    CHECK_AND_ASSERT_THROW_MES(std::find(signers.begin(), signers.end(), m_base_pubkey) != signers.end(),
+      "multisig account: tried to set signers, but did not find the account's base pubkey in signer list.");
+
+    // sort signers
+    std::sort(signers.begin(), signers.end(),
+      [](const crypto::public_key &key1, const crypto::public_key &key2) -> bool
+        {
+          return memcmp(&key1, &key2, sizeof(crypto::public_key)) < 0;
+        }
+      );
+
+    // set
+    m_threshold = threshold;
+    m_signers = std::move(signers);
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::initialize_kex(const std::uint32_t threshold,
+    std::vector<crypto::public_key> signers,
+    const std::vector<multisig_kex_msg> &expanded_msgs_rnd1)
+  {
+    CHECK_AND_ASSERT_THROW_MES(!account_is_active(), "multisig account: tried to initialize kex, but already initialized");
+
+    // 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);
+    *this = std::move(temp_account);
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::kex_update(const std::vector<multisig_kex_msg> &expanded_msgs)
+  {
+    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);
+    *this = std::move(temp_account);
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  std::uint32_t multisig_kex_rounds_required(const std::uint32_t num_signers, const std::uint32_t threshold)
+  {
+    CHECK_AND_ASSERT_THROW_MES(num_signers >= threshold, "num_signers must be >= threshold");
+    CHECK_AND_ASSERT_THROW_MES(threshold >= 1, "threshold must be >= 1");
+    return num_signers - threshold + 1;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+} //namespace multisig
diff --git a/src/multisig/multisig_account.h b/src/multisig/multisig_account.h
new file mode 100644
index 000000000..b01ae6c88
--- /dev/null
+++ b/src/multisig/multisig_account.h
@@ -0,0 +1,246 @@
+// Copyright (c) 2021, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include "crypto/crypto.h"
+#include "multisig_kex_msg.h"
+
+#include <cstdint>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+
+namespace multisig
+{
+  /**
+  * multisig account:
+  * 
+  * - handles account keys for an M-of-N multisig participant (M <= N; M >= 1; N >= 2)
+  * - encapsulates multisig account construction process (via key exchange [kex])
+  * - TODO: encapsulates key preparation for aggregation-style signing
+  *
+  * :: multisig pubkey: the private key is split, M group participants are required to reassemble (e.g. to sign something)
+  *    - in cryptonote, this is the multisig spend key
+  * :: multisig common pubkey: the private key is known to all participants (e.g. for authenticating as a group member)
+  *    - in cryptonote, this is the multisig view key
+  * 
+  * 
+  * multisig key exchange:
+  * 
+  * An 'M-of-N' (M <= N; M >= 1; N >= 2) multisignature key is a public key where at least 'M' out of 'N'
+  * possible co-signers must collaborate in order to create a signature.
+  * 
+  * Constructing a multisig key involves a series of Diffie-Hellman exchanges between participants.
+  * At the end of key exchange (kex), each participant will hold a number of private keys. Each private
+  * key is shared by a group of (N - M + 1) participants. This way if (N - M) co-signers are missing, every
+  * private key will be held by at least one of the remaining M people.
+  * 
+  * Note on MULTISIG_MAX_SIGNERS: During key exchange, participants will have up to '(N - 1) choose (N - M)'
+  *   key shares. If N is large, then the max number of key shares (when M = (N-1)/2) can be huge. A limit of N <= 16 was
+  *   arbitrarily chosen as a power of 2 that can accomodate the vast majority of practical use-cases. To increase the
+  *   limit, FROST-style key aggregation should be used instead (it is more efficient than DH-based key generation
+  *   when N - M > 1).
+  * 
+  * - Further reading
+  *   - MRL-0009: https://www.getmonero.org/resources/research-lab/pubs/MRL-0009.pdf
+  *   - MuSig2: https://eprint.iacr.org/2020/1261
+  *   - ZtM2: https://web.getmonero.org/library/Zero-to-Monero-2-0-0.pdf Ch. 9, especially Section 9.6.3
+  *   - FROST: https://eprint.iacr.org/2018/417
+  */
+  class multisig_account final
+  {
+  public:
+  //member types
+    using kex_origins_map_t = std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>>;
+
+  //constructors
+    // default constructor
+    multisig_account() = default;
+
+    /**
+    * construct from base privkeys
+    * 
+    * - 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
+    */
+    multisig_account(const crypto::secret_key &base_privkey,
+      const crypto::secret_key &base_common_privkey);
+
+    // reconstruct from full account details (not recommended)
+    multisig_account(const std::uint32_t threshold,
+      std::vector<crypto::public_key> signers,
+      const crypto::secret_key &base_privkey,
+      const crypto::secret_key &base_common_privkey,
+      std::vector<crypto::secret_key> multisig_privkeys,
+      const crypto::secret_key &common_privkey,
+      const crypto::public_key &multisig_pubkey,
+      const crypto::public_key &common_pubkey,
+      const std::uint32_t kex_rounds_complete,
+      kex_origins_map_t kex_origins_map,
+      std::string next_round_kex_message);
+
+    // copy constructor: default
+
+  //destructor: default
+    ~multisig_account() = default;
+
+  //overloaded operators: none
+
+  //getters
+    // get threshold
+    std::uint32_t get_threshold() const { return m_threshold; }
+    // get signers
+    const std::vector<crypto::public_key>& get_signers() const { return m_signers; }
+    // get base privkey
+    const crypto::secret_key& get_base_privkey() const { return m_base_privkey; }
+    // get base pubkey
+    const crypto::public_key& get_base_pubkey() const { return m_base_pubkey; }
+    // get base common privkey
+    const crypto::secret_key& get_base_common_privkey() const { return m_base_common_privkey; }
+    // get multisig privkeys
+    const std::vector<crypto::secret_key>& get_multisig_privkeys() const { return m_multisig_privkeys; }
+    // get common privkey
+    const crypto::secret_key& get_common_privkey() const { return m_common_privkey; }
+    // get multisig pubkey
+    const crypto::public_key& get_multisig_pubkey() const { return m_multisig_pubkey; }
+    // get common pubkey
+    const crypto::public_key& get_common_pubkey() const { return m_common_pubkey; }
+    // get kex rounds complete
+    std::uint32_t get_kex_rounds_complete() const { return m_kex_rounds_complete; }
+    // get kex keys to origins map
+    const kex_origins_map_t& get_kex_keys_to_origins_map() const { return m_kex_keys_to_origins_map; }
+    // get the kex msg for the next round
+    const std::string& get_next_kex_round_msg() const { return m_next_round_kex_message; }
+
+  //account status functions
+    // account has been intialized, and the account holder can use the 'common' key
+    bool account_is_active() const;
+    // account is ready to make multisig signatures
+    bool multisig_is_ready() const;
+
+  //account helpers
+  private:
+    // set the threshold (M) and signers (N)
+    void set_multisig_config(const std::size_t threshold, std::vector<crypto::public_key> signers);
+
+  //account mutators: key exchange to set up account
+  public:
+    /**
+    * brief: initialize_kex - initialize key exchange
+    *    - Updates the account with a 'transactional' model. This account will only be mutated if the update succeeds.
+    */
+    void initialize_kex(const std::uint32_t threshold,
+      std::vector<crypto::public_key> signers,
+      const std::vector<multisig_kex_msg> &expanded_msgs_rnd1);
+    /**
+    * brief: kex_update - Complete the 'in progress' kex round and set the kex message for the next round.
+    *    - Updates the account with a 'transactional' model. This account will only be mutated if the update succeeds.
+    *    - 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
+    */
+    void kex_update(const std::vector<multisig_kex_msg> &expanded_msgs);
+
+  private:
+    // implementation of kex_update() (non-transactional)
+    void kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs);
+    /**
+    * 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.
+    * param: expanded_msgs - set of multisig kex messages to process
+    * param: rounds_required - number of rounds required for kex
+    * 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 rounds_required,
+      std::vector<crypto::public_key> &exclude_pubkeys_out);
+    /**
+    * brief: finalize_kex_update - Helper for kex_update_impl()
+    * param: rounds_required - number of rounds required for kex
+    * param: result_keys_to_origins_map - map between keys for the next round and the other participants they correspond to
+    * inoutparam: temp_account_inout - account to perform last update steps on
+    */
+    void finalize_kex_update(const std::uint32_t rounds_required,
+      kex_origins_map_t result_keys_to_origins_map);
+
+  //member variables
+  private:
+    /// misc. account details
+    // [M] minimum number of co-signers to sign a message with the aggregate pubkey
+    std::uint32_t m_threshold{0};
+    // [N] base keys of all participants in the multisig (used to initiate key exchange, and as participant ids for msg signing)
+    std::vector<crypto::public_key> m_signers;
+
+    /// local participant's personal keys
+    // base keypair of the participant
+    // - used for signing messages, as the initial base key for key exchange, and to make DH derivations for key exchange
+    crypto::secret_key m_base_privkey;
+    crypto::public_key m_base_pubkey;
+    // common base privkey, used to produce the aggregate common privkey
+    crypto::secret_key m_base_common_privkey;
+
+    /// core multisig account keys
+    // the account's private key shares of the multisig address
+    // TODO: also record which other signers have these privkeys, to enable aggregation signing (instead of round-robin)
+    std::vector<crypto::secret_key> m_multisig_privkeys;
+    // a privkey owned by all multisig participants (e.g. a cryptonote view key)
+    crypto::secret_key m_common_privkey;
+    // the multisig public key (e.g. a cryptonote spend key)
+    crypto::public_key m_multisig_pubkey;
+    // the common public key (e.g. a view spend key)
+    crypto::public_key m_common_pubkey;
+
+    /// kex variables
+    // number of key exchange rounds that have been completed (all messages for the round collected and processed)
+    std::uint32_t m_kex_rounds_complete{0};
+    // this account's pubkeys for the in-progress key exchange round
+    // - either DH derivations (intermediate rounds), H(derivation)*G (final round), empty (when kex is done)
+    kex_origins_map_t m_kex_keys_to_origins_map;
+    // the account's message for the in-progress key exchange round
+    std::string m_next_round_kex_message;
+  };
+
+  /**
+  * brief: multisig_kex_rounds_required - The number of key exchange rounds required to produce an M-of-N shared key.
+  *    - Key exchange (kex) is a synchronous series of 'rounds'. In an 'active round', participants send messages
+  *      to each other.
+  *    - A participant considers a round 'complete' when they have collected sufficient messages
+  *      from other participants, processed those messages, and updated their multisig account state.
+  *    - Typically (as implemented in this module), completing a round coincides with making a message for the next round.
+  * param: num_signers - number of participants in multisig (N)
+  * param: threshold - threshold of multisig (M)
+  * return: number of kex rounds required
+  */
+  std::uint32_t multisig_kex_rounds_required(const std::uint32_t num_signers, const std::uint32_t threshold);
+} //namespace multisig
diff --git a/src/multisig/multisig_account_kex_impl.cpp b/src/multisig/multisig_account_kex_impl.cpp
new file mode 100644
index 000000000..0a0ca7bdc
--- /dev/null
+++ b/src/multisig/multisig_account_kex_impl.cpp
@@ -0,0 +1,726 @@
+// Copyright (c) 2021, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "multisig_account.h"
+
+#include "crypto/crypto.h"
+#include "cryptonote_config.h"
+#include "include_base_utils.h"
+#include "multisig.h"
+#include "multisig_kex_msg.h"
+#include "ringct/rctOps.h"
+
+#include <boost/math/special_functions/binomial.hpp>
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+#include <limits>
+#include <memory>
+#include <unordered_map>
+#include <unordered_set>
+#include <utility>
+#include <vector>
+
+
+#undef MONERO_DEFAULT_LOG_CATEGORY
+#define MONERO_DEFAULT_LOG_CATEGORY "multisig"
+
+namespace multisig
+{
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  * 
+  * brief: calculate_multisig_keypair_from_derivation - wrapper on calculate_multisig_keypair() for an input public key
+  *    Converts an input public key into a crypto private key (type cast, does not change serialization),
+  *    then passes it to get_multisig_blinded_secret_key().
+  * 
+  *    Result:
+  *      - privkey = H(derivation)
+  *      - pubkey = privkey * G
+  * param: derivation - a curve point
+  * outparam: derived_pubkey_out - public key of the resulting privkey
+  * return: multisig private key
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static crypto::secret_key calculate_multisig_keypair_from_derivation(const crypto::public_key_memsafe &derivation,
+    crypto::public_key &derived_pubkey_out)
+  {
+    crypto::secret_key blinded_skey = get_multisig_blinded_secret_key(rct::rct2sk(rct::pk2rct(derivation)));
+    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(blinded_skey, derived_pubkey_out), "Failed to derive public key");
+
+    return blinded_skey;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  *
+  * brief: make_multisig_common_privkey - Create the 'common' multisig privkey, owned by all multisig participants.
+  *    - common privkey = H(sorted base common privkeys)
+  * param: participant_base_common_privkeys - Base common privkeys contributed by multisig participants.
+  * outparam: common_privkey_out - result
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static void make_multisig_common_privkey(std::vector<crypto::secret_key> participant_base_common_privkeys,
+    crypto::secret_key &common_privkey_out)
+  {
+    // sort the privkeys for consistency
+    //TODO: need a constant-time operator< for sorting secret keys
+    std::sort(participant_base_common_privkeys.begin(), participant_base_common_privkeys.end(),
+        [](const crypto::secret_key &key1, const crypto::secret_key &key2) -> bool
+        {
+          return memcmp(&key1, &key2, sizeof(crypto::secret_key)) < 0;
+        }
+      );
+
+    // privkey = H(sorted ancillary base privkeys)
+    crypto::hash_to_scalar(participant_base_common_privkeys.data(),
+      participant_base_common_privkeys.size()*sizeof(crypto::secret_key),
+      common_privkey_out);
+
+    CHECK_AND_ASSERT_THROW_MES(common_privkey_out != crypto::null_skey, "Unexpected null secret key (danger!).");
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  * 
+  * brief: compute_multisig_aggregation_coefficient - creates aggregation coefficient for a specific public key in a set
+  *    of public keys
+  *    
+  *    WARNING: The coefficient will only be deterministic if...
+  *      1) input keys are pre-sorted
+  *         - tested here
+  *      2) input keys are in canonical form (compressed points in the prime-order subgroup of Ed25519)
+  *         - untested here for performance
+  * param: sorted_keys - set of component public keys that will be merged into a multisig public spend key
+  * param: aggregation_key - one of the component public keys
+  * return: aggregation coefficient
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static rct::key compute_multisig_aggregation_coefficient(const std::vector<crypto::public_key> &sorted_keys,
+    const crypto::public_key &aggregation_key)
+  {
+    CHECK_AND_ASSERT_THROW_MES(std::is_sorted(sorted_keys.begin(), sorted_keys.end()),
+      "Keys for aggregation coefficient aren't sorted.");
+
+    // aggregation key must be in sorted_keys
+    CHECK_AND_ASSERT_THROW_MES(std::find(sorted_keys.begin(), sorted_keys.end(), aggregation_key) != sorted_keys.end(),
+      "Aggregation key expected to be in input keyset.");
+
+    // aggregation coefficient salt
+    rct::key salt = rct::zero();
+    static_assert(sizeof(rct::key) >= sizeof(config::HASH_KEY_MULTISIG_KEY_AGGREGATION), "Hash domain separator is too big.");
+    memcpy(salt.bytes, config::HASH_KEY_MULTISIG_KEY_AGGREGATION, sizeof(config::HASH_KEY_MULTISIG_KEY_AGGREGATION));
+
+    // coeff = H(aggregation_key, sorted_keys, domain-sep)
+    rct::keyV data;
+    data.reserve(sorted_keys.size() + 2);
+    data.push_back(rct::pk2rct(aggregation_key));
+    for (const auto &key : sorted_keys)
+      data.push_back(rct::pk2rct(key));
+    data.push_back(salt);
+
+    // note: coefficient is considered public knowledge, no need to memwipe data
+    return rct::hash_to_scalar(data);
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  * 
+  * brief: generate_multisig_aggregate_key - generates a multisig public spend key via key aggregation
+  *    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)
+  * return: final multisig public spend key for the account
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static crypto::public_key generate_multisig_aggregate_key(std::vector<crypto::public_key> final_keys,
+    std::vector<crypto::secret_key> &privkeys_inout)
+  {
+    // collect all public keys that will go into the spend key (these don't need to be memsafe)
+    final_keys.reserve(final_keys.size() + privkeys_inout.size());
+
+    // 1. convert local multisig private keys to pub keys
+    // 2. insert to final keyset if not there yet
+    // 3. save the corresponding index of input priv key set for later reference
+    std::unordered_map<crypto::public_key, std::size_t> own_keys_mapping;
+
+    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");
+
+      own_keys_mapping[pubkey] = multisig_keys_index;
+
+      final_keys.push_back(pubkey);
+    }
+
+    // sort input final keys for computing aggregation coefficients (lowest to highest)
+    // note: input should be sanitized (no duplicates)
+    std::sort(final_keys.begin(), final_keys.end());
+    CHECK_AND_ASSERT_THROW_MES(std::adjacent_find(final_keys.begin(), final_keys.end()) == final_keys.end(),
+        "Unexpected duplicate found in input list.");
+
+    // key aggregation
+    rct::key aggregate_key = rct::identity();
+
+    for (const crypto::public_key &key : final_keys)
+    {
+      // get aggregation coefficient
+      rct::key coeff = compute_multisig_aggregation_coefficient(final_keys, key);
+
+      // convert private key if possible
+      // note: retain original priv key index in input list, in case order matters upstream
+      auto found_key = own_keys_mapping.find(key);
+      if (found_key != own_keys_mapping.end())
+      {
+        // k_agg = coeff*k_base
+        sc_mul((unsigned char*)&(privkeys_inout[found_key->second]),
+          coeff.bytes,
+          (const unsigned char*)&(privkeys_inout[found_key->second]));
+
+        CHECK_AND_ASSERT_THROW_MES(privkeys_inout[found_key->second] != crypto::null_skey,
+          "Multisig privkey with aggregation coefficient unexpectedly null.");
+      }
+
+      // convert public key (pre-merge operation)
+      // K_agg = coeff*K_base
+      rct::key converted_pubkey = rct::scalarmultKey(rct::pk2rct(key), coeff);
+
+      // build aggregate key (merge operation)
+      rct::addKeys(aggregate_key, aggregate_key, converted_pubkey);
+    }
+
+    return rct::rct2pk(aggregate_key);
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  *
+  * brief: multisig_kex_make_next_msg - Construct a kex msg for any round > 1 of multisig key construction.
+  *    - Involves DH exchanges with pubkeys provided by other participants.
+  *    - Conserves mapping [pubkey -> DH derivation] : [origin keys of participants that share this secret with you].
+  * param: base_privkey - account's base private key, for performing DH exchanges and signing messages
+  * param: round - the round of the message that should be produced
+  * param: threshold - threshold for multisig (M in M-of-N)
+  * param: num_signers - number of participants in multisig (N)
+  * param: pubkey_origins_map - map between pubkeys to produce DH derivations with and identity keys of
+  *    participants who will share each derivation with you
+  * outparam: derivation_origins_map_out - map between DH derivations (shared secrets) and identity keys
+  *        - If msg is not for the last round, then these derivations are also stored in the output message
+  *          so they can be sent to other participants, who will make more DH derivations for the next kex round.
+  *        - If msg is for the last round, then these derivations won't be sent to other participants.
+  *          Instead, they are converted to share secrets (i.e. s = H(derivation)) and multiplied by G.
+  *          The keys s*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().
+  *            - The values s are the local account's shares of the final multisig key's private key. The caller can
+  *              compute those values with calculate_multisig_keypair_from_derivation() (or compute them directly).
+  * return: multisig kex message for the specified round
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static multisig_kex_msg multisig_kex_make_next_msg(const crypto::secret_key &base_privkey,
+    const std::uint32_t round,
+    const std::uint32_t threshold,
+    const std::uint32_t num_signers,
+    const std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> &pubkey_origins_map,
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> &derivation_origins_map_out)
+  {
+    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 (limit = 16 to prevent dangerous combinatorial explosion during key exchange).");
+    CHECK_AND_ASSERT_THROW_MES(num_signers >= threshold,
+      "Multisig threshold may not be larger than number of signers.");
+    CHECK_AND_ASSERT_THROW_MES(threshold > 0, "Multisig threshold must be > 0.");
+    CHECK_AND_ASSERT_THROW_MES(round > 1, "Round for next msg must be > 1.");
+    CHECK_AND_ASSERT_THROW_MES(round <= multisig_kex_rounds_required(num_signers, threshold),
+      "Trying to make key exchange message for an invalid round.");
+
+    // make shared secrets with input pubkeys
+    std::vector<crypto::public_key> msg_pubkeys;
+    msg_pubkeys.reserve(pubkey_origins_map.size());
+    derivation_origins_map_out.clear();
+
+    for (const auto &pubkey_and_origins : pubkey_origins_map)
+    {
+      // D = 8 * k_base * K_pubkey
+      // note: must be mul8 (cofactor), otherwise it is possible to leak to a malicious participant if the local
+      //       base_privkey is a multiple of 8 or not
+      // note2: avoid making temporaries that won't be memwiped
+      rct::key derivation_rct;
+      auto a_wiper = epee::misc_utils::create_scope_leave_handler([&]{
+        memwipe(&derivation_rct, sizeof(rct::key));
+      });
+
+      rct::scalarmultKey(derivation_rct, rct::pk2rct(pubkey_and_origins.first), rct::sk2rct(base_privkey));
+      rct::scalarmultKey(derivation_rct, derivation_rct, rct::EIGHT);
+
+      crypto::public_key_memsafe derivation{rct::rct2pk(derivation_rct)};
+
+      // retain mapping between pubkey's origins and the DH derivation
+      // note: if msg for last round, then caller must know how to handle these derivations properly
+      derivation_origins_map_out[derivation] = pubkey_and_origins.second;
+
+      // if the last round, convert derivations to public keys for the output message
+      if (round == multisig_kex_rounds_required(num_signers, threshold))
+      {
+        // derived_pubkey = H(derivation)*G
+        crypto::public_key derived_pubkey;
+        calculate_multisig_keypair_from_derivation(derivation, derived_pubkey);
+        msg_pubkeys.push_back(derived_pubkey);
+      }
+      // otherwise, put derivations in message directly, so other signers can in turn create derivations (shared secrets)
+      //  with them for the next round
+      else
+        msg_pubkeys.push_back(derivation);
+    }
+
+    return multisig_kex_msg{round, base_privkey, std::move(msg_pubkeys)};
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * 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',
+  *      constructs map of pubkey:origins.
+  *    - Requires that all input msgs have the same round number.
+  *
+  *    origins = all the signing pubkeys that recommended a given pubkey found in input msgs
+  *
+  *    - 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,
+    const std::vector<crypto::public_key> &exclude_pubkeys,
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> &sanitized_pubkeys_out)
+  {
+    CHECK_AND_ASSERT_THROW_MES(expanded_msgs.size() > 0, "At least one input message expected.");
+
+    std::uint32_t round = expanded_msgs[0].get_round();
+    sanitized_pubkeys_out.clear();
+
+    // get all pubkeys from input messages, add them to pubkey:origins map
+    // - origins = all the signing pubkeys that recommended a given msg pubkey
+    for (const auto &expanded_msg : expanded_msgs)
+    {
+      CHECK_AND_ASSERT_THROW_MES(expanded_msg.get_round() == round, "All messages must have the same kex round number.");
+
+      // 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)
+      {
+        // note: ignores duplicates
+        sanitized_pubkeys_out[expanded_msg.get_signing_pubkey()].insert(expanded_msg.get_signing_pubkey());
+      }
+      // in other rounds, only the msg pubkeys are treated as msg pubkeys
+      else
+      {
+        // 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;
+
+          // note: ignores duplicates
+          sanitized_pubkeys_out[pubkey].insert(expanded_msg.get_signing_pubkey());
+        }
+      }
+    }
+
+    return round;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * 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: 'signers', 'exclude_pubkeys'.
+  *    - Requires each input pubkey be recommended by 'num_recommendations = expected_round' msg signers.
+  *      - For a final multisig key to be truly 'M-of-N', each of the the private key's components must be
+  *        shared by (N - M + 1) signers.
+  *    - Requires that msgs are signed by only keys in 'signers'.
+  *    - Requires that each key in 'signers' recommends [num_signers - 2 CHOOSE (expected_round - 1)] pubkeys.
+  *      - These should be derivations each signer recommends for round 'expected_round', excluding derivations shared
+  *        with the local account.
+  *    - Requires that 'exclude_pubkeys' has [num_signers - 1 CHOOSE (expected_round - 1)] pubkeys.
+  *      - These should be derivations the local account has corresponding to round 'expected_round'.
+  * param: base_privkey - multisig account's base private key
+  * param: expected_round - expected kex round of input messages
+  * param: threshold - threshold for multisig (M in M-of-N)
+  * 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'
+  * return: fully sanitized and validated pubkey:origins map for building the account's next kex round message
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> evaluate_multisig_kex_round_msgs(
+    const crypto::public_key &base_pubkey,
+    const std::uint32_t expected_round,
+    const std::uint32_t threshold,
+    const std::vector<crypto::public_key> &signers,
+    const std::vector<multisig_kex_msg> &expanded_msgs,
+    const std::vector<crypto::public_key> &exclude_pubkeys)
+  {
+    CHECK_AND_ASSERT_THROW_MES(signers.size() > 1, "Must be at least one other multisig signer.");
+    CHECK_AND_ASSERT_THROW_MES(signers.size() <= config::MULTISIG_MAX_SIGNERS,
+      "Too many multisig signers specified (limit = 16 to prevent dangerous combinatorial explosion during key exchange).");
+    CHECK_AND_ASSERT_THROW_MES(signers.size() >= threshold, "Multisig threshold may not be larger than number of signers.");
+    CHECK_AND_ASSERT_THROW_MES(threshold > 0, "Multisig threshold must be > 0.");
+    CHECK_AND_ASSERT_THROW_MES(expected_round > 0, "Expected round must be > 0.");
+    CHECK_AND_ASSERT_THROW_MES(expected_round <= multisig_kex_rounds_required(signers.size(), threshold),
+      "Expecting key exchange messages for an invalid round.");
+
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> pubkey_origins_map;
+
+    // leave early in the last round of 1-of-N, where all signers share a key so the local signer doesn't care about
+    // recommendations from other signers
+    if (threshold == 1 && expected_round == multisig_kex_rounds_required(signers.size(), threshold))
+      return pubkey_origins_map;
+
+    // exclude_pubkeys should all be unique
+    for (auto it = exclude_pubkeys.begin(); it != exclude_pubkeys.end(); ++it)
+    {
+      CHECK_AND_ASSERT_THROW_MES(std::find(exclude_pubkeys.begin(), it, *it) == it,
+        "Found duplicate pubkeys for exclusion unexpectedly.");
+    }
+
+    // sanitize input messages
+    std::uint32_t round = multisig_kex_msgs_sanitize_pubkeys(base_pubkey, 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 << "]");
+
+    // evaluate pubkeys collected
+    std::unordered_map<crypto::public_key, std::unordered_set<crypto::public_key>> origin_pubkeys_map;
+
+    // 1. each pubkey should be recommended by a precise number of signers
+    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,
+        "A pubkey recommended by multisig kex messages had an unexpected number of recommendations.");
+
+      // map (sanitized) pubkeys back to origins
+      for (const auto &origin : pubkey_and_origins.second)
+        origin_pubkeys_map[origin].insert(pubkey_and_origins.first);
+    }
+
+    // 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.");
+
+    // 3. each origin should recommend a precise number of pubkeys
+
+    // TODO: move to a 'math' library, with unit tests
+    auto n_choose_k_f =
+      [](const std::uint32_t n, const std::uint32_t k) -> std::uint32_t
+      {
+        static_assert(std::numeric_limits<std::int32_t>::digits <= std::numeric_limits<double>::digits,
+          "n_choose_k requires no rounding issues when converting between int32 <-> double.");
+
+        if (n < k)
+          return 0;
+
+        double fp_result = boost::math::binomial_coefficient<double>(n, k);
+
+        if (fp_result < 0)
+          return 0;
+
+        if (fp_result > std::numeric_limits<std::int32_t>::max())  // note: std::round() returns std::int32_t
+          return 0;
+
+        return static_cast<std::uint32_t>(std::round(fp_result));
+      };
+
+    // 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
+      //       have boosted the local signer (implying they didn't have access to the local signer's previous round msg).
+    std::uint32_t expected_recommendations_others = n_choose_k_f(signers.size() - 2, round - 1);
+
+    // local: (N - 1) choose (msg_round_num - 1)
+    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
+    CHECK_AND_ASSERT_THROW_MES(expected_recommendations_self > 0 && expected_recommendations_others > 0,
+      "Bad num signers or round num (possibly numerical limits exceeded).");
+
+    // check that local account recommends expected number of keys
+    CHECK_AND_ASSERT_THROW_MES(exclude_pubkeys.size() == expected_recommendations_self,
+      "Local account did not recommend expected number of multisig keys.");
+
+    // check that other signers recommend expected number of keys
+    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.");
+
+      // 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(),
+        "Multisig kex message with unexpected signer encountered.");
+    }
+
+    // note: above tests implicitly detect if the total number of recommended keys is correct or not
+    return pubkey_origins_map;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  /**
+  * INTERNAL
+  *
+  * brief: multisig_kex_process_round - Process kex messages for the active kex round.
+  *    - A wrapper around evaluate_multisig_kex_round_msgs() -> multisig_kex_make_next_msg().
+  *      - 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.
+  * param: base_privkey - multisig account's base private key
+  * param: current_round - round of kex the input messages should be designed for
+  * param: threshold - threshold for multisig (M in M-of-N)
+  * param: signers - expected participants in multisig kex
+  * 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.
+  * 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.
+  * return: multisig kex message for next round, or empty message if 'current_round' is the final round
+  */
+  //----------------------------------------------------------------------------------------------------------------------
+  static multisig_kex_msg multisig_kex_process_round(const crypto::secret_key &base_privkey,
+    const crypto::public_key &base_pubkey,
+    const std::uint32_t current_round,
+    const std::uint32_t threshold,
+    const std::vector<crypto::public_key> &signers,
+    const std::vector<multisig_kex_msg> &expanded_msgs,
+    const std::vector<crypto::public_key> &exclude_pubkeys,
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> &keys_to_origins_map_out)
+  {
+    // evaluate messages
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> evaluated_pubkeys =
+      evaluate_multisig_kex_round_msgs(base_pubkey, current_round, threshold, signers, expanded_msgs, exclude_pubkeys);
+
+    // produce message for next round (if there is one)
+    if (current_round < multisig_kex_rounds_required(signers.size(), threshold))
+    {
+      return multisig_kex_make_next_msg(base_privkey,
+        current_round + 1,
+        threshold,
+        signers.size(),
+        evaluated_pubkeys,
+        keys_to_origins_map_out);
+    }
+    else
+    {
+      // no more rounds, so collect the key shares recommended by other signers for the final aggregate key
+      keys_to_origins_map_out.clear();
+      keys_to_origins_map_out = std::move(evaluated_pubkeys);
+
+      return multisig_kex_msg{};
+    }
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::initialize_kex_update(const std::vector<multisig_kex_msg> &expanded_msgs,
+    const std::uint32_t rounds_required,
+    std::vector<crypto::public_key> &exclude_pubkeys_out)
+  {
+    if (m_kex_rounds_complete == 0)
+    {
+      // the first round of kex msgs will contain each participant's base pubkeys and ancillary privkeys
+
+      // 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);
+
+      // add local ancillary base privkey
+      participant_base_common_privkeys.emplace_back(m_base_common_privkey);
+
+      // 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());
+        }
+      }
+
+      // make common privkey
+      make_multisig_common_privkey(std::move(participant_base_common_privkeys), m_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");
+
+      // if N-of-N, then the base privkey will be used directly to make the account's share of the final key
+      if (rounds_required == 1)
+      {
+        m_multisig_privkeys.clear();
+        m_multisig_privkeys.emplace_back(m_base_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
+
+      // 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);
+      }
+    }
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::finalize_kex_update(const std::uint32_t rounds_required,
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> result_keys_to_origins_map)
+  {
+    // prepare for next round (or complete the multisig account fully)
+    if (rounds_required == m_kex_rounds_complete + 1)
+    {
+      // finished (have set of msgs to complete address)
+
+      // when 'completing the final round', result keys are other signers' shares of the final key
+      std::vector<crypto::public_key> result_keys;
+      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);
+
+      // no longer need the account's pubkeys saved for this round (they were only used to build exclude_pubkeys)
+      // TODO: record [pre-aggregation pubkeys : origins] map for aggregation-style signing
+      m_kex_keys_to_origins_map.clear();
+    }
+    else if (rounds_required == m_kex_rounds_complete + 2)
+    {
+      // one more round (must send/receive one more set of kex msgs)
+      // - at this point, have local signer's pre-aggregation private key shares of the final address
+
+      // result keys are the local signer's DH derivations for the next round
+
+      // derivations are shared secrets between each group of N - M + 1 signers of which the local account is a member
+      // - convert them to private keys: multisig_key = H(derivation)
+      // - note: shared key = multisig_key[i]*G is recorded in the kex msg for sending to other participants
+      //   instead of the original 'derivation' value (which MUST be kept secret!)
+      m_multisig_privkeys.clear();
+      m_multisig_privkeys.reserve(result_keys_to_origins_map.size());
+
+      m_kex_keys_to_origins_map.clear();
+
+      for (const auto &derivation_and_origins : result_keys_to_origins_map)
+      {
+        // multisig_privkey = H(derivation)
+        // 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));
+
+        // 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);
+      }
+    }
+    else
+    {
+      // next round is an 'intermediate' key exchange round, so there is nothing special to do here
+
+      // save the account's kex keys for this round [DH derivation : other signers who will have the same derivation]
+      m_kex_keys_to_origins_map = std::move(result_keys_to_origins_map);
+    }
+
+    // a full set of msgs has been collected and processed, so the 'round is complete'
+    ++m_kex_rounds_complete;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_account: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_account::kex_update_impl(const std::vector<multisig_kex_msg> &expanded_msgs)
+  {
+    CHECK_AND_ASSERT_THROW_MES(expanded_msgs.size() > 0, "No key exchange messages passed in.");
+
+    const std::uint32_t rounds_required = multisig_kex_rounds_required(m_signers.size(), m_threshold);
+    CHECK_AND_ASSERT_THROW_MES(rounds_required > 0, "Multisig kex rounds required unexpectedly 0.");
+
+    // initialize account update
+    std::vector<crypto::public_key> exclude_pubkeys;
+    initialize_kex_update(expanded_msgs, rounds_required, exclude_pubkeys);
+
+    // evaluate messages and get this account's kex msg for the next round
+    std::unordered_map<crypto::public_key_memsafe, std::unordered_set<crypto::public_key>> result_keys_to_origins_map;
+
+    m_next_round_kex_message = multisig_kex_process_round(
+      m_base_privkey,
+      m_base_pubkey,
+      m_kex_rounds_complete + 1,
+      m_threshold,
+      m_signers,
+      expanded_msgs,
+      exclude_pubkeys,
+      result_keys_to_origins_map).get_msg();
+
+    // finish account update
+    finalize_kex_update(rounds_required, std::move(result_keys_to_origins_map));
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+} //namespace multisig
diff --git a/src/multisig/multisig_kex_msg.cpp b/src/multisig/multisig_kex_msg.cpp
new file mode 100644
index 000000000..2bbceb19d
--- /dev/null
+++ b/src/multisig/multisig_kex_msg.cpp
@@ -0,0 +1,290 @@
+// Copyright (c) 2021, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "multisig_kex_msg.h"
+#include "multisig_kex_msg_serialization.h"
+
+#include "common/base58.h"
+#include "crypto/crypto.h"
+extern "C"
+{
+#include "crypto/crypto-ops.h"
+}
+#include "cryptonote_basic/cryptonote_format_utils.h"
+#include "include_base_utils.h"
+#include "ringct/rctOps.h"
+#include "serialization/binary_archive.h"
+#include "serialization/serialization.h"
+
+#include <boost/utility/string_ref.hpp> 
+
+#include <sstream>
+#include <utility>
+#include <vector>
+
+
+#undef MONERO_DEFAULT_LOG_CATEGORY
+#define MONERO_DEFAULT_LOG_CATEGORY "multisig"
+
+const boost::string_ref MULTISIG_KEX_V1_MAGIC{"MultisigV1"};
+const boost::string_ref MULTISIG_KEX_MSG_V1_MAGIC{"MultisigxV1"};
+const boost::string_ref MULTISIG_KEX_MSG_V2_MAGIC_1{"MultisigxV2R1"};  //round 1
+const boost::string_ref MULTISIG_KEX_MSG_V2_MAGIC_N{"MultisigxV2Rn"};  //round n > 1
+
+namespace multisig
+{
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_kex_msg: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  multisig_kex_msg::multisig_kex_msg(const std::uint32_t round,
+    const crypto::secret_key &signing_privkey,
+    std::vector<crypto::public_key> msg_pubkeys,
+    const crypto::secret_key &msg_privkey) :
+      m_kex_round{round}
+  {
+    CHECK_AND_ASSERT_THROW_MES(round > 0, "Kex round must be > 0.");
+    CHECK_AND_ASSERT_THROW_MES(sc_check((const unsigned char*)&signing_privkey) == 0 &&
+      signing_privkey != crypto::null_skey, "Invalid msg signing key.");
+
+    if (round == 1)
+    {
+      CHECK_AND_ASSERT_THROW_MES(sc_check((const unsigned char*)&msg_privkey) == 0 &&
+        msg_privkey != crypto::null_skey, "Invalid msg privkey.");
+
+      m_msg_privkey = msg_privkey;
+    }
+    else
+    {
+      for (const auto &pubkey : msg_pubkeys)
+      {
+        CHECK_AND_ASSERT_THROW_MES(pubkey != crypto::null_pkey && pubkey != rct::rct2pk(rct::identity()),
+          "Pubkey for message was invalid.");
+        CHECK_AND_ASSERT_THROW_MES((rct::scalarmultKey(rct::pk2rct(pubkey), rct::curveOrder()) == rct::identity()),
+          "Pubkey for message was not in prime subgroup.");
+      }
+
+      m_msg_pubkeys = std::move(msg_pubkeys);
+    }
+    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(signing_privkey, m_signing_pubkey),
+      "Failed to derive public key");
+
+    // sets message and signing pub key
+    construct_msg(signing_privkey);
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_kex_msg: EXTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  multisig_kex_msg::multisig_kex_msg(std::string msg) : m_msg{std::move(msg)}
+  {
+    parse_and_validate_msg();
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_kex_msg: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  crypto::hash multisig_kex_msg::get_msg_to_sign() const
+  {
+    ////
+    // msg_content = kex_round | signing_pubkey | expand(msg_pubkeys) | OPTIONAL msg_privkey
+    // sign_msg = versioning-domain-sep | msg_content
+    ///
+
+    std::string data;
+    CHECK_AND_ASSERT_THROW_MES(MULTISIG_KEX_MSG_V2_MAGIC_1.size() == MULTISIG_KEX_MSG_V2_MAGIC_N.size(),
+      "Multisig kex msg magic inconsistency.");
+    data.reserve(MULTISIG_KEX_MSG_V2_MAGIC_1.size() + 4 + 32*(1 + (m_kex_round == 1 ? 1 : 0) + m_msg_pubkeys.size()));
+
+    // versioning domain-sep
+    if (m_kex_round == 1)
+      data.append(MULTISIG_KEX_MSG_V2_MAGIC_1.data(), MULTISIG_KEX_MSG_V2_MAGIC_1.size());
+    else
+      data.append(MULTISIG_KEX_MSG_V2_MAGIC_N.data(), MULTISIG_KEX_MSG_V2_MAGIC_N.size());
+
+    // kex_round as little-endian bytes
+    for (std::size_t i{0}; i < 4; ++i)
+    {
+      data += static_cast<char>(m_kex_round >> i*8);
+    }
+
+    // signing pubkey
+    data.append((const char *)&m_signing_pubkey, sizeof(crypto::public_key));
+
+    // add msg privkey if kex_round == 1
+    if (m_kex_round == 1)
+      data.append((const char *)&m_msg_privkey, sizeof(crypto::secret_key));
+    else
+    {
+      // only add pubkeys if not round 1
+
+      // msg pubkeys
+      for (const auto &key : m_msg_pubkeys)
+        data.append((const char *)&key, sizeof(crypto::public_key));
+    }
+
+    // message to sign
+    crypto::hash hash;
+    crypto::cn_fast_hash(data.data(), data.size(), hash);
+
+    return hash;
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_kex_msg: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_kex_msg::construct_msg(const crypto::secret_key &signing_privkey)
+  {
+    ////
+    // msg_content = kex_round | signing_pubkey | expand(msg_pubkeys) | OPTIONAL msg_privkey
+    // sign_msg = versioning-domain-sep | msg_content
+    // msg = versioning-domain-sep | serialize(msg_content | crypto_sig[signing_privkey](sign_msg))
+    ///
+
+    // sign the message
+    crypto::signature msg_signature;
+    crypto::hash msg_to_sign{get_msg_to_sign()};
+    crypto::generate_signature(msg_to_sign, m_signing_pubkey, signing_privkey, msg_signature);
+
+    // assemble the message
+    m_msg.clear();
+
+    std::stringstream serialized_msg_ss;
+    binary_archive<true> b_archive(serialized_msg_ss);
+
+    if (m_kex_round == 1)
+    {
+      m_msg.append(MULTISIG_KEX_MSG_V2_MAGIC_1.data(), MULTISIG_KEX_MSG_V2_MAGIC_1.size());
+
+      multisig_kex_msg_serializable_round1 msg_serializable;
+      msg_serializable.msg_privkey    = m_msg_privkey;
+      msg_serializable.signing_pubkey = m_signing_pubkey;
+      msg_serializable.signature      = msg_signature;
+
+      CHECK_AND_ASSERT_THROW_MES(::serialization::serialize(b_archive, msg_serializable),
+        "Failed to serialize multisig kex msg");
+    }
+    else
+    {
+      m_msg.append(MULTISIG_KEX_MSG_V2_MAGIC_N.data(), MULTISIG_KEX_MSG_V2_MAGIC_N.size());
+
+      multisig_kex_msg_serializable_general msg_serializable;
+      msg_serializable.kex_round      = m_kex_round;
+      msg_serializable.msg_pubkeys    = m_msg_pubkeys;
+      msg_serializable.signing_pubkey = m_signing_pubkey;
+      msg_serializable.signature      = msg_signature;
+
+      CHECK_AND_ASSERT_THROW_MES(::serialization::serialize(b_archive, msg_serializable),
+        "Failed to serialize multisig kex msg");
+    }
+
+    m_msg.append(tools::base58::encode(serialized_msg_ss.str()));
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+  // multisig_kex_msg: INTERNAL
+  //----------------------------------------------------------------------------------------------------------------------
+  void multisig_kex_msg::parse_and_validate_msg()
+  {
+    // check message type
+    CHECK_AND_ASSERT_THROW_MES(m_msg.size() > 0, "Kex message unexpectedly empty.");
+    CHECK_AND_ASSERT_THROW_MES(m_msg.substr(0, MULTISIG_KEX_V1_MAGIC.size()) != MULTISIG_KEX_V1_MAGIC,
+      "V1 multisig kex messages are deprecated (unsafe).");
+    CHECK_AND_ASSERT_THROW_MES(m_msg.substr(0, MULTISIG_KEX_MSG_V1_MAGIC.size()) != MULTISIG_KEX_MSG_V1_MAGIC,
+      "V1 multisig kex messages are deprecated (unsafe).");
+
+    // deserialize the message
+    std::string msg_no_magic;
+    CHECK_AND_ASSERT_THROW_MES(MULTISIG_KEX_MSG_V2_MAGIC_1.size() == MULTISIG_KEX_MSG_V2_MAGIC_N.size(),
+      "Multisig kex msg magic inconsistency.");
+    CHECK_AND_ASSERT_THROW_MES(tools::base58::decode(m_msg.substr(MULTISIG_KEX_MSG_V2_MAGIC_1.size()), msg_no_magic),
+      "Multisig kex msg decoding error.");
+    binary_archive<false> b_archive{epee::strspan<std::uint8_t>(msg_no_magic)};
+    crypto::signature msg_signature;
+
+    if (m_msg.substr(0, MULTISIG_KEX_MSG_V2_MAGIC_1.size()) == MULTISIG_KEX_MSG_V2_MAGIC_1)
+    {
+      // try round 1 message
+      multisig_kex_msg_serializable_round1 kex_msg_rnd1;
+
+      if (::serialization::serialize(b_archive, kex_msg_rnd1))
+      {
+        // in round 1 the message stores a private ancillary key component for the multisig account
+        // that will be shared by all participants (e.g. a shared private view key)
+        m_kex_round      = 1;
+        m_msg_privkey    = kex_msg_rnd1.msg_privkey;
+        m_signing_pubkey = kex_msg_rnd1.signing_pubkey;
+        msg_signature    = kex_msg_rnd1.signature;
+      }
+      else
+      {
+        CHECK_AND_ASSERT_THROW_MES(false, "Deserializing kex msg failed.");
+      }
+    }
+    else if (m_msg.substr(0, MULTISIG_KEX_MSG_V2_MAGIC_N.size()) == MULTISIG_KEX_MSG_V2_MAGIC_N)
+    {
+      // try general message
+      multisig_kex_msg_serializable_general kex_msg_general;
+
+      if (::serialization::serialize(b_archive, kex_msg_general))
+      {
+        m_kex_round      = kex_msg_general.kex_round;
+        m_msg_privkey    = crypto::null_skey;
+        m_msg_pubkeys    = std::move(kex_msg_general.msg_pubkeys);
+        m_signing_pubkey = kex_msg_general.signing_pubkey;
+        msg_signature    = kex_msg_general.signature;
+
+        CHECK_AND_ASSERT_THROW_MES(m_kex_round > 1, "Invalid kex message round (must be > 1 for the general msg type).");
+      }
+      else
+      {
+        CHECK_AND_ASSERT_THROW_MES(false, "Deserializing kex msg failed.");
+      }
+    }
+    else
+    {
+      // unknown message type
+      CHECK_AND_ASSERT_THROW_MES(false, "Only v2 multisig kex messages are supported.");
+    }
+
+    // checks
+    for (const auto &pubkey: m_msg_pubkeys)
+    {
+      CHECK_AND_ASSERT_THROW_MES(pubkey != crypto::null_pkey && pubkey != rct::rct2pk(rct::identity()),
+        "Pubkey from message was invalid.");
+      CHECK_AND_ASSERT_THROW_MES(rct::isInMainSubgroup(rct::pk2rct(pubkey)),
+        "Pubkey from message was not in prime subgroup.");
+    }
+
+    CHECK_AND_ASSERT_THROW_MES(m_signing_pubkey != crypto::null_pkey && m_signing_pubkey != rct::rct2pk(rct::identity()),
+      "Message signing key was invalid.");
+    CHECK_AND_ASSERT_THROW_MES(rct::isInMainSubgroup(rct::pk2rct(m_signing_pubkey)),
+      "Message signing key was not in prime subgroup.");
+
+    // validate signature
+    crypto::hash signed_msg{get_msg_to_sign()};
+    CHECK_AND_ASSERT_THROW_MES(crypto::check_signature(signed_msg, m_signing_pubkey, msg_signature),
+      "Multisig kex msg signature invalid.");
+  }
+  //----------------------------------------------------------------------------------------------------------------------
+} //namespace multisig
diff --git a/src/multisig/multisig_kex_msg.h b/src/multisig/multisig_kex_msg.h
new file mode 100644
index 000000000..23e3042f2
--- /dev/null
+++ b/src/multisig/multisig_kex_msg.h
@@ -0,0 +1,109 @@
+// Copyright (c) 2021, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include "crypto/crypto.h"
+
+#include <cstdint>
+#include <vector>
+
+
+namespace multisig
+{
+  ////
+  // multisig key exchange message
+  // - can parse and validate an input message
+  // - can construct and sign a new message
+  //
+  // msg_content = kex_round | signing_pubkey | expand(msg_pubkeys) | OPTIONAL msg_privkey
+  // msg_to_sign = versioning-domain-sep | msg_content
+  // msg = versioning-domain-sep | b58(msg_content | crypto_sig[signing_privkey](msg_to_sign))
+  //
+  // note: round 1 messages will contain a private key (e.g. for the aggregate multisig private view key)
+  ///
+  class multisig_kex_msg final
+  {
+  //member types: none
+
+  //constructors
+  public:
+    // default constructor
+    multisig_kex_msg() = default;
+
+    // construct from info
+    multisig_kex_msg(const std::uint32_t round,
+      const crypto::secret_key &signing_privkey,
+      std::vector<crypto::public_key> msg_pubkeys,
+      const crypto::secret_key &msg_privkey = crypto::null_skey);
+
+    // construct from string
+    multisig_kex_msg(std::string msg);
+
+    // copy constructor: default
+
+  //destructor: default
+    ~multisig_kex_msg() = default;
+
+  //overloaded operators: none
+
+  //member functions
+    // get msg string
+    const std::string& get_msg() const { return m_msg; }
+    // get kex round
+    std::uint32_t get_round() const { return m_kex_round; }
+    // get msg pubkeys
+    const std::vector<crypto::public_key>& get_msg_pubkeys() const { return m_msg_pubkeys; }
+    // get msg privkey
+    const crypto::secret_key& get_msg_privkey() const { return m_msg_privkey; }
+    // get msg signing pubkey
+    const crypto::public_key& get_signing_pubkey() const { return m_signing_pubkey; }
+
+  private:
+    // msg_to_sign = versioning-domain-sep | kex_round | signing_pubkey | expand(msg_pubkeys) | OPTIONAL msg_privkey
+    crypto::hash get_msg_to_sign() const;
+    // set: msg string based on msg contents, signing pubkey based on input privkey
+    void construct_msg(const crypto::secret_key &signing_privkey);
+    // parse msg string into parts, validate contents and signature
+    void parse_and_validate_msg();
+
+  //member variables
+  private:
+    // message as string
+    std::string m_msg;
+
+    // key exchange round this msg was produced for
+    std::uint32_t m_kex_round;
+    // pubkeys stored in msg
+    std::vector<crypto::public_key> m_msg_pubkeys;
+    // privkey stored in msg (if kex round 1)
+    crypto::secret_key m_msg_privkey;
+    // pubkey used to sign this msg
+    crypto::public_key m_signing_pubkey;
+  };
+} //namespace multisig
diff --git a/src/multisig/multisig_kex_msg_serialization.h b/src/multisig/multisig_kex_msg_serialization.h
new file mode 100644
index 000000000..9c7b993a7
--- /dev/null
+++ b/src/multisig/multisig_kex_msg_serialization.h
@@ -0,0 +1,78 @@
+// Copyright (c) 2021, The Monero Project
+// 
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+// 
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+// 
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+// 
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include "crypto/crypto.h"
+#include "serialization/containers.h"
+#include "serialization/crypto.h"
+#include "serialization/serialization.h"
+
+#include <cstdint>
+#include <vector>
+
+
+namespace multisig
+{
+  /// round 1 kex message
+  struct multisig_kex_msg_serializable_round1
+  {
+    // privkey stored in msg
+    crypto::secret_key msg_privkey;
+    // pubkey used to sign this msg
+    crypto::public_key signing_pubkey;
+    // message signature
+    crypto::signature signature;
+
+    BEGIN_SERIALIZE()
+      FIELD(msg_privkey)
+      FIELD(signing_pubkey)
+      FIELD(signature)
+    END_SERIALIZE()
+  };
+
+  /// general kex message (if round > 1)
+  struct multisig_kex_msg_serializable_general
+  {
+    // key exchange round this msg was produced for
+    std::uint32_t kex_round;
+    // pubkeys stored in msg
+    std::vector<crypto::public_key> msg_pubkeys;
+    // pubkey used to sign this msg
+    crypto::public_key signing_pubkey;
+    // message signature
+    crypto::signature signature;
+
+    BEGIN_SERIALIZE()
+      VARINT_FIELD(kex_round)
+      FIELD(msg_pubkeys)
+      FIELD(signing_pubkey)
+      FIELD(signature)
+    END_SERIALIZE()
+  };
+} //namespace multisig
diff --git a/src/simplewallet/simplewallet.cpp b/src/simplewallet/simplewallet.cpp
index fd784c5ae..891253830 100644
--- a/src/simplewallet/simplewallet.cpp
+++ b/src/simplewallet/simplewallet.cpp
@@ -235,7 +235,6 @@ 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_FINALIZE_MULTISIG("finalize_multisig <string> [<string>...]");
   const char* USAGE_EXCHANGE_MULTISIG_KEYS("exchange_multisig_keys <string> [<string>...]");
   const char* USAGE_EXPORT_MULTISIG_INFO("export_multisig_info <filename>");
   const char* USAGE_IMPORT_MULTISIG_INFO("import_multisig_info <filename> [<filename>...]");
@@ -1021,7 +1020,7 @@ bool simple_wallet::prepare_multisig_main(const std::vector<std::string> &args,
 
   SCOPED_WALLET_UNLOCK_ON_BAD_PASSWORD(return false;);
 
-  std::string multisig_info = m_wallet->get_multisig_info();
+  std::string multisig_info = m_wallet->get_multisig_first_kex_msg();
   success_msg_writer() << multisig_info;
   success_msg_writer() << tr("Send this multisig info to all other participants, then use make_multisig <threshold> <info1> [<info2>...] with others' multisig info");
   success_msg_writer() << tr("This includes the PRIVATE view key, so needs to be disclosed only to that multisig wallet's participants ");
@@ -1122,58 +1121,6 @@ bool simple_wallet::make_multisig_main(const std::vector<std::string> &args, boo
   return true;
 }
 
-bool simple_wallet::finalize_multisig(const std::vector<std::string> &args)
-{
-  bool ready;
-  if (m_wallet->key_on_device())
-  {
-    fail_msg_writer() << tr("command not supported by HW wallet");
-    return true;
-  }
-
-  const auto pwd_container = get_and_verify_password();
-  if(pwd_container == boost::none)
-  {
-    fail_msg_writer() << tr("Your original password was incorrect.");
-    return true;
-  }
-
-  if (!m_wallet->multisig(&ready))
-  {
-    fail_msg_writer() << tr("This wallet is not multisig");
-    return true;
-  }
-  if (ready)
-  {
-    fail_msg_writer() << tr("This wallet is already finalized");
-    return true;
-  }
-
-  LOCK_IDLE_SCOPE();
-
-  if (args.size() < 2)
-  {
-    PRINT_USAGE(USAGE_FINALIZE_MULTISIG);
-    return true;
-  }
-
-  try
-  {
-    if (!m_wallet->finalize_multisig(pwd_container->password(), args))
-    {
-      fail_msg_writer() << tr("Failed to finalize multisig");
-      return true;
-    }
-  }
-  catch (const std::exception &e)
-  {
-    fail_msg_writer() << tr("Failed to finalize multisig: ") << e.what();
-    return true;
-  }
-
-  return true;
-}
-
 bool simple_wallet::exchange_multisig_keys(const std::vector<std::string> &args)
 {
   exchange_multisig_keys_main(args, false);
@@ -3627,10 +3574,6 @@ simple_wallet::simple_wallet()
   m_cmd_binder.set_handler("make_multisig", boost::bind(&simple_wallet::on_command, this, &simple_wallet::make_multisig, _1),
                            tr(USAGE_MAKE_MULTISIG),
                            tr("Turn this wallet into a multisig wallet"));
-  m_cmd_binder.set_handler("finalize_multisig",
-                           boost::bind(&simple_wallet::on_command, this, &simple_wallet::finalize_multisig, _1),
-                           tr(USAGE_FINALIZE_MULTISIG),
-                           tr("Turn this wallet into a multisig wallet, extra step for N-1/N wallets"));
   m_cmd_binder.set_handler("exchange_multisig_keys",
                            boost::bind(&simple_wallet::on_command, this, &simple_wallet::exchange_multisig_keys, _1),
                            tr(USAGE_EXCHANGE_MULTISIG_KEYS),
@@ -10971,8 +10914,8 @@ void simple_wallet::mms_init(const std::vector<std::string> &args)
   std::vector<std::string> numbers;
   boost::split(numbers, mn, boost::is_any_of("/"));
   bool mn_ok = (numbers.size() == 2)
-               && get_number_from_arg(numbers[1], num_authorized_signers, 2, 100)
-               && get_number_from_arg(numbers[0], num_required_signers, 2, num_authorized_signers);
+               && get_number_from_arg(numbers[1], num_authorized_signers, 2, config::MULTISIG_MAX_SIGNERS)
+               && get_number_from_arg(numbers[0], num_required_signers, 1, num_authorized_signers);
   if (!mn_ok)
   {
     fail_msg_writer() << tr("Error in the number of required signers and/or authorized signers");
diff --git a/src/simplewallet/simplewallet.h b/src/simplewallet/simplewallet.h
index 8780bee1d..4bb2c6a71 100644
--- a/src/simplewallet/simplewallet.h
+++ b/src/simplewallet/simplewallet.h
@@ -233,7 +233,6 @@ namespace cryptonote
     bool prepare_multisig_main(const std::vector<std::string>& args, bool called_by_mms);
     bool make_multisig(const std::vector<std::string>& args);
     bool make_multisig_main(const std::vector<std::string>& args, bool called_by_mms);
-    bool finalize_multisig(const std::vector<std::string> &args);
     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 export_multisig(const std::vector<std::string>& args);
diff --git a/src/wallet/api/wallet.cpp b/src/wallet/api/wallet.cpp
index 989061250..b058619a3 100644
--- a/src/wallet/api/wallet.cpp
+++ b/src/wallet/api/wallet.cpp
@@ -1332,7 +1332,7 @@ MultisigState WalletImpl::multisig() const {
 string WalletImpl::getMultisigInfo() const {
     try {
         clearStatus();
-        return m_wallet->get_multisig_info();
+        return m_wallet->get_multisig_first_kex_msg();
     } catch (const exception& e) {
         LOG_ERROR("Error on generating multisig info: " << e.what());
         setStatusError(string(tr("Failed to get multisig info: ")) + e.what());
@@ -1341,7 +1341,7 @@ string WalletImpl::getMultisigInfo() const {
     return string();
 }
 
-string WalletImpl::makeMultisig(const vector<string>& info, uint32_t threshold) {
+string WalletImpl::makeMultisig(const vector<string>& info, const uint32_t threshold) {
     try {
         clearStatus();
 
@@ -1366,30 +1366,12 @@ std::string WalletImpl::exchangeMultisigKeys(const std::vector<std::string> &inf
         return m_wallet->exchange_multisig_keys(epee::wipeable_string(m_password), info);
     } catch (const exception& e) {
         LOG_ERROR("Error on exchanging multisig keys: " << e.what());
-        setStatusError(string(tr("Failed to make multisig: ")) + e.what());
+        setStatusError(string(tr("Failed to exchange multisig keys: ")) + e.what());
     }
 
     return string();
 }
 
-bool WalletImpl::finalizeMultisig(const vector<string>& extraMultisigInfo) {
-    try {
-        clearStatus();
-        checkMultisigWalletNotReady(m_wallet);
-
-        if (m_wallet->finalize_multisig(epee::wipeable_string(m_password), extraMultisigInfo)) {
-            return true;
-        }
-
-        setStatusError(tr("Failed to finalize multisig wallet creation"));
-    } catch (const exception& e) {
-        LOG_ERROR("Error on finalizing multisig wallet creation: " << e.what());
-        setStatusError(string(tr("Failed to finalize multisig wallet creation: ")) + e.what());
-    }
-
-    return false;
-}
-
 bool WalletImpl::exportMultisigImages(string& images) {
     try {
         clearStatus();
diff --git a/src/wallet/api/wallet.h b/src/wallet/api/wallet.h
index 67fc2c08a..7e1e62081 100644
--- a/src/wallet/api/wallet.h
+++ b/src/wallet/api/wallet.h
@@ -147,7 +147,6 @@ 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) override;
-    bool finalizeMultisig(const std::vector<std::string>& extraMultisigInfo) override;
     bool exportMultisigImages(std::string& images) override;
     size_t importMultisigImages(const std::vector<std::string>& images) override;
     bool hasMultisigPartialKeyImages() const override;
diff --git a/src/wallet/api/wallet2_api.h b/src/wallet/api/wallet2_api.h
index f9c421a93..6da547054 100644
--- a/src/wallet/api/wallet2_api.h
+++ b/src/wallet/api/wallet2_api.h
@@ -790,7 +790,7 @@ struct Wallet
     /**
      * @brief makeMultisig - switches wallet in multisig state. The one and only creation phase for N / N wallets
      * @param info - vector of multisig infos from other participants obtained with getMulitisInfo call
-     * @param threshold - number of required signers to make valid transaction. Must be equal to number of participants (N) or N - 1
+     * @param threshold - number of required signers to make valid transaction. Must be <= number of participants
      * @return in case of N / N wallets returns empty string since no more key exchanges needed. For N - 1 / N wallets returns base58 encoded extra multisig info
      */
     virtual std::string makeMultisig(const std::vector<std::string>& info, uint32_t threshold) = 0;
@@ -801,12 +801,6 @@ struct Wallet
      */
     virtual std::string exchangeMultisigKeys(const std::vector<std::string> &info) = 0;
     /**
-     * @brief finalizeMultisig - finalizes N - 1 / N multisig wallets creation
-     * @param extraMultisigInfo - wallet participants' extra multisig info obtained with makeMultisig call
-     * @return true if success
-     */
-    virtual bool finalizeMultisig(const std::vector<std::string>& extraMultisigInfo) = 0;
-    /**
      * @brief exportMultisigImages - exports transfers' key images
      * @param images - output paramter for hex encoded array of images
      * @return true if success
diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index f36fa73e0..e95e53e0c 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -28,6 +28,7 @@
 // 
 // Parts of this file are originally copyright (c) 2012-2013 The Cryptonote developers
 
+#include <algorithm>
 #include <numeric>
 #include <tuple>
 #include <queue>
@@ -59,6 +60,8 @@ using namespace epee;
 #include "misc_language.h"
 #include "cryptonote_basic/cryptonote_basic_impl.h"
 #include "multisig/multisig.h"
+#include "multisig/multisig_account.h"
+#include "multisig/multisig_kex_msg.h"
 #include "common/boost_serialization_helper.h"
 #include "common/command_line.h"
 #include "common/threadpool.h"
@@ -149,7 +152,6 @@ using namespace cryptonote;
 #define RECENT_SPEND_WINDOW (15 * DIFFICULTY_TARGET_V2)
 
 static const std::string MULTISIG_SIGNATURE_MAGIC = "SigMultisigPkV1";
-static const std::string MULTISIG_EXTRA_INFO_MAGIC = "MultisigxV1";
 
 static const std::string ASCII_OUTPUT_MAGIC = "MoneroAsciiDataV1";
 
@@ -167,42 +169,6 @@ namespace
     return dir.string();
   }
 
-  std::string pack_multisignature_keys(const std::string& prefix, const std::vector<crypto::public_key>& keys, const crypto::secret_key& signer_secret_key)
-  {
-    std::string data;
-    crypto::public_key signer;
-    CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(signer_secret_key, signer), "Failed to derive public spend key");
-    data += std::string((const char *)&signer, sizeof(crypto::public_key));
-
-    for (const auto &key: keys)
-    {
-      data += std::string((const char *)&key, sizeof(crypto::public_key));
-    }
-
-    data.resize(data.size() + sizeof(crypto::signature));
-
-    crypto::hash hash;
-    crypto::cn_fast_hash(data.data(), data.size() - sizeof(crypto::signature), hash);
-    crypto::signature &signature = *(crypto::signature*)&data[data.size() - sizeof(crypto::signature)];
-    crypto::generate_signature(hash, signer, signer_secret_key, signature);
-
-    return MULTISIG_EXTRA_INFO_MAGIC + tools::base58::encode(data);
-  }
-
-  std::vector<crypto::public_key> secret_keys_to_public_keys(const std::vector<crypto::secret_key>& keys)
-  {
-    std::vector<crypto::public_key> public_keys;
-    public_keys.reserve(keys.size());
-
-    std::transform(keys.begin(), keys.end(), std::back_inserter(public_keys), [] (const crypto::secret_key& k) -> crypto::public_key {
-      crypto::public_key p;
-      CHECK_AND_ASSERT_THROW_MES(crypto::secret_key_to_public_key(k, p), "Failed to derive public spend key");
-      return p;
-    });
-
-    return public_keys;
-  }
-
   bool keys_intersect(const std::unordered_set<crypto::public_key>& s1, const std::unordered_set<crypto::public_key>& s2)
   {
     if (s1.empty() || s2.empty())
@@ -4763,7 +4729,6 @@ void wallet2::generate(const std::string& wallet_, const epee::wipeable_string&
   memwipe(&skey, sizeof(rct::key));
 
   m_account.make_multisig(view_secret_key, spend_secret_key, spend_public_key, multisig_keys);
-  m_account.finalize_multisig(spend_public_key);
 
   // Not possible to restore a multisig wallet that is able to activate the MMS
   // (because the original keys are not (yet) part of the restore info), so
@@ -4978,24 +4943,12 @@ void wallet2::restore(const std::string& wallet_, const epee::wipeable_string& p
     store();
   }
 }
-
+//----------------------------------------------------------------------------------------------------
 std::string wallet2::make_multisig(const epee::wipeable_string &password,
-  const std::vector<crypto::secret_key> &view_keys,
-  const std::vector<crypto::public_key> &spend_keys,
-  uint32_t threshold)
+  const std::vector<std::string> &initial_kex_msgs,
+  const std::uint32_t threshold)
 {
-  CHECK_AND_ASSERT_THROW_MES(!view_keys.empty(), "empty view keys");
-  CHECK_AND_ASSERT_THROW_MES(view_keys.size() == spend_keys.size(), "Mismatched view/spend key sizes");
-  CHECK_AND_ASSERT_THROW_MES(threshold > 1 && threshold <= spend_keys.size() + 1, "Invalid threshold");
-
-  std::string extra_multisig_info;
-  std::vector<crypto::secret_key> multisig_keys;
-  rct::key spend_pkey = rct::identity();
-  rct::key spend_skey;
-  auto wiper = epee::misc_utils::create_scope_leave_handler([&](){memwipe(&spend_skey, sizeof(spend_skey));});
-  std::vector<crypto::public_key> multisig_signers;
-
-  // decrypt keys
+  // decrypt account keys
   epee::misc_utils::auto_scope_leave_caller keys_reencryptor;
   if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
   {
@@ -5003,104 +4956,89 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
     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); });
+    keys_reencryptor = epee::misc_utils::create_scope_leave_handler(
+        [&, this, chacha_key]()
+        {
+          m_account.encrypt_keys(chacha_key);
+          m_account.decrypt_viewkey(chacha_key);
+        }
+      );
   }
 
-  // In common multisig scheme there are 4 types of key exchange rounds:
-  // 1. First round is exchange of view secret keys and public spend keys.
-  // 2. Middle round is exchange of derivations: Ki = b * Mj, where b - spend secret key,
-  //    M - public multisig key (in first round it equals to public spend key), K - new public multisig key.
-  // 3. Secret spend establishment round sets your secret multisig keys as follows: kl = H(Ml), where M - is *your* public multisig key,
-  //    k - secret multisig key used to sign transactions. k and M are sets of keys, of course.
-  //    And secret spend key as the sum of all participant's secret multisig keys
-  // 4. Last round establishes multisig wallet's public spend key. Participants exchange their public multisig keys
-  //    and calculate common spend public key as sum of all unique participants' public multisig keys.
-  // Note that N/N scheme has only first round. N-1/N has 2 rounds: first and last. Common M/N has all 4 rounds.
-
-  // IMPORTANT: wallet's public spend key is not equal to secret_spend_key * G!
-  // Wallet's public spend key is the sum of unique public multisig keys of all participants.
-  // secret_spend_key * G = public signer key
-
-  if (threshold == spend_keys.size() + 1)
-  {
-    // In N / N case we only need to do one round and calculate secret multisig keys and new secret spend key
-    MINFO("Creating spend key...");
+  // 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)
+    };
 
-    // Calculates all multisig keys and spend key
-    cryptonote::generate_multisig_N_N(get_account().get_keys(), spend_keys, multisig_keys, spend_skey, spend_pkey);
+  // open initial kex messages, validate them, extract signers
+  std::vector<multisig::multisig_kex_msg> expanded_msgs;
+  std::vector<crypto::public_key> signers;
+  expanded_msgs.reserve(initial_kex_msgs.size());
+  signers.reserve(initial_kex_msgs.size() + 1);
 
-    // Our signer key is b * G, where b is secret spend key.
-    multisig_signers = spend_keys;
-    multisig_signers.push_back(get_multisig_signer_public_key(get_account().get_keys().m_spend_secret_key));
-  }
-  else
+  for (const auto &msg : initial_kex_msgs)
   {
-    // We just got public spend keys of all participants and deriving multisig keys (set of Mi = b * Bi).
-    // note that derivations are public keys as DH exchange suppose it to be
-    auto derivations = cryptonote::generate_multisig_derivations(get_account().get_keys(), spend_keys);
-
-    spend_pkey = rct::identity();
-    multisig_signers = std::vector<crypto::public_key>(spend_keys.size() + 1, crypto::null_pkey);
-
-    if (threshold == spend_keys.size())
-    {
-      // N - 1 / N case
+    expanded_msgs.emplace_back(msg);
 
-      // We need an extra step, so we package all the composite public keys
-      // we know about, and make a signed string out of them
-      MINFO("Creating spend key...");
+    // validate each message
+    // 1. must be 'round 1'
+    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').");
 
-      // Calculating set of our secret multisig keys as follows: mi = H(Mi),
-      // where mi - secret multisig key, Mi - others' participants public multisig key
-      multisig_keys = cryptonote::calculate_multisig_keys(derivations);
+    // 2. duplicate signers not allowed
+    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.");
 
-      // calculating current participant's spend secret key as sum of all secret multisig keys for current participant.
-      // IMPORTANT: participant's secret spend key is not an entire wallet's secret spend!
-      //            Entire wallet's secret spend is sum of all unique secret multisig keys
-      //            among all of participants and is not held by anyone!
-      spend_skey = rct::sk2rct(cryptonote::calculate_multisig_signer_key(multisig_keys));
+    // 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());
+  }
 
-      // Preparing data for the last round to calculate common public spend key. The data contains public multisig keys.
-      extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, secret_keys_to_public_keys(multisig_keys), rct::rct2sk(spend_skey));
-    }
-    else
-    {
-      // M / N case
-      MINFO("Preparing keys for next exchange round...");
+  // add self to signers
+  signers.push_back(multisig_account.get_base_pubkey());
 
-      // Preparing data for middle round - packing new public multisig keys to exchage with others.
-      extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, derivations, m_account.get_keys().m_spend_secret_key);
-      spend_skey = rct::sk2rct(m_account.get_keys().m_spend_secret_key);
+  // intialize key exchange
+  multisig_account.initialize_kex(threshold, signers, expanded_msgs);
+  CHECK_AND_ASSERT_THROW_MES(multisig_account.account_is_active(), "Failed to activate multisig account.");
 
-      // Need to store middle keys to be able to proceed in case of wallet shutdown.
-      m_multisig_derivations = derivations;
-    }
-  }
-  
+  // update wallet state
   if (!m_original_keys_available)
   {
     // 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 = m_account.get_keys().m_account_address;
-    m_original_view_secret_key = m_account.get_keys().m_view_secret_key;
+    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_keys_available = true;
   }
 
   clear();
-  MINFO("Creating view key...");
-  crypto::secret_key view_skey = cryptonote::generate_multisig_view_secret_key(get_account().get_keys().m_view_secret_key, view_keys);
 
+  // account base
   MINFO("Creating multisig address...");
-  CHECK_AND_ASSERT_THROW_MES(m_account.make_multisig(view_skey, rct::rct2sk(spend_skey), rct::rct2pk(spend_pkey), multisig_keys),
-      "Failed to create multisig wallet due to bad keys");
-  memwipe(&spend_skey, sizeof(rct::key));
+  CHECK_AND_ASSERT_THROW_MES(m_account.make_multisig(multisig_account.get_common_privkey(),
+    multisig_account.get_base_privkey(),
+    multisig_account.get_multisig_pubkey(),
+    multisig_account.get_multisig_privkeys()),
+      "Failed to create multisig wallet account due to bad keys");
 
   init_type(hw::device::device_type::SOFTWARE);
   m_original_keys_available = true;
   m_multisig = true;
   m_multisig_threshold = threshold;
-  m_multisig_signers = multisig_signers;
-  ++m_multisig_rounds_passed;
+  m_multisig_signers = signers;
+  m_multisig_rounds_passed = 1;
+
+  // derivations stored (should be empty in last round)
+  // TODO: make use of the origins map for aggregation-style signing (instead of round-robin)
+  m_multisig_derivations.clear();
+  m_multisig_derivations.reserve(multisig_account.get_kex_keys_to_origins_map().size());
+
+  for (const auto &key_to_origins : multisig_account.get_kex_keys_to_origins_map())
+    m_multisig_derivations.push_back(key_to_origins.first);
+
+  // address
+  m_account_public_address.m_spend_public_key = multisig_account.get_multisig_pubkey();
 
   // re-encrypt keys
   keys_reencryptor = epee::misc_utils::auto_scope_leave_caller();
@@ -5113,42 +5051,18 @@ std::string wallet2::make_multisig(const epee::wipeable_string &password,
   if (!m_wallet_file.empty())
     store();
 
-  return extra_multisig_info;
-}
-
-std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &password,
-  const std::vector<std::string> &info)
-{
-  THROW_WALLET_EXCEPTION_IF(info.empty(),
-    error::wallet_internal_error, "Empty multisig info");
-
-  if (info[0].substr(0, MULTISIG_EXTRA_INFO_MAGIC.size()) != MULTISIG_EXTRA_INFO_MAGIC)
-  {
-    THROW_WALLET_EXCEPTION_IF(false,
-      error::wallet_internal_error, "Unsupported info string");
-  }
-
-  std::vector<crypto::public_key> signers;
-  std::unordered_set<crypto::public_key> pkeys;
-
-  THROW_WALLET_EXCEPTION_IF(!unpack_extra_multisig_info(info, signers, pkeys),
-    error::wallet_internal_error, "Bad extra multisig info");
-
-  return exchange_multisig_keys(password, pkeys, signers);
+  return multisig_account.get_next_kex_round_msg();
 }
-
+//----------------------------------------------------------------------------------------------------
 std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &password,
-  std::unordered_set<crypto::public_key> derivations,
-  std::vector<crypto::public_key> signers)
+  const std::vector<std::string> &kex_messages)
 {
-  CHECK_AND_ASSERT_THROW_MES(!derivations.empty(), "empty pkeys");
-  CHECK_AND_ASSERT_THROW_MES(!signers.empty(), "empty signers");
-
-  bool ready = 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.");
 
-  // keys are decrypted
+  // decrypt account keys
   epee::misc_utils::auto_scope_leave_caller keys_reencryptor;
   if (m_ask_password == AskPasswordToDecrypt && !m_unattended && !m_watch_only)
   {
@@ -5156,37 +5070,72 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
     crypto::generate_chacha_key(password.data(), password.size(), chacha_key, m_kdf_rounds);
     m_account.encrypt_viewkey(chacha_key);
     m_account.decrypt_keys(chacha_key);
-    keys_reencryptor = epee::misc_utils::create_scope_leave_handler([&, this, chacha_key]() { m_account.encrypt_keys(chacha_key); m_account.decrypt_viewkey(chacha_key); });
-  }
-
-  if (m_multisig_rounds_passed == multisig_rounds_required(m_multisig_signers.size(), m_multisig_threshold) - 1)
-  {
-    // the last round is passed and we have to calculate spend public key
-    // add ours if not included
-    crypto::public_key local_signer = get_multisig_signer_public_key();
-
-    if (std::find(signers.begin(), signers.end(), local_signer) == signers.end())
-    {
-        signers.push_back(local_signer);
-        for (const auto &msk: get_account().get_multisig_keys())
+    keys_reencryptor = epee::misc_utils::create_scope_leave_handler(
+        [&, this, chacha_key]()
         {
-            derivations.insert(rct::rct2pk(rct::scalarmultBase(rct::sk2rct(msk))));
+          m_account.encrypt_keys(chacha_key);
+          m_account.decrypt_viewkey(chacha_key);
         }
-    }
+      );
+  }
+
+  // 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
+  crypto::public_key dummy;
+  multisig::multisig_account::kex_origins_map_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,
+      dummy,              //common pubkey: not used
+      m_multisig_rounds_passed,
+      std::move(kex_origins_map),
+      ""
+    };
 
-    CHECK_AND_ASSERT_THROW_MES(signers.size() == m_multisig_signers.size(), "Bad signers size");
+  // update multisig kex
+  multisig_account.kex_update(expanded_msgs);
 
-    // Summing all of unique public multisig keys to calculate common public spend key
-    crypto::public_key spend_public_key = cryptonote::generate_multisig_M_N_spend_public_key(std::vector<crypto::public_key>(derivations.begin(), derivations.end()));
-    m_account_public_address.m_spend_public_key = spend_public_key;
-    m_account.finalize_multisig(spend_public_key);
+  // update wallet state
 
-    m_multisig_signers = signers;
-    std::sort(m_multisig_signers.begin(), m_multisig_signers.end(), [](const crypto::public_key &e0, const crypto::public_key &e1){ return memcmp(&e0, &e1, sizeof(e0)) < 0; });
+  // address
+  m_account_public_address.m_spend_public_key = multisig_account.get_multisig_pubkey();
 
-    ++m_multisig_rounds_passed;
-    m_multisig_derivations.clear();
+  // account base
+  CHECK_AND_ASSERT_THROW_MES(m_account.make_multisig(multisig_account.get_common_privkey(),
+    multisig_account.get_base_privkey(),
+    multisig_account.get_multisig_pubkey(),
+    multisig_account.get_multisig_privkeys()),
+      "Failed to update multisig wallet account due to bad keys");
+
+  // derivations stored (should be empty in last round)
+  // TODO: make use of the origins map for aggregation-style signing (instead of round-robin)
+  m_multisig_derivations.clear();
+  m_multisig_derivations.reserve(multisig_account.get_kex_keys_to_origins_map().size());
+
+  for (const auto &key_to_origins : multisig_account.get_kex_keys_to_origins_map())
+    m_multisig_derivations.push_back(key_to_origins.first);
+
+  // rounds passed
+  m_multisig_rounds_passed = multisig_account.get_kex_rounds_complete();
 
+  // why is this necessary? who knows...
+  if (multisig_account.multisig_is_ready())
+  {
     // keys are encrypted again
     keys_reencryptor = epee::misc_utils::auto_scope_leave_caller();
 
@@ -5208,270 +5157,28 @@ std::string wallet2::exchange_multisig_keys(const epee::wipeable_string &passwor
 
     if (!m_wallet_file.empty())
       store();
-
-    return {};
-  }
-
-  // Below are either middle or secret spend key establishment rounds
-
-  for (const auto& key: m_multisig_derivations)
-    derivations.erase(key);
-
-  // Deriving multisig keys (set of Mi = b * Bi) according to DH from other participants' multisig keys.
-  auto new_derivations = cryptonote::generate_multisig_derivations(get_account().get_keys(), std::vector<crypto::public_key>(derivations.begin(), derivations.end()));
-
-  std::string extra_multisig_info;
-  if (m_multisig_rounds_passed == multisig_rounds_required(m_multisig_signers.size(), m_multisig_threshold) - 2) // next round is last
-  {
-    // Next round is last therefore we are performing secret spend establishment round as described above.
-    MINFO("Creating spend key...");
-
-    // Calculating our secret multisig keys by hashing our public multisig keys.
-    auto multisig_keys = cryptonote::calculate_multisig_keys(std::vector<crypto::public_key>(new_derivations.begin(), new_derivations.end()));
-    // And summing it to get personal secret spend key
-    crypto::secret_key spend_skey = cryptonote::calculate_multisig_signer_key(multisig_keys);
-
-    m_account.make_multisig(m_account.get_keys().m_view_secret_key, spend_skey, rct::rct2pk(rct::identity()), multisig_keys);
-
-    // Packing public multisig keys to exchange with others and calculate common public spend key in the last round
-    extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, secret_keys_to_public_keys(multisig_keys), spend_skey);
   }
-  else
-  {
-    // This is just middle round
-    MINFO("Preparing keys for next exchange round...");
-    extra_multisig_info = pack_multisignature_keys(MULTISIG_EXTRA_INFO_MAGIC, new_derivations, m_account.get_keys().m_spend_secret_key);
-    m_multisig_derivations = new_derivations;
-  }
-
-  ++m_multisig_rounds_passed;
 
+  // wallet/file relationship
   if (!m_wallet_file.empty())
     create_keys_file(m_wallet_file, false, password, boost::filesystem::exists(m_wallet_file + ".address.txt"));
 
-  return extra_multisig_info;
-}
-
-void wallet2::unpack_multisig_info(const std::vector<std::string>& info,
-  std::vector<crypto::public_key> &public_keys,
-  std::vector<crypto::secret_key> &secret_keys) const
-{
-  // parse all multisig info
-  public_keys.resize(info.size());
-  secret_keys.resize(info.size());
-  for (size_t i = 0; i < info.size(); ++i)
-  {
-    THROW_WALLET_EXCEPTION_IF(!verify_multisig_info(info[i], secret_keys[i], public_keys[i]),
-        error::wallet_internal_error, "Bad multisig info: " + info[i]);
-  }
-
-  // remove duplicates
-  for (size_t i = 0; i < secret_keys.size(); ++i)
-  {
-    for (size_t j = i + 1; j < secret_keys.size(); ++j)
-    {
-      if (rct::sk2rct(secret_keys[i]) == rct::sk2rct(secret_keys[j]))
-      {
-        MDEBUG("Duplicate key found, ignoring");
-        secret_keys[j] = secret_keys.back();
-        public_keys[j] = public_keys.back();
-        secret_keys.pop_back();
-        public_keys.pop_back();
-        --j;
-      }
-    }
-  }
-
-  // people may include their own, weed it out
-  const crypto::secret_key local_skey = cryptonote::get_multisig_blinded_secret_key(get_account().get_keys().m_view_secret_key);
-  const crypto::public_key local_pkey = get_multisig_signer_public_key(get_account().get_keys().m_spend_secret_key);
-  for (size_t i = 0; i < secret_keys.size(); ++i)
-  {
-    if (secret_keys[i] == local_skey)
-    {
-      MDEBUG("Local key is present, ignoring");
-      secret_keys[i] = secret_keys.back();
-      public_keys[i] = public_keys.back();
-      secret_keys.pop_back();
-      public_keys.pop_back();
-      --i;
-    }
-    else
-    {
-      THROW_WALLET_EXCEPTION_IF(public_keys[i] == local_pkey, error::wallet_internal_error,
-          "Found local spend public key, but not local view secret key - something very weird");
-    }
-  }
+  return multisig_account.get_next_kex_round_msg();
 }
-
-std::string wallet2::make_multisig(const epee::wipeable_string &password,
-  const std::vector<std::string> &info,
-  uint32_t threshold)
-{
-  std::vector<crypto::secret_key> secret_keys(info.size());
-  std::vector<crypto::public_key> public_keys(info.size());
-  unpack_multisig_info(info, public_keys, secret_keys);
-  return make_multisig(password, secret_keys, public_keys, threshold);
-}
-
-bool wallet2::finalize_multisig(const epee::wipeable_string &password, const std::unordered_set<crypto::public_key> &pkeys, std::vector<crypto::public_key> signers)
-{
-  bool ready;
-  uint32_t threshold, total;
-  if (!multisig(&ready, &threshold, &total))
-  {
-    MERROR("This is not a multisig wallet");
-    return false;
-  }
-  if (ready)
-  {
-    MERROR("This multisig wallet is already finalized");
-    return false;
-  }
-  if (threshold + 1 != total)
-  {
-    MERROR("finalize_multisig should only be used for N-1/N wallets, use exchange_multisig_keys instead");
-    return false;
-  }
-  exchange_multisig_keys(password, pkeys, signers);
-  return true;
-}
-
-bool wallet2::unpack_extra_multisig_info(const std::vector<std::string>& info,
-  std::vector<crypto::public_key> &signers,
-  std::unordered_set<crypto::public_key> &pkeys) const
-{
-  // parse all multisig info
-  signers.resize(info.size(), crypto::null_pkey);
-  for (size_t i = 0; i < info.size(); ++i)
-  {
-      if (!verify_extra_multisig_info(info[i], pkeys, signers[i]))
-      {
-          return false;
-      }
-  }
-
-  return true;
-}
-
-bool wallet2::finalize_multisig(const epee::wipeable_string &password, const std::vector<std::string> &info)
-{
-  std::unordered_set<crypto::public_key> public_keys;
-  std::vector<crypto::public_key> signers;
-  if (!unpack_extra_multisig_info(info, signers, public_keys))
-  {
-    MERROR("Bad multisig info");
-    return false;
-  }
-
-  return finalize_multisig(password, public_keys, signers);
-}
-
-std::string wallet2::get_multisig_info() const
-{
-  // It's a signed package of private view key and public spend key
-  const crypto::secret_key skey = cryptonote::get_multisig_blinded_secret_key(get_account().get_keys().m_view_secret_key);
-  const crypto::public_key pkey = get_multisig_signer_public_key(get_account().get_keys().m_spend_secret_key);
-  crypto::hash hash;
-
-  std::string data;
-  data += std::string((const char *)&skey, sizeof(crypto::secret_key));
-  data += std::string((const char *)&pkey, sizeof(crypto::public_key));
-
-  data.resize(data.size() + sizeof(crypto::signature));
-  crypto::cn_fast_hash(data.data(), data.size() - sizeof(signature), hash);
-  crypto::signature &signature = *(crypto::signature*)&data[data.size() - sizeof(crypto::signature)];
-  crypto::generate_signature(hash, pkey, get_multisig_blinded_secret_key(get_account().get_keys().m_spend_secret_key), signature);
-
-  return std::string("MultisigV1") + tools::base58::encode(data);
-}
-
-bool wallet2::verify_multisig_info(const std::string &data, crypto::secret_key &skey, crypto::public_key &pkey)
-{
-  const size_t header_len = strlen("MultisigV1");
-  if (data.size() < header_len || data.substr(0, header_len) != "MultisigV1")
-  {
-    MERROR("Multisig info header check error");
-    return false;
-  }
-  std::string decoded;
-  if (!tools::base58::decode(data.substr(header_len), decoded))
-  {
-    MERROR("Multisig info decoding error");
-    return false;
-  }
-  if (decoded.size() != sizeof(crypto::secret_key) + sizeof(crypto::public_key) + sizeof(crypto::signature))
-  {
-    MERROR("Multisig info is corrupt");
-    return false;
-  }
-
-  size_t offset = 0;
-  skey = *(const crypto::secret_key*)(decoded.data() + offset);
-  offset += sizeof(skey);
-  pkey = *(const crypto::public_key*)(decoded.data() + offset);
-  offset += sizeof(pkey);
-  const crypto::signature &signature = *(const crypto::signature*)(decoded.data() + offset);
-
-  crypto::hash hash;
-  crypto::cn_fast_hash(decoded.data(), decoded.size() - sizeof(signature), hash);
-  if (!crypto::check_signature(hash, pkey, signature))
-  {
-    MERROR("Multisig info signature is invalid");
-    return false;
-  }
-
-  return true;
-}
-
-bool wallet2::verify_extra_multisig_info(const std::string &data, std::unordered_set<crypto::public_key> &pkeys, crypto::public_key &signer)
+//----------------------------------------------------------------------------------------------------
+std::string wallet2::get_multisig_first_kex_msg() const
 {
-  if (data.size() < MULTISIG_EXTRA_INFO_MAGIC.size() || data.substr(0, MULTISIG_EXTRA_INFO_MAGIC.size()) != MULTISIG_EXTRA_INFO_MAGIC)
-  {
-    MERROR("Multisig info header check error");
-    return false;
-  }
-  std::string decoded;
-  if (!tools::base58::decode(data.substr(MULTISIG_EXTRA_INFO_MAGIC.size()), decoded))
-  {
-    MERROR("Multisig info decoding error");
-    return false;
-  }
-  if (decoded.size() < sizeof(crypto::public_key) + sizeof(crypto::signature))
-  {
-    MERROR("Multisig info is corrupt");
-    return false;
-  }
-  if ((decoded.size() - (sizeof(crypto::public_key) + sizeof(crypto::signature))) % sizeof(crypto::public_key))
-  {
-    MERROR("Multisig info is corrupt");
-    return false;
-  }
-
-  const size_t n_keys = (decoded.size() - (sizeof(crypto::public_key) + sizeof(crypto::signature))) / sizeof(crypto::public_key);
-  size_t offset = 0;
-  signer = *(const crypto::public_key*)(decoded.data() + offset);
-  offset += sizeof(signer);
-  const crypto::signature &signature = *(const crypto::signature*)(decoded.data() + offset + n_keys * sizeof(crypto::public_key));
-
-  crypto::hash hash;
-  crypto::cn_fast_hash(decoded.data(), decoded.size() - sizeof(signature), hash);
-  if (!crypto::check_signature(hash, signer, signature))
-  {
-    MERROR("Multisig info signature is invalid");
-    return false;
-  }
-
-  for (size_t n = 0; n < n_keys; ++n)
-  {
-    crypto::public_key mspk = *(const crypto::public_key*)(decoded.data() + offset);
-    pkeys.insert(mspk);
-    offset += sizeof(mspk);
-  }
+  // 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)
+    };
 
-  return true;
+  return multisig_account.get_next_kex_round_msg();
 }
-
+//----------------------------------------------------------------------------------------------------
 bool wallet2::multisig(bool *ready, uint32_t *threshold, uint32_t *total) const
 {
   if (!m_multisig)
@@ -5484,7 +5191,7 @@ bool wallet2::multisig(bool *ready, uint32_t *threshold, uint32_t *total) const
     *ready = !(get_account().get_keys().m_account_address.m_spend_public_key == rct::rct2pk(rct::identity()));
   return true;
 }
-
+//----------------------------------------------------------------------------------------------------
 bool wallet2::has_multisig_partial_key_images() const
 {
   if (!m_multisig)
@@ -5494,7 +5201,7 @@ bool wallet2::has_multisig_partial_key_images() const
       return true;
   return false;
 }
-
+//----------------------------------------------------------------------------------------------------
 bool wallet2::has_unknown_key_images() const
 {
   for (const auto &td: m_transfers)
@@ -13318,13 +13025,6 @@ size_t wallet2::import_outputs_from_str(const std::string &outputs_st)
   return imported_outputs;
 }
 //----------------------------------------------------------------------------------------------------
-crypto::public_key wallet2::get_multisig_signer_public_key(const crypto::secret_key &spend_skey) const
-{
-  crypto::public_key pkey;
-  crypto::secret_key_to_public_key(get_multisig_blinded_secret_key(spend_skey), pkey);
-  return pkey;
-}
-//----------------------------------------------------------------------------------------------------
 crypto::public_key wallet2::get_multisig_signer_public_key() const
 {
   CHECK_AND_ASSERT_THROW_MES(m_multisig, "Wallet is not multisig");
@@ -13368,7 +13068,7 @@ rct::multisig_kLRki wallet2::get_multisig_kLRki(size_t n, const rct::key &k) con
   CHECK_AND_ASSERT_THROW_MES(n < m_transfers.size(), "Bad m_transfers index");
   rct::multisig_kLRki kLRki;
   kLRki.k = k;
-  cryptonote::generate_multisig_LR(m_transfers[n].get_public_key(), rct::rct2sk(kLRki.k), (crypto::public_key&)kLRki.L, (crypto::public_key&)kLRki.R);
+  multisig::generate_multisig_LR(m_transfers[n].get_public_key(), rct::rct2sk(kLRki.k), (crypto::public_key&)kLRki.L, (crypto::public_key&)kLRki.R);
   kLRki.ki = rct::ki2rct(m_transfers[n].m_key_image);
   return kLRki;
 }
@@ -13415,7 +13115,7 @@ crypto::key_image wallet2::get_multisig_composite_key_image(size_t n) const
   for (const auto &info: td.m_multisig_info)
     for (const auto &pki: info.m_partial_key_images)
       pkis.push_back(pki);
-  bool r = cryptonote::generate_multisig_composite_key_image(get_account().get_keys(), m_subaddresses, td.get_public_key(), tx_key, additional_tx_keys, td.m_internal_output_index, pkis, ki);
+  bool r = multisig::generate_multisig_composite_key_image(get_account().get_keys(), m_subaddresses, td.get_public_key(), tx_key, additional_tx_keys, td.m_internal_output_index, pkis, ki);
   THROW_WALLET_EXCEPTION_IF(!r, error::wallet_internal_error, "Failed to generate key image");
   return ki;
 }
@@ -13438,7 +13138,7 @@ cryptonote::blobdata wallet2::export_multisig()
     for (size_t m = 0; m < get_account().get_multisig_keys().size(); ++m)
     {
       // we want to export the partial key image, not the full one, so we can't use td.m_key_image
-      bool r = generate_multisig_key_image(get_account().get_keys(), m, td.get_public_key(), ki);
+      bool r = multisig::generate_multisig_key_image(get_account().get_keys(), m, td.get_public_key(), ki);
       CHECK_AND_ASSERT_THROW_MES(r, "Failed to generate key image");
       info[n].m_partial_key_images.push_back(ki);
     }
diff --git a/src/wallet/wallet2.h b/src/wallet/wallet2.h
index 7648becc8..d64832b13 100644
--- a/src/wallet/wallet2.h
+++ b/src/wallet/wallet2.h
@@ -757,45 +757,20 @@ private:
      * to other participants
      */
     std::string make_multisig(const epee::wipeable_string &password,
-      const std::vector<std::string> &info,
-      uint32_t threshold);
+      const std::vector<std::string> &kex_messages,
+      const std::uint32_t threshold);
     /*!
-     * \brief Creates a multisig wallet
+     * \brief Increment the multisig key exchange round
      * \return empty if done, non empty if we need to send another string
      * to other participants
      */
-    std::string make_multisig(const epee::wipeable_string &password,
-      const std::vector<crypto::secret_key> &view_keys,
-      const std::vector<crypto::public_key> &spend_keys,
-      uint32_t threshold);
     std::string exchange_multisig_keys(const epee::wipeable_string &password,
-      const std::vector<std::string> &info);
-    /*!
-     * \brief Any but first round of keys exchange
-     */
-    std::string exchange_multisig_keys(const epee::wipeable_string &password,
-      std::unordered_set<crypto::public_key> pkeys,
-      std::vector<crypto::public_key> signers);
-    /*!
-     * \brief Finalizes creation of a multisig wallet
-     */
-    bool finalize_multisig(const epee::wipeable_string &password, const std::vector<std::string> &info);
-    /*!
-     * \brief Finalizes creation of a multisig wallet
-     */
-    bool finalize_multisig(const epee::wipeable_string &password, const std::unordered_set<crypto::public_key> &pkeys, std::vector<crypto::public_key> signers);
-    /*!
-     * Get a packaged multisig information string
-     */
-    std::string get_multisig_info() const;
-    /*!
-     * Verifies and extracts keys from a packaged multisig information string
-     */
-    static bool verify_multisig_info(const std::string &data, crypto::secret_key &skey, crypto::public_key &pkey);
+      const std::vector<std::string> &kex_messages);
     /*!
-     * Verifies and extracts keys from a packaged multisig information string
+     * \brief Get initial message to start multisig key exchange (before 'make_multisig()' is called)
+     * \return string to send to other participants
      */
-    static bool verify_extra_multisig_info(const std::string &data, std::unordered_set<crypto::public_key> &pkeys, crypto::public_key &signer);
+    std::string get_multisig_first_kex_msg() const;
     /*!
      * Export multisig info
      * This will generate and remember new k values
@@ -1477,7 +1452,6 @@ private:
     void set_attribute(const std::string &key, const std::string &value);
     bool get_attribute(const std::string &key, std::string &value) const;
 
-    crypto::public_key get_multisig_signer_public_key(const crypto::secret_key &spend_skey) const;
     crypto::public_key get_multisig_signer_public_key() const;
     crypto::public_key get_multisig_signing_public_key(size_t idx) const;
     crypto::public_key get_multisig_signing_public_key(const crypto::secret_key &skey) const;
@@ -1641,12 +1615,6 @@ private:
     bool get_rct_distribution(uint64_t &start_height, std::vector<uint64_t> &distribution);
 
     uint64_t get_segregation_fork_height() const;
-    void unpack_multisig_info(const std::vector<std::string>& info,
-      std::vector<crypto::public_key> &public_keys,
-      std::vector<crypto::secret_key> &secret_keys) const;
-    bool unpack_extra_multisig_info(const std::vector<std::string>& info,
-      std::vector<crypto::public_key> &signers,
-      std::unordered_set<crypto::public_key> &pkeys) const;
 
     void cache_tx_data(const cryptonote::transaction& tx, const crypto::hash &txid, tx_cache_data &tx_cache_data) const;
     std::shared_ptr<std::map<std::pair<uint64_t, uint64_t>, size_t>> create_output_tracker_cache() const;
diff --git a/src/wallet/wallet_rpc_server.cpp b/src/wallet/wallet_rpc_server.cpp
index 4655e24cd..c3e8166b7 100644
--- a/src/wallet/wallet_rpc_server.cpp
+++ b/src/wallet/wallet_rpc_server.cpp
@@ -3938,7 +3938,7 @@ namespace tools
       return false;
     }
 
-    res.multisig_info = m_wallet->get_multisig_info();
+    res.multisig_info = m_wallet->get_multisig_first_kex_msg();
     return true;
   }
   //------------------------------------------------------------------------------------------------------------------------------
@@ -4069,7 +4069,7 @@ namespace tools
     catch (const std::exception &e)
     {
       er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
-      er.message = "Error calling import_multisig";
+      er.message = std::string{"Error calling import_multisig: "} + e.what();
       return false;
     }
 
@@ -4094,53 +4094,7 @@ namespace tools
   //------------------------------------------------------------------------------------------------------------------------------
   bool wallet_rpc_server::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)
   {
-    if (!m_wallet) return not_open(er);
-    if (m_restricted)
-    {
-      er.code = WALLET_RPC_ERROR_CODE_DENIED;
-      er.message = "Command unavailable in restricted mode.";
-      return false;
-    }
-    bool ready;
-    uint32_t threshold, total;
-    if (!m_wallet->multisig(&ready, &threshold, &total))
-    {
-      er.code = WALLET_RPC_ERROR_CODE_NOT_MULTISIG;
-      er.message = "This wallet is not multisig";
-      return false;
-    }
-    if (ready)
-    {
-      er.code = WALLET_RPC_ERROR_CODE_ALREADY_MULTISIG;
-      er.message = "This wallet is multisig, and already finalized";
-      return false;
-    }
-
-    if (req.multisig_info.size() < 1 || req.multisig_info.size() > total)
-    {
-      er.code = WALLET_RPC_ERROR_CODE_THRESHOLD_NOT_REACHED;
-      er.message = "Needs multisig info from more participants";
-      return false;
-    }
-
-    try
-    {
-      if (!m_wallet->finalize_multisig(req.password, req.multisig_info))
-      {
-        er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
-        er.message = "Error calling finalize_multisig";
-        return false;
-      }
-    }
-    catch (const std::exception &e)
-    {
-      er.code = WALLET_RPC_ERROR_CODE_UNKNOWN_ERROR;
-      er.message = std::string("Error calling finalize_multisig: ") + e.what();
-      return false;
-    }
-    res.address = m_wallet->get_account().get_public_address_str(m_wallet->nettype());
-
-    return true;
+    return false;
   }
   //------------------------------------------------------------------------------------------------------------------------------
   bool wallet_rpc_server::on_exchange_multisig_keys(const wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS::request& req, wallet_rpc::COMMAND_RPC_EXCHANGE_MULTISIG_KEYS::response& res, epee::json_rpc::error& er, const connection_context *ctx)
@@ -4168,7 +4122,7 @@ namespace tools
       return false;
     }
 
-    if (req.multisig_info.size() < 1 || req.multisig_info.size() > total)
+    if (req.multisig_info.size() + 1 < total)
     {
       er.code = WALLET_RPC_ERROR_CODE_THRESHOLD_NOT_REACHED;
       er.message = "Needs multisig info from more participants";
diff --git a/src/wallet/wallet_rpc_server_commands_defs.h b/src/wallet/wallet_rpc_server_commands_defs.h
index 248d31aa4..40bcb2c49 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 23
+#define WALLET_RPC_VERSION_MINOR 24
 #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
@@ -2504,24 +2504,17 @@ namespace wallet_rpc
 
   struct COMMAND_RPC_FINALIZE_MULTISIG
   {
+    // NOP
     struct request_t
     {
-      std::string password;
-      std::vector<std::string> multisig_info;
-
       BEGIN_KV_SERIALIZE_MAP()
-        KV_SERIALIZE(password)
-        KV_SERIALIZE(multisig_info)
       END_KV_SERIALIZE_MAP()
     };
     typedef epee::misc_utils::struct_init<request_t> request;
 
     struct response_t
     {
-      std::string address;
-
       BEGIN_KV_SERIALIZE_MAP()
-        KV_SERIALIZE(address)
       END_KV_SERIALIZE_MAP()
     };
     typedef epee::misc_utils::struct_init<response_t> response;