Arbitrary M/N multisig schemes:

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

2 files changed, 66 insertions, 2 deletions

diff --git a/src/multisig/multisig.cpp b/src/multisig/multisig.cpp
index a0a788b7d..33d0a312f 100644
--- a/src/multisig/multisig.cpp
+++ b/src/multisig/multisig.cpp
@@ -84,6 +84,43 @@ namespace cryptonote
     }
   }
   //-----------------------------------------------------------------
+  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)
   {
     rct::key view_skey = rct::sk2rct(get_multisig_blinded_secret_key(skey));
@@ -92,7 +129,7 @@ namespace cryptonote
     return rct::rct2sk(view_skey);
   }
   //-----------------------------------------------------------------
-  crypto::public_key generate_multisig_N1_N_spend_public_key(const std::vector<crypto::public_key> &pkeys)
+  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)
@@ -141,4 +178,9 @@ namespace cryptonote
     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;
+  }
 }
diff --git a/src/multisig/multisig.h b/src/multisig/multisig.h
index f95611441..93a756812 100644
--- a/src/multisig/multisig.h
+++ b/src/multisig/multisig.h
@@ -41,9 +41,31 @@ namespace cryptonote
   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);
-  crypto::public_key generate_multisig_N1_N_spend_public_key(const std::vector<crypto::public_key> &pkeys);
+  /**
+   * @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);
 }