Fix issue 4793 - M/N multisig transaction signature

2 files changed, 63 insertions, 18 deletions

diff --git a/src/wallet/wallet2.cpp b/src/wallet/wallet2.cpp
index ebf857b78..8fcecb402 100644
--- a/src/wallet/wallet2.cpp
+++ b/src/wallet/wallet2.cpp
@@ -67,6 +67,7 @@ using namespace epee;
 #include "common/json_util.h"
 #include "memwipe.h"
 #include "common/base58.h"
+#include "common/combinator.h"
 #include "common/dns_utils.h"
 #include "common/notify.h"
 #include "common/perf_timer.h"
@@ -177,6 +178,20 @@ namespace
 
     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())
+      return false;
+
+    for (const auto& e: s1)
+    {
+      if (s2.find(e) != s2.end())
+        return true;
+    }
+
+    return false;
+  }
 }
 
 namespace
@@ -6045,7 +6060,7 @@ bool wallet2::sign_multisig_tx(multisig_tx_set &exported_txs, std::vector<crypto
 
     for (auto &sig: ptx.multisig_sigs)
     {
-      if (sig.ignore != local_signer)
+      if (sig.ignore.find(local_signer) == sig.ignore.end())
       {
         ptx.tx.rct_signatures = sig.sigs;
 
@@ -6079,7 +6094,7 @@ bool wallet2::sign_multisig_tx(multisig_tx_set &exported_txs, std::vector<crypto
       bool found = false;
       for (const auto &sig: ptx.multisig_sigs)
       {
-        if (sig.ignore != local_signer && exported_txs.m_signers.find(sig.ignore) == exported_txs.m_signers.end())
+        if (sig.ignore.find(local_signer) == sig.ignore.end() && !keys_intersect(sig.ignore, exported_txs.m_signers))
         {
           THROW_WALLET_EXCEPTION_IF(found, error::wallet_internal_error, "More than one transaction is final");
           ptx.tx.rct_signatures = sig.sigs;
@@ -7512,30 +7527,56 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
   // if this is a multisig wallet, create a list of multisig signers we can use
   std::deque<crypto::public_key> multisig_signers;
   size_t n_multisig_txes = 0;
+  std::vector<std::unordered_set<crypto::public_key>> ignore_sets;
   if (m_multisig && !m_transfers.empty())
   {
     const crypto::public_key local_signer = get_multisig_signer_public_key();
     size_t n_available_signers = 1;
+
+    // At this step we need to define set of participants available for signature,
+    // i.e. those of them who exchanged with multisig info's
     for (const crypto::public_key &signer: m_multisig_signers)
     {
       if (signer == local_signer)
         continue;
-      multisig_signers.push_front(signer);
       for (const auto &i: m_transfers[0].m_multisig_info)
       {
         if (i.m_signer == signer)
         {
-          multisig_signers.pop_front();
           multisig_signers.push_back(signer);
           ++n_available_signers;
           break;
         }
       }
     }
-    multisig_signers.push_back(local_signer);
+    // n_available_signers includes the transaction creator, but multisig_signers doesn't
     MDEBUG("We can use " << n_available_signers << "/" << m_multisig_signers.size() <<  " other signers");
-    THROW_WALLET_EXCEPTION_IF(n_available_signers+1 < m_multisig_threshold, error::multisig_import_needed);
-    n_multisig_txes = n_available_signers == m_multisig_signers.size() ? m_multisig_threshold : 1;
+    THROW_WALLET_EXCEPTION_IF(n_available_signers < m_multisig_threshold, error::multisig_import_needed);
+    if (n_available_signers > m_multisig_threshold)
+    {
+      // If there more potential signers (those who exchanged with multisig info)
+      // than threshold needed some of them should be skipped since we don't know
+      // who will sign tx and who won't. Hence we don't contribute their LR pairs to the signature.
+
+      // We create as many transactions as many combinations of excluded signers may be.
+      // For example, if we have 2/4 wallet and wallets are: A, B, C and D. Let A be
+      // transaction creator, so we need just 1 signature from set of B, C, D.
+      // Using "excluding" logic here we have to exclude 2-of-3 wallets. Combinations go as follows:
+      // BC, BD, and CD. We save these sets to use later and counting the number of required txs.
+      tools::Combinator<crypto::public_key> c(std::vector<crypto::public_key>(multisig_signers.begin(), multisig_signers.end()));
+      auto ignore_combinations = c.combine(multisig_signers.size() + 1 - m_multisig_threshold);
+      for (const auto& combination: ignore_combinations)
+      {
+        ignore_sets.push_back(std::unordered_set<crypto::public_key>(combination.begin(), combination.end()));
+      }
+
+      n_multisig_txes = ignore_sets.size();
+    }
+    else
+    {
+      // If we have exact count of signers just to fit in threshold we don't exclude anyone and create 1 transaction
+      n_multisig_txes = 1;
+    }
     MDEBUG("We will create " << n_multisig_txes << " txes");
   }
 
@@ -7603,8 +7644,8 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
     src.mask = td.m_mask;
     if (m_multisig)
     {
-      crypto::public_key ignore = m_multisig_threshold == m_multisig_signers.size() ? crypto::null_pkey : multisig_signers.front();
-      src.multisig_kLRki = get_multisig_composite_kLRki(idx, ignore, used_L, used_L);
+      auto ignore_set = ignore_sets.empty() ? std::unordered_set<crypto::public_key>() : ignore_sets.front();
+      src.multisig_kLRki = get_multisig_composite_kLRki(idx, ignore_set, used_L, used_L);
     }
     else
       src.multisig_kLRki = rct::multisig_kLRki({rct::zero(), rct::zero(), rct::zero(), rct::zero()});
@@ -7666,7 +7707,7 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
   std::vector<tools::wallet2::multisig_sig> multisig_sigs;
   if (m_multisig)
   {
-    crypto::public_key ignore = m_multisig_threshold == m_multisig_signers.size() ? crypto::null_pkey : multisig_signers.front();
+    auto ignore = ignore_sets.empty() ? std::unordered_set<crypto::public_key>() : ignore_sets.front();
     multisig_sigs.push_back({tx.rct_signatures, ignore, used_L, std::unordered_set<crypto::public_key>(), msout});
 
     if (m_multisig_threshold < m_multisig_signers.size())
@@ -7674,7 +7715,7 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
       const crypto::hash prefix_hash = cryptonote::get_transaction_prefix_hash(tx);
 
       // create the other versions, one for every other participant (the first one's already done above)
-      for (size_t signer_index = 1; signer_index < n_multisig_txes; ++signer_index)
+      for (size_t ignore_index = 1; ignore_index < ignore_sets.size(); ++ignore_index)
       {
         std::unordered_set<rct::key> new_used_L;
         size_t src_idx = 0;
@@ -7682,7 +7723,7 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
         for(size_t idx: selected_transfers)
         {
           cryptonote::tx_source_entry& src = sources_copy[src_idx];
-          src.multisig_kLRki = get_multisig_composite_kLRki(idx, multisig_signers[signer_index], used_L, new_used_L);
+          src.multisig_kLRki = get_multisig_composite_kLRki(idx, ignore_sets[ignore_index], used_L, new_used_L);
           ++src_idx;
         }
 
@@ -7694,7 +7735,7 @@ void wallet2::transfer_selected_rct(std::vector<cryptonote::tx_destination_entry
         THROW_WALLET_EXCEPTION_IF(!r, error::tx_not_constructed, sources, splitted_dsts, unlock_time, m_nettype);
         THROW_WALLET_EXCEPTION_IF(upper_transaction_weight_limit <= get_transaction_weight(tx), error::tx_too_big, tx, upper_transaction_weight_limit);
         THROW_WALLET_EXCEPTION_IF(cryptonote::get_transaction_prefix_hash(ms_tx) != prefix_hash, error::wallet_internal_error, "Multisig txes do not share prefix");
-        multisig_sigs.push_back({ms_tx.rct_signatures, multisig_signers[signer_index], new_used_L, std::unordered_set<crypto::public_key>(), msout});
+        multisig_sigs.push_back({ms_tx.rct_signatures, ignore_sets[ignore_index], new_used_L, std::unordered_set<crypto::public_key>(), msout});
 
         ms_tx.rct_signatures = tx.rct_signatures;
         THROW_WALLET_EXCEPTION_IF(cryptonote::get_transaction_hash(ms_tx) != cryptonote::get_transaction_hash(tx), error::wallet_internal_error, "Multisig txes differ by more than the signatures");
@@ -11265,7 +11306,7 @@ rct::multisig_kLRki wallet2::get_multisig_kLRki(size_t n, const rct::key &k) con
   return kLRki;
 }
 //----------------------------------------------------------------------------------------------------
-rct::multisig_kLRki wallet2::get_multisig_composite_kLRki(size_t n, const crypto::public_key &ignore, std::unordered_set<rct::key> &used_L, std::unordered_set<rct::key> &new_used_L) const
+rct::multisig_kLRki wallet2::get_multisig_composite_kLRki(size_t n, const std::unordered_set<crypto::public_key> &ignore_set, std::unordered_set<rct::key> &used_L, std::unordered_set<rct::key> &new_used_L) const
 {
   CHECK_AND_ASSERT_THROW_MES(n < m_transfers.size(), "Bad transfer index");
 
@@ -11276,8 +11317,9 @@ rct::multisig_kLRki wallet2::get_multisig_composite_kLRki(size_t n, const crypto
   size_t n_signers_used = 1;
   for (const auto &p: m_transfers[n].m_multisig_info)
   {
-    if (p.m_signer == ignore)
+    if (ignore_set.find(p.m_signer) != ignore_set.end())
       continue;
+
     for (const auto &lr: p.m_LR)
     {
       if (used_L.find(lr.m_L) != used_L.end())
@@ -11336,7 +11378,10 @@ cryptonote::blobdata wallet2::export_multisig()
       info[n].m_partial_key_images.push_back(ki);
     }
 
-    size_t nlr = m_multisig_threshold < m_multisig_signers.size() ? m_multisig_threshold - 1 : 1;
+    // Wallet tries to create as many transactions as many signers combinations. We calculate the maximum number here as follows:
+    // if we have 2/4 wallet with signers: A, B, C, D and A is a transaction creator it will need to pick up 1 signer from 3 wallets left.
+    // That means counting combinations for excluding 2-of-3 wallets (k = total signers count - threshold, n = total signers count - 1).
+    size_t nlr = tools::combinations_count(m_multisig_signers.size() - m_multisig_threshold, m_multisig_signers.size() - 1);
     for (size_t m = 0; m < nlr; ++m)
     {
       td.m_multisig_k.push_back(rct::skGen());
diff --git a/src/wallet/wallet2.h b/src/wallet/wallet2.h
index dbfd45c53..823da27a8 100644
--- a/src/wallet/wallet2.h
+++ b/src/wallet/wallet2.h
@@ -374,7 +374,7 @@ namespace tools
     struct multisig_sig
     {
       rct::rctSig sigs;
-      crypto::public_key ignore;
+      std::unordered_set<crypto::public_key> ignore;
       std::unordered_set<rct::key> used_L;
       std::unordered_set<crypto::public_key> signing_keys;
       rct::multisig_out msout;
@@ -1256,7 +1256,7 @@ namespace tools
     void scan_output(const cryptonote::transaction &tx, const crypto::public_key &tx_pub_key, size_t i, tx_scan_info_t &tx_scan_info, int &num_vouts_received, std::unordered_map<cryptonote::subaddress_index, uint64_t> &tx_money_got_in_outs, std::vector<size_t> &outs);
     void trim_hashchain();
     crypto::key_image get_multisig_composite_key_image(size_t n) const;
-    rct::multisig_kLRki get_multisig_composite_kLRki(size_t n, const crypto::public_key &ignore, std::unordered_set<rct::key> &used_L, std::unordered_set<rct::key> &new_used_L) const;
+    rct::multisig_kLRki get_multisig_composite_kLRki(size_t n,  const std::unordered_set<crypto::public_key> &ignore_set, std::unordered_set<rct::key> &used_L, std::unordered_set<rct::key> &new_used_L) const;
     rct::multisig_kLRki get_multisig_kLRki(size_t n, const rct::key &k) const;
     rct::key get_multisig_k(size_t idx, const std::unordered_set<rct::key> &used_L) const;
     void update_multisig_rescan_info(const std::vector<std::vector<rct::key>> &multisig_k, const std::vector<std::vector<tools::wallet2::multisig_info>> &info, size_t n);