From fa5697127f5a99ddd20311cec8180f6a89b31ceb Mon Sep 17 00:00:00 2001
From: moneromooo-monero <moneromooo-monero@users.noreply.github.com>
Date: Sat, 21 Oct 2017 12:14:31 +0100
Subject: make multisig work with subaddresses

Thanks to kenshi84 for help getting this work
---
 src/cryptonote_basic/cryptonote_format_utils.cpp | 53 ++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 4 deletions(-)

(limited to 'src/cryptonote_basic')

diff --git a/src/cryptonote_basic/cryptonote_format_utils.cpp b/src/cryptonote_basic/cryptonote_format_utils.cpp
index a22c3bdea..8f7ab94db 100644
--- a/src/cryptonote_basic/cryptonote_format_utils.cpp
+++ b/src/cryptonote_basic/cryptonote_format_utils.cpp
@@ -80,6 +80,31 @@ static std::atomic<uint64_t> tx_hashes_cached_count(0);
 static std::atomic<uint64_t> block_hashes_calculated_count(0);
 static std::atomic<uint64_t> block_hashes_cached_count(0);
 
+#define CHECK_AND_ASSERT_THROW_MES_L1(expr, message) {if(!(expr)) {MWARNING(message); throw std::runtime_error(message);}}
+
+namespace cryptonote
+{
+  static inline unsigned char *operator &(ec_point &point) {
+    return &reinterpret_cast<unsigned char &>(point);
+  }
+  static inline const unsigned char *operator &(const ec_point &point) {
+    return &reinterpret_cast<const unsigned char &>(point);
+  }
+
+  // a copy of rct::addKeys, since we can't link to libringct to avoid circular dependencies
+  static void add_public_key(crypto::public_key &AB, const crypto::public_key &A, const crypto::public_key &B) {
+      ge_p3 B2, A2;
+      CHECK_AND_ASSERT_THROW_MES_L1(ge_frombytes_vartime(&B2, &B) == 0, "ge_frombytes_vartime failed at "+boost::lexical_cast<std::string>(__LINE__));
+      CHECK_AND_ASSERT_THROW_MES_L1(ge_frombytes_vartime(&A2, &A) == 0, "ge_frombytes_vartime failed at "+boost::lexical_cast<std::string>(__LINE__));
+      ge_cached tmp2;
+      ge_p3_to_cached(&tmp2, &B2);
+      ge_p1p1 tmp3;
+      ge_add(&tmp3, &A2, &tmp2);
+      ge_p1p1_to_p3(&A2, &tmp3);
+      ge_p3_tobytes(&AB, &A2);
+  }
+}
+
 namespace cryptonote
 {
   //---------------------------------------------------------------
@@ -182,6 +207,7 @@ namespace cryptonote
       crypto::derive_secret_key(recv_derivation, real_output_index, ack.m_spend_secret_key, scalar_step1); // computes Hs(a*R || idx) + b
 
       // step 2: add Hs(a || index_major || index_minor)
+      crypto::secret_key subaddr_sk;
       crypto::secret_key scalar_step2;
       if (received_index.is_zero())
       {
@@ -189,13 +215,32 @@ namespace cryptonote
       }
       else
       {
-        crypto::secret_key m = get_subaddress_secret_key(ack.m_view_secret_key, received_index);
-        sc_add((unsigned char*)&scalar_step2, (unsigned char*)&scalar_step1, (unsigned char*)&m);
+        subaddr_sk = get_subaddress_secret_key(ack.m_view_secret_key, received_index);
+        sc_add((unsigned char*)&scalar_step2, (unsigned char*)&scalar_step1, (unsigned char*)&subaddr_sk);
       }
 
       in_ephemeral.sec = scalar_step2;
-      crypto::secret_key_to_public_key(in_ephemeral.sec, in_ephemeral.pub);
-      CHECK_AND_ASSERT_MES(in_ephemeral.pub == out_key, false, "key image helper precomp: given output pubkey doesn't match the derived one");
+
+      if (ack.m_multisig_keys.empty())
+      {
+        // when not in multisig, we know the full spend secret key, so the output pubkey can be obtained by scalarmultBase
+        CHECK_AND_ASSERT_MES(crypto::secret_key_to_public_key(in_ephemeral.sec, in_ephemeral.pub), false, "Failed to derive public key");
+      }
+      else
+      {
+        // when in multisig, we only know the partial spend secret key. but we do know the full spend public key, so the output pubkey can be obtained by using the standard CN key derivation
+        CHECK_AND_ASSERT_MES(crypto::derive_public_key(recv_derivation, real_output_index, ack.m_account_address.m_spend_public_key, in_ephemeral.pub), false, "Failed to derive public key");
+        // and don't forget to add the contribution from the subaddress part
+        if (!received_index.is_zero())
+        {
+          crypto::public_key subaddr_pk;
+          CHECK_AND_ASSERT_MES(crypto::secret_key_to_public_key(subaddr_sk, subaddr_pk), false, "Failed to derive public key");
+          add_public_key(in_ephemeral.pub, in_ephemeral.pub, subaddr_pk);
+        }
+      }
+
+      CHECK_AND_ASSERT_MES(in_ephemeral.pub == out_key,
+           false, "key image helper precomp: given output pubkey doesn't match the derived one");
     }
 
     crypto::generate_key_image(in_ephemeral.pub, in_ephemeral.sec, ki);
-- 
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