rct: avoid the need for the last II element

This element is used in the generation of the MLSAG, but isn't
needed in verification.
Also misc changes in the cryptonote code to match, by mooo.

src/cryptonote_core/blockchain.cpp

@@ -2475,12 +2475,12 @@ bool Blockchain::check_tx_inputs(const transaction& tx, tx_verification_context
       const rct::ctkeyM &mixRing = tx.rct_signatures.mixRing.empty() ? reconstructed_mixRing : tx.rct_signatures.mixRing;
       // always do II, because it's split in the simple version, and always do outPk
 
-      // all MGs should have the same II size (1)
+      // all MGs should have empty II
       for (size_t n = 0; n < tx.rct_signatures.MGs.size(); ++n)
       {
-        if (tx.rct_signatures.MGs[n].II.size() != 1)
+        if (tx.rct_signatures.MGs[n].II.size() != 0)
         {
-          LOG_PRINT_L1("Failed to check ringct signatures: mismatched MGs II sizes");
+          LOG_PRINT_L1("Failed to check ringct signatures: non empty MGs II");
           return false;
         }
       }
@@ -2489,7 +2489,6 @@ bool Blockchain::check_tx_inputs(const transaction& tx, tx_verification_context
       for (size_t n = 0; n < tx.vin.size(); ++n)
       {
         reconstructed_II[n].push_back(rct::ki2rct(boost::get<txin_to_key>(tx.vin[n]).k_image));
-        reconstructed_II[n].push_back(tx.rct_signatures.MGs[n].II[0]);
       }
 
       if (tx.rct_signatures.outPk.size() != tx.vout.size())
@@ -2579,7 +2578,7 @@ bool Blockchain::check_tx_inputs(const transaction& tx, tx_verification_context
       // if the tx already has a non empty mixRing and/or II, use them,
       // else reconstruct them. Always do outPk.
       const rct::ctkeyM &mixRing = tx.rct_signatures.mixRing.empty() ? reconstructed_mixRing : tx.rct_signatures.mixRing;
-      const rct::keyV &II = tx.rct_signatures.MG.II.size() == 1 ? reconstructed_II : tx.rct_signatures.MG.II;
+      const rct::keyV &II = tx.rct_signatures.MG.II.empty() ? reconstructed_II : tx.rct_signatures.MG.II;
       const rct::ctkeyV outPk = reconstructed_outPk;
 
       // RCT needs the same mixin for all inputs
@@ -2604,14 +2603,13 @@ bool Blockchain::check_tx_inputs(const transaction& tx, tx_verification_context
         }
       }
 
-      if (tx.rct_signatures.MG.II.size() == 1)
+      if (tx.rct_signatures.MG.II.empty())
       {
         reconstructed_II.resize(tx.vin.size());
         for (size_t n = 0; n < tx.vin.size(); ++n)
         {
           reconstructed_II[n] = rct::ki2rct(boost::get<txin_to_key>(tx.vin[n]).k_image);
         }
-        reconstructed_II.push_back(tx.rct_signatures.MG.II.back());
       }
 
       if (tx.rct_signatures.outPk.size() != tx.vout.size())
@@ -2657,7 +2655,7 @@ bool Blockchain::check_tx_inputs(const transaction& tx, tx_verification_context
         }
       }
 
-      if (II.size() != 1 + tx.vin.size())
+      if (II.size() != tx.vin.size())
       {
         LOG_PRINT_L1("Failed to check ringct signatures: mismatched II/vin sizes");
         return false;

src/cryptonote_core/cryptonote_boost_serialization.h

@@ -209,19 +209,12 @@ namespace boost
     a & x.s;
   }
 
-  inline void serialize(boost::archive::binary_iarchive &a, rct::mgSig &x, const boost::serialization::version_type ver)
+  template <class Archive>
+  inline void serialize(Archive &a, rct::mgSig &x, const boost::serialization::version_type ver)
   {
     a & x.ss;
     a & x.cc;
-    x.II.resize(1);
+    // a & x.II; // not serialized, we can recover it from the tx vin
-    a & x.II[0];
-  }
-
-  inline void serialize(boost::archive::binary_oarchive &a, rct::mgSig &x, const boost::serialization::version_type ver)
-  {
-    a & x.ss;
-    a & x.cc;
-    a & x.II.back();
   }
 
   template <class Archive>

src/ringct/rctSigs.cpp

@@ -148,7 +148,7 @@ namespace rct {
     // Gen creates a signature which proves that for some column in the keymatrix "pk"
     //   the signer knows a secret key for each row in that column
     // Ver verifies that the MG sig was created correctly        
-    mgSig MLSAG_Gen(key message, const keyM & pk, const keyV & xx, const unsigned int index) {
+    mgSig MLSAG_Gen(key message, const keyM & pk, const keyV & xx, const unsigned int index, size_t dsRows) {
         mgSig rv;
         size_t cols = pk.size();
         CHECK_AND_ASSERT_THROW_MES(cols >= 2, "Error! What is c if cols = 1!");
@@ -159,20 +159,21 @@ namespace rct {
           CHECK_AND_ASSERT_THROW_MES(pk[i].size() == rows, "pk is not rectangular");
         }
         CHECK_AND_ASSERT_THROW_MES(xx.size() == rows, "Bad xx size");
+        CHECK_AND_ASSERT_THROW_MES(dsRows <= rows, "Bad dsRows size");
 
-        size_t i = 0, j = 0;
+        size_t i = 0, j = 0, ii = 0;
         key c, c_old, L, R, Hi;
         sc_0(c_old.bytes);
-        vector<geDsmp> Ip(rows);
+        vector<geDsmp> Ip(dsRows);
-        rv.II = keyV(rows);
+        rv.II = keyV(dsRows);
-        rv.ss = keyM(cols, rv.II);
         keyV alpha(rows);
         keyV aG(rows);
-        keyV aHP(rows);
+        rv.ss = keyM(cols, aG);
-        keyV toHash(1 + 3 * rows);
+        keyV aHP(dsRows);
+        keyV toHash(1 + 3 * dsRows + 2 * (rows - dsRows));
         toHash[0] = message;
         DP("here1");
-        for (i = 0; i < rows; i++) {
+        for (i = 0; i < dsRows; i++) {
             skpkGen(alpha[i], aG[i]); //need to save alphas for later..
             Hi = hashToPoint(pk[index][i]);
             aHP[i] = scalarmultKey(Hi, alpha[i]);
@@ -182,6 +183,13 @@ namespace rct {
             rv.II[i] = scalarmultKey(Hi, xx[i]);
             precomp(Ip[i].k, rv.II[i]);
         }
+        size_t ndsRows = 3 * dsRows; //non Double Spendable Rows (see identity chains paper)
+        for (i = dsRows, ii = 0 ; i < rows ; i++, ii++) {
+            skpkGen(alpha[i], aG[i]); //need to save alphas for later..
+            toHash[ndsRows + 2 * ii + 1] = pk[index][i];
+            toHash[ndsRows + 2 * ii + 2] = aG[i];
+        }
+
         c_old = hash_to_scalar(toHash);
 
         
@@ -193,7 +201,7 @@ namespace rct {
 
             rv.ss[i] = skvGen(rows);            
             sc_0(c.bytes);
-            for (j = 0; j < rows; j++) {
+            for (j = 0; j < dsRows; j++) {
                 addKeys2(L, rv.ss[i][j], c_old, pk[i][j]);
                 hashToPoint(Hi, pk[i][j]);
                 addKeys3(R, rv.ss[i][j], Hi, c_old, Ip[j].k);
@@ -201,6 +209,11 @@ namespace rct {
                 toHash[3 * j + 2] = L; 
                 toHash[3 * j + 3] = R;
             }
+            for (j = dsRows, ii = 0; j < rows; j++, ii++) {
+                addKeys2(L, rv.ss[i][j], c_old, pk[i][j]);
+                toHash[ndsRows + 2 * ii + 1] = pk[i][j];
+                toHash[ndsRows + 2 * ii + 2] = L;
+            }
             c = hash_to_scalar(toHash);
             copy(c_old, c);
             i = (i + 1) % cols;
@@ -224,7 +237,7 @@ namespace rct {
     // Gen creates a signature which proves that for some column in the keymatrix "pk"
     //   the signer knows a secret key for each row in that column
     // Ver verifies that the MG sig was created correctly            
-    bool MLSAG_Ver(key message, const keyM & pk, const mgSig & rv, const keyV &II) {
+    bool MLSAG_Ver(key message, const keyM & pk, const mgSig & rv, const keyV &II, size_t dsRows) {
 
         size_t cols = pk.size();
         CHECK_AND_ASSERT_MES(cols >= 2, false, "Error! What is c if cols = 1!");
@@ -233,25 +246,27 @@ namespace rct {
         for (size_t i = 1; i < cols; ++i) {
           CHECK_AND_ASSERT_MES(pk[i].size() == rows, false, "pk is not rectangular");
         }
-        CHECK_AND_ASSERT_MES(II.size() == rows, false, "Bad II size");
+        CHECK_AND_ASSERT_MES(II.size() == dsRows, false, "Bad II size");
         CHECK_AND_ASSERT_MES(rv.ss.size() == cols, false, "Bad rv.ss size");
         for (size_t i = 0; i < cols; ++i) {
           CHECK_AND_ASSERT_MES(rv.ss[i].size() == rows, false, "rv.ss is not rectangular");
         }
+        CHECK_AND_ASSERT_MES(dsRows <= rows, false, "Bad dsRows value");
 
-        size_t i = 0, j = 0;
+        size_t i = 0, j = 0, ii = 0;
         key c,  L, R, Hi;
         key c_old = copy(rv.cc);
-        vector<geDsmp> Ip(rows);
+        vector<geDsmp> Ip(dsRows);
-        for (i= 0 ; i< rows ; i++) {
+        for (i = 0 ; i < dsRows ; i++) {
             precomp(Ip[i].k, II[i]);
         }
-        keyV toHash(1 + 3 * rows);
+        size_t ndsRows = 3 * dsRows; //non Double Spendable Rows (see identity chains paper
+        keyV toHash(1 + 3 * dsRows + 2 * (rows - dsRows));
         toHash[0] = message;
         i = 0;
         while (i < cols) {
             sc_0(c.bytes);
-            for (j = 0; j < rows; j++) {
+            for (j = 0; j < dsRows; j++) {
                 addKeys2(L, rv.ss[i][j], c_old, pk[i][j]);
                 hashToPoint(Hi, pk[i][j]);
                 addKeys3(R, rv.ss[i][j], Hi, c_old, Ip[j].k);
@@ -259,6 +274,11 @@ namespace rct {
                 toHash[3 * j + 2] = L; 
                 toHash[3 * j + 3] = R;
             }
+            for (j = dsRows, ii = 0 ; j < rows ; j++, ii++) {
+                addKeys2(L, rv.ss[i][j], c_old, pk[i][j]);
+                toHash[ndsRows + 2 * ii + 1] = pk[i][j];
+                toHash[ndsRows + 2 * ii + 2] = L;
+            }
             c = hash_to_scalar(toHash);
             copy(c_old, c);
             i = (i + 1);
@@ -376,7 +396,7 @@ namespace rct {
         ctkeyV signed_data = outPk;
         signed_data.push_back(ctkey({message, identity()}));
         key msg = cn_fast_hash(signed_data);
-        return MLSAG_Gen(msg, M, sk, index);
+        return MLSAG_Gen(msg, M, sk, index, rows);
     }
 
 
@@ -403,7 +423,7 @@ namespace rct {
             sk[0] = copy(inSk.dest);
             sc_sub(sk[1].bytes, inSk.mask.bytes, a.bytes);  
         }
-        return MLSAG_Gen(message, M, sk, index);
+        return MLSAG_Gen(message, M, sk, index, rows);
     }
 
 
@@ -451,7 +471,7 @@ namespace rct {
         key msg = cn_fast_hash(signed_data);
         DP("message:");
         DP(msg);
-        return MLSAG_Ver(msg, M, mg, II);
+        return MLSAG_Ver(msg, M, mg, II, rows);
     }
 
     //Ring-ct Simple MG sigs
@@ -472,7 +492,7 @@ namespace rct {
                     subKeys(M[i][1], pubs[i].mask, C);
             }
             //DP(C);
-            return MLSAG_Ver(message, M, mg, II);
+            return MLSAG_Ver(message, M, mg, II, rows);
     }
 
     //These functions get keys from blockchain
@@ -729,7 +749,7 @@ namespace rct {
         {
           for (size_t n = 0; n < II->size(); ++n)
           {
-            CHECK_AND_ASSERT_MES((*II)[n].size() == 2, false, "Bad II size");
+            CHECK_AND_ASSERT_MES((*II)[n].size() == 1, false, "Bad II size");
           }
         }
 

src/ringct/rctSigs.h

@@ -90,8 +90,8 @@ namespace rct {
     //   the signer knows a secret key for each row in that column
     // Ver verifies that the MG sig was created correctly
     keyV keyImageV(const keyV &xx);
-    mgSig MLSAG_Gen(key message, const keyM & pk, const keyV & xx, const unsigned int index);
+    mgSig MLSAG_Gen(key message, const keyM & pk, const keyV & xx, const unsigned int index, size_t dsRows);
-    bool MLSAG_Ver(key message, const keyM &pk, const mgSig &sig, const keyV &II);
+    bool MLSAG_Ver(key message, const keyM &pk, const mgSig &sig, const keyV &II, size_t dsRows);
     //mgSig MLSAG_Gen_Old(const keyM & pk, const keyV & xx, const int index);
 
     //proveRange and verRange

src/ringct/rctTypes.h

@@ -148,16 +148,7 @@ namespace rct {
         BEGIN_SERIALIZE_OBJECT()
             FIELD(ss)
             FIELD(cc)
-            if (II.size() == 0) {
+            // FIELD(II) - not serialized, it can be reconstructed
-              // loading
-              FIELD(II)
-            }
-            else {
-              // saving
-              keyV II;
-              II.push_back(this->II.back());
-              FIELD(II)
-            }
         END_SERIALIZE()
     };
     //contains the data for an asnl sig

tests/unit_tests/serialization.cpp

@@ -582,8 +582,7 @@ TEST(Serialization, serializes_ringct_types)
   ASSERT_TRUE(mg0.cc == mg1.cc);
 
   // mixRing and II are not serialized, they are meant to be reconstructed
-  ASSERT_TRUE(mg1.II.size() == 1);
+  ASSERT_TRUE(mg1.II.empty());
-  ASSERT_TRUE(mg1.II[0] == mg0.II.back());
 
   rg0 = s0.rangeSigs.front();
   ASSERT_TRUE(serialization::dump_binary(rg0, blob));
@@ -605,8 +604,7 @@ TEST(Serialization, serializes_ringct_types)
   }
   ASSERT_TRUE(s0.MG.cc == s1.MG.cc);
   // mixRing and II are not serialized, they are meant to be reconstructed
-  ASSERT_TRUE(s1.MG.II.size() == 1);
+  ASSERT_TRUE(s1.MGs[0].II.empty());
-  ASSERT_TRUE(s1.MG.II[0] == s0.MG.II.back());
 
   // mixRing and II are not serialized, they are meant to be reconstructed
   ASSERT_TRUE(s1.mixRing.size() == 0);