ringct: the commitment mask is now deterministic

saves space in the tx and is safe

Found by knaccc

4 files changed, 51 insertions, 26 deletions

diff --git a/src/ringct/rctOps.cpp b/src/ringct/rctOps.cpp
index b28aa4fe6..e39ba16fd 100644
--- a/src/ringct/rctOps.cpp
+++ b/src/ringct/rctOps.cpp
@@ -672,36 +672,56 @@ namespace rct {
     // where C= aG + bH
     static key ecdhHash(const key &k)
     {
-      char data[38];
-      rct::key hash;
-      memcpy(data, "amount", 6);
-      memcpy(data + 6, &k, sizeof(k));
-      cn_fast_hash(hash, data, sizeof(data));
-      return hash;
+        char data[38];
+        rct::key hash;
+        memcpy(data, "amount", 6);
+        memcpy(data + 6, &k, sizeof(k));
+        cn_fast_hash(hash, data, sizeof(data));
+        return hash;
     }
     static void xor8(key &v, const key &k)
     {
-      for (int i = 0; i < 8; ++i)
-        v.bytes[i] ^= k.bytes[i];
+        for (int i = 0; i < 8; ++i)
+            v.bytes[i] ^= k.bytes[i];
+    }
+    key genCommitmentMask(const key &sk)
+    {
+        char data[15 + sizeof(key)];
+        memcpy(data, "commitment_mask", 15);
+        memcpy(data + 15, &sk, sizeof(sk));
+        key scalar;
+        hash_to_scalar(scalar, data, sizeof(data));
+        return scalar;
     }
-    void ecdhEncode(ecdhTuple & unmasked, const key & sharedSec, bool short_amount) {
-        key sharedSec1 = hash_to_scalar(sharedSec);
-        key sharedSec2 = hash_to_scalar(sharedSec1);
+
+    void ecdhEncode(ecdhTuple & unmasked, const key & sharedSec, bool v2) {
         //encode
-        sc_add(unmasked.mask.bytes, unmasked.mask.bytes, sharedSec1.bytes);
-        if (short_amount)
+        if (v2)
+        {
+          unmasked.mask = zero();
           xor8(unmasked.amount, ecdhHash(sharedSec));
+        }
         else
+        {
+          key sharedSec1 = hash_to_scalar(sharedSec);
+          key sharedSec2 = hash_to_scalar(sharedSec1);
+          sc_add(unmasked.mask.bytes, unmasked.mask.bytes, sharedSec1.bytes);
           sc_add(unmasked.amount.bytes, unmasked.amount.bytes, sharedSec2.bytes);
+        }
     }
-    void ecdhDecode(ecdhTuple & masked, const key & sharedSec, bool short_amount) {
-        key sharedSec1 = hash_to_scalar(sharedSec);
-        key sharedSec2 = hash_to_scalar(sharedSec1);
+    void ecdhDecode(ecdhTuple & masked, const key & sharedSec, bool v2) {
         //decode
-        sc_sub(masked.mask.bytes, masked.mask.bytes, sharedSec1.bytes);
-        if (short_amount)
+        if (v2)
+        {
+          masked.mask = genCommitmentMask(sharedSec);
           xor8(masked.amount, ecdhHash(sharedSec));
+        }
         else
+        {
+          key sharedSec1 = hash_to_scalar(sharedSec);
+          key sharedSec2 = hash_to_scalar(sharedSec1);
+          sc_sub(masked.mask.bytes, masked.mask.bytes, sharedSec1.bytes);
           sc_sub(masked.amount.bytes, masked.amount.bytes, sharedSec2.bytes);
+        }
     }
 }
diff --git a/src/ringct/rctOps.h b/src/ringct/rctOps.h
index 01cdd6fd7..dd6d87593 100644
--- a/src/ringct/rctOps.h
+++ b/src/ringct/rctOps.h
@@ -182,7 +182,8 @@ namespace rct {
 
     //Elliptic Curve Diffie Helman: encodes and decodes the amount b and mask a
     // where C= aG + bH
-    void ecdhEncode(ecdhTuple & unmasked, const key & sharedSec, bool short_amount);
-    void ecdhDecode(ecdhTuple & masked, const key & sharedSec, bool short_amount);
+    key genCommitmentMask(const key &sk);
+    void ecdhEncode(ecdhTuple & unmasked, const key & sharedSec, bool v2);
+    void ecdhDecode(ecdhTuple & masked, const key & sharedSec, bool v2);
 }
 #endif  /* RCTOPS_H */
diff --git a/src/ringct/rctSigs.cpp b/src/ringct/rctSigs.cpp
index 6687c91cd..81bec487c 100644
--- a/src/ringct/rctSigs.cpp
+++ b/src/ringct/rctSigs.cpp
@@ -79,9 +79,12 @@ namespace
 }
 
 namespace rct {
-    Bulletproof proveRangeBulletproof(keyV &C, keyV &masks, const std::vector<uint64_t> &amounts)
+    Bulletproof proveRangeBulletproof(keyV &C, keyV &masks, const std::vector<uint64_t> &amounts, epee::span<const key> sk)
     {
-        masks = rct::skvGen(amounts.size());
+        CHECK_AND_ASSERT_THROW_MES(amounts.size() == sk.size(), "Invalid amounts/sk sizes");
+        masks.resize(amounts.size());
+        for (size_t i = 0; i < masks.size(); ++i)
+            masks[i] = genCommitmentMask(sk[i]);
         Bulletproof proof = bulletproof_PROVE(amounts, masks);
         CHECK_AND_ASSERT_THROW_MES(proof.V.size() == amounts.size(), "V does not have the expected size");
         C = proof.V;
@@ -803,7 +806,8 @@ namespace rct {
                 }
                 else
                 {
-                    rv.p.bulletproofs.push_back(proveRangeBulletproof(C, masks, outamounts));
+                    const epee::span<const key> keys{&amount_keys[0], amount_keys.size()};
+                    rv.p.bulletproofs.push_back(proveRangeBulletproof(C, masks, outamounts, keys));
                     #ifdef DBG
                     CHECK_AND_ASSERT_THROW_MES(verBulletproof(rv.p.bulletproofs.back()), "verBulletproof failed on newly created proof");
                     #endif
@@ -831,7 +835,8 @@ namespace rct {
                 }
                 else
                 {
-                    rv.p.bulletproofs.push_back(proveRangeBulletproof(C, masks, batch_amounts));
+                    const epee::span<const key> keys{&amount_keys[amounts_proved], batch_size};
+                    rv.p.bulletproofs.push_back(proveRangeBulletproof(C, masks, batch_amounts, keys));
                 #ifdef DBG
                     CHECK_AND_ASSERT_THROW_MES(verBulletproof(rv.p.bulletproofs.back()), "verBulletproof failed on newly created proof");
                 #endif
diff --git a/src/ringct/rctTypes.h b/src/ringct/rctTypes.h
index 54fca1d05..a02c338e9 100644
--- a/src/ringct/rctTypes.h
+++ b/src/ringct/rctTypes.h
@@ -128,7 +128,7 @@ namespace rct {
         key senderPk;
 
         BEGIN_SERIALIZE_OBJECT()
-          FIELD(mask)
+          FIELD(mask) // not saved from v2 BPs
           FIELD(amount)
           // FIELD(senderPk) // not serialized, as we do not use it in monero currently
         END_SERIALIZE()
@@ -286,7 +286,6 @@ namespace rct {
             if (type == RCTTypeBulletproof2)
             {
               ar.begin_object();
-              FIELD_N("mask", ecdhInfo[i].mask);
               if (!typename Archive<W>::is_saving())
                 memset(ecdhInfo[i].amount.bytes, 0, sizeof(ecdhInfo[i].amount.bytes));
               crypto::hash8 &amount = (crypto::hash8&)ecdhInfo[i].amount;