ringct: encode 8 byte amount, saving 24 bytes per output

Found by knaccc

4 files changed, 44 insertions, 11 deletions

diff --git a/src/ringct/rctOps.cpp b/src/ringct/rctOps.cpp
index 0ec654af6..b28aa4fe6 100644
--- a/src/ringct/rctOps.cpp
+++ b/src/ringct/rctOps.cpp
@@ -670,18 +670,38 @@ 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) {
+    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;
+    }
+    static void xor8(key &v, const key &k)
+    {
+      for (int i = 0; i < 8; ++i)
+        v.bytes[i] ^= k.bytes[i];
+    }
+    void ecdhEncode(ecdhTuple & unmasked, const key & sharedSec, bool short_amount) {
         key sharedSec1 = hash_to_scalar(sharedSec);
         key sharedSec2 = hash_to_scalar(sharedSec1);
         //encode
         sc_add(unmasked.mask.bytes, unmasked.mask.bytes, sharedSec1.bytes);
-        sc_add(unmasked.amount.bytes, unmasked.amount.bytes, sharedSec2.bytes);
+        if (short_amount)
+          xor8(unmasked.amount, ecdhHash(sharedSec));
+        else
+          sc_add(unmasked.amount.bytes, unmasked.amount.bytes, sharedSec2.bytes);
     }
-    void ecdhDecode(ecdhTuple & masked, const key & sharedSec) {
+    void ecdhDecode(ecdhTuple & masked, const key & sharedSec, bool short_amount) {
         key sharedSec1 = hash_to_scalar(sharedSec);
         key sharedSec2 = hash_to_scalar(sharedSec1);
         //decode
         sc_sub(masked.mask.bytes, masked.mask.bytes, sharedSec1.bytes);
-        sc_sub(masked.amount.bytes, masked.amount.bytes, sharedSec2.bytes);
+        if (short_amount)
+          xor8(masked.amount, ecdhHash(sharedSec));
+        else
+          sc_sub(masked.amount.bytes, masked.amount.bytes, sharedSec2.bytes);
     }
 }
diff --git a/src/ringct/rctOps.h b/src/ringct/rctOps.h
index 60e920b3a..01cdd6fd7 100644
--- a/src/ringct/rctOps.h
+++ b/src/ringct/rctOps.h
@@ -182,7 +182,7 @@ 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);
-    void ecdhDecode(ecdhTuple & masked, const key & sharedSec);
+    void ecdhEncode(ecdhTuple & unmasked, const key & sharedSec, bool short_amount);
+    void ecdhDecode(ecdhTuple & masked, const key & sharedSec, bool short_amount);
 }
 #endif  /* RCTOPS_H */
diff --git a/src/ringct/rctSigs.cpp b/src/ringct/rctSigs.cpp
index 298afd0d9..6687c91cd 100644
--- a/src/ringct/rctSigs.cpp
+++ b/src/ringct/rctSigs.cpp
@@ -716,7 +716,7 @@ namespace rct {
             //mask amount and mask
             rv.ecdhInfo[i].mask = copy(outSk[i].mask);
             rv.ecdhInfo[i].amount = d2h(amounts[i]);
-            hwdev.ecdhEncode(rv.ecdhInfo[i], amount_keys[i]);
+            hwdev.ecdhEncode(rv.ecdhInfo[i], amount_keys[i], rv.type == RCTTypeBulletproof2);
         }
 
         //set txn fee
@@ -853,7 +853,7 @@ namespace rct {
             //mask amount and mask
             rv.ecdhInfo[i].mask = copy(outSk[i].mask);
             rv.ecdhInfo[i].amount = d2h(outamounts[i]);
-            hwdev.ecdhEncode(rv.ecdhInfo[i], amount_keys[i]);
+            hwdev.ecdhEncode(rv.ecdhInfo[i], amount_keys[i], rv.type == RCTTypeBulletproof2);
         }
             
         //set txn fee
@@ -1151,7 +1151,7 @@ namespace rct {
 
         //mask amount and mask
         ecdhTuple ecdh_info = rv.ecdhInfo[i];
-        hwdev.ecdhDecode(ecdh_info, sk);
+        hwdev.ecdhDecode(ecdh_info, sk, rv.type == RCTTypeBulletproof2);
         mask = ecdh_info.mask;
         key amount = ecdh_info.amount;
         key C = rv.outPk[i].mask;
@@ -1181,7 +1181,7 @@ namespace rct {
 
         //mask amount and mask
         ecdhTuple ecdh_info = rv.ecdhInfo[i];
-        hwdev.ecdhDecode(ecdh_info, sk);
+        hwdev.ecdhDecode(ecdh_info, sk, rv.type == RCTTypeBulletproof2);
         mask = ecdh_info.mask;
         key amount = ecdh_info.amount;
         key C = rv.outPk[i].mask;
diff --git a/src/ringct/rctTypes.h b/src/ringct/rctTypes.h
index 5578a51dc..54fca1d05 100644
--- a/src/ringct/rctTypes.h
+++ b/src/ringct/rctTypes.h
@@ -283,7 +283,20 @@ namespace rct {
             return false;
           for (size_t i = 0; i < outputs; ++i)
           {
-            FIELDS(ecdhInfo[i])
+            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;
+              FIELD(amount);
+              ar.end_object();
+            }
+            else
+            {
+              FIELDS(ecdhInfo[i])
+            }
             if (outputs - i > 1)
               ar.delimit_array();
           }