rct: make the amount key derivable by a third party with the tx key

Scheme design from luigi1114.

1 files changed, 13 insertions, 7 deletions

diff --git a/src/ringct/rctOps.cpp b/src/ringct/rctOps.cpp
index 06267fd8e..b8a0d26ad 100644
--- a/src/ringct/rctOps.cpp
+++ b/src/ringct/rctOps.cpp
@@ -741,22 +741,28 @@ void fe_mul(fe h,const fe f,const fe g)
 
     //Elliptic Curve Diffie Helman: encodes and decodes the amount b and mask a
     // where C= aG + bH
+    void ecdhEncodeFromSharedSecret(ecdhTuple & unmasked, const key & sharedSec1) {
+        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);
+    }
     void ecdhEncode(ecdhTuple & unmasked, const key & receiverPk) {
         key esk;
         //compute shared secret
         skpkGen(esk, unmasked.senderPk);
         key sharedSec1 = hash_to_scalar(scalarmultKey(receiverPk, esk));
+        ecdhEncodeFromSharedSecret(unmasked, sharedSec1);
+    }
+    void ecdhDecodeFromSharedSecret(ecdhTuple & masked, const key & sharedSec1) {
         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);
+        //decode
+        sc_sub(masked.mask.bytes, masked.mask.bytes, sharedSec1.bytes);
+        sc_sub(masked.amount.bytes, masked.amount.bytes, sharedSec2.bytes);
     }
     void ecdhDecode(ecdhTuple & masked, const key & receiverSk) {
         //compute shared secret
         key sharedSec1 = hash_to_scalar(scalarmultKey(masked.senderPk, receiverSk));
-        key sharedSec2 = hash_to_scalar(sharedSec1);
-        //encode
-        sc_sub(masked.mask.bytes, masked.mask.bytes, sharedSec1.bytes);
-        sc_sub(masked.amount.bytes, masked.amount.bytes, sharedSec2.bytes);
+        ecdhDecodeFromSharedSecret(masked, sharedSec1);
     }
 }