ringct: new {gen,decode}Rct APIs for convenience

A new version of genRct takes the mixRing as parameter, instead
of the inPk. inPk are part of the mixRing, and it is cleaner to
pass the mixRing data than to fetch it from the RingCT code.

A new version of decodeRct also returns the mask.

Also, failure to decode throws, so errors are properly detected.

src/ringct/rctSigs.cpp

@@ -466,11 +466,13 @@ namespace rct {
     //decodeRct: (c.f. http://eprint.iacr.org/2015/1098 section 5.1.1)
     //   uses the attached ecdh info to find the amounts represented by each output commitment 
     //   must know the destination private key to find the correct amount, else will return a random number
-    rctSig genRct(const ctkeyV & inSk, const ctkeyV  & inPk, const keyV & destinations, const vector<xmr_amount> amounts, const int mixin) {
-        CHECK_AND_ASSERT_THROW_MES(mixin >= 0, "Mixin must be positive");
+    rctSig genRct(const ctkeyV & inSk, const keyV & destinations, const vector<xmr_amount> amounts, const ctkeyM &mixRing, unsigned int index) {
         CHECK_AND_ASSERT_THROW_MES(amounts.size() > 0, "Amounts must not be empty");
-        CHECK_AND_ASSERT_THROW_MES(inSk.size() == inPk.size(), "Different number of public/private keys");
         CHECK_AND_ASSERT_THROW_MES(amounts.size() == destinations.size(), "Different number of amounts/destinations");
+        CHECK_AND_ASSERT_THROW_MES(index < mixRing.size(), "Bad index into mixRing");
+        for (size_t n = 0; n < mixRing.size(); ++n) {
+          CHECK_AND_ASSERT_THROW_MES(mixRing[n].size() == inSk.size(), "Bad mixRing size");
+        }
 
         rctSig rv;
         rv.outPk.resize(destinations.size());
@@ -496,11 +498,18 @@ namespace rct {
 
         }
 
-        unsigned int index;
-        tie(rv.mixRing, index) = populateFromBlockchain(inPk, mixin);
+        rv.mixRing = mixRing;
         rv.MG = proveRctMG(rv.mixRing, inSk, outSk, rv.outPk, index);
+        if (!verRctMG(rv.MG, rv.mixRing, rv.outPk)) { printf("proveRctMG genreated bad data\n"); }
         return rv;
     }
+
+    rctSig genRct(const ctkeyV & inSk, const ctkeyV  & inPk, const keyV & destinations, const vector<xmr_amount> amounts, const int mixin) {
+        unsigned int index;
+        ctkeyM mixRing;
+        tie(mixRing, index) = populateFromBlockchain(inPk, mixin);
+        return genRct(inSk, destinations, amounts, mixRing, index);
+    }
     
     //RingCT protocol
     //genRct: 
@@ -542,15 +551,16 @@ namespace rct {
     //decodeRct: (c.f. http://eprint.iacr.org/2015/1098 section 5.1.1)
     //   uses the attached ecdh info to find the amounts represented by each output commitment 
     //   must know the destination private key to find the correct amount, else will return a random number    
-    xmr_amount decodeRct(rctSig & rv, const key & sk, unsigned int i) {
+    xmr_amount decodeRct(const rctSig & rv, const key & sk, unsigned int i, key & mask) {
         CHECK_AND_ASSERT_THROW_MES(rv.rangeSigs.size() > 0, "Empty rv.rangeSigs");
         CHECK_AND_ASSERT_THROW_MES(rv.outPk.size() == rv.rangeSigs.size(), "Mismatched sizes of rv.outPk and rv.rangeSigs");
         CHECK_AND_ASSERT_THROW_MES(i < rv.ecdhInfo.size(), "Bad index");
 
         //mask amount and mask
-        ecdhDecode(rv.ecdhInfo[i], sk);
-        key mask = rv.ecdhInfo[i].mask;
-        key amount = rv.ecdhInfo[i].amount;
+        ecdhTuple ecdh_info = rv.ecdhInfo[i];
+        ecdhDecode(ecdh_info, sk);
+        mask = ecdh_info.mask;
+        key amount = ecdh_info.amount;
         key C = rv.outPk[i].mask;
         DP("C");
         DP(C);
@@ -559,9 +569,13 @@ namespace rct {
         DP("Ctmp");
         DP(Ctmp);
         if (equalKeys(C, Ctmp) == false) {
-            printf("warning, amount decoded incorrectly, will be unable to spend");
+            CHECK_AND_ASSERT_THROW_MES(false, "warning, amount decoded incorrectly, will be unable to spend");
         }
         return h2d(amount);
     }
 
+    xmr_amount decodeRct(const rctSig & rv, const key & sk, unsigned int i) {
+      key mask;
+      return decodeRct(rv, sk, i, mask);
+    }
 }

src/ringct/rctSigs.h

@@ -133,9 +133,11 @@ namespace rct {
     //decodeRct: (c.f. http://eprint.iacr.org/2015/1098 section 5.1.1)
     //   uses the attached ecdh info to find the amounts represented by each output commitment
     //   must know the destination private key to find the correct amount, else will return a random number
+    rctSig genRct(const ctkeyV & inSk, const keyV & destinations, const vector<xmr_amount> amounts, const ctkeyM &mixRing, unsigned int index);
     rctSig genRct(const ctkeyV & inSk, const ctkeyV  & inPk, const keyV & destinations, const vector<xmr_amount> amounts, const int mixin);
     bool verRct(const rctSig & rv);
-    xmr_amount decodeRct(rctSig & rv, const key & sk, unsigned int i);
+    xmr_amount decodeRct(const rctSig & rv, const key & sk, unsigned int i, key & mask);
+    xmr_amount decodeRct(const rctSig & rv, const key & sk, unsigned int i);