ringct: add functions to commit to an amount

One to commit to an amount with zero key (for use with fake
commitments for pre-rct outputs), and one with an arbitrary
key (for rct outputs).

src/ringct/rctOps.cpp

@@ -170,6 +170,23 @@ namespace rct {
         return make_tuple(sk, pk);
     }
     
+    key zeroCommit(xmr_amount amount) {
+        key mask = identity();
+        mask = scalarmultBase(mask);
+        key am = d2h(amount);
+        key bH = scalarmultH(am);
+        addKeys(mask, mask, bH);
+        return mask;
+    }
+
+    key commit(xmr_amount amount, key mask) {
+        mask = scalarmultBase(mask);
+        key am = d2h(amount);
+        key bH = scalarmultH(am);
+        addKeys(mask, mask, bH);
+        return mask;
+    }
+
     //generates a random uint long long (for testing)
     xmr_amount randXmrAmount(xmr_amount upperlimit) {
         return h2d(skGen()) % (upperlimit);

src/ringct/rctOps.h

@@ -96,6 +96,10 @@ namespace rct {
     tuple<ctkey, ctkey> ctskpkGen(xmr_amount amount);
     //this one is mainly for testing, can take arbitrary amounts..
     tuple<ctkey, ctkey> ctskpkGen(key bH);
+    // make a pedersen commitment with given key
+    key commit(xmr_amount amount, key mask);
+    // make a pedersen commitment with zero key
+    key zeroCommit(xmr_amount amount);
     //generates a random uint long long
     xmr_amount randXmrAmount(xmr_amount upperlimit);