Add explicit constrains for recepient, relayer and fee

Author: pertsev

circuits/withdraw.circom

@@ -50,6 +50,18 @@ template Withdraw(levels) {
         tree.pathElements[i] <== pathElements[i];
         tree.pathIndices[i] <== pathIndices[i];
     }
+
+    // Add hidden signals to make sure that tampering with recipient or fee will invalidate the snark proof
+    // Most likely it is not required, but it's better to stay on the safe side and it only takes 2 constraints
+    // Squares are used to prevent optimizer from removing those constraints
+    signal recipientSquare;
+    signal feeSquare;
+    signal relayerSquare;
+    signal refundSquare;
+    recipientSquare <== recipient * recipient;
+    feeSquare <== fee * fee;
+    relayerSquare <== relayer * relayer;
+    refundSquare <== refund * refund;
 }
 
 component main = Withdraw(20);