function confirmOrder(bytes32 _orderID, bytes32 _matchedOrderID) external onlyDarknode(msg.sender) {
        require(orders[_orderID].state == OrderState.Open, "invalid order status");
        require(orders[_matchedOrderID].state == OrderState.Open, "invalid order status");

        orders[_orderID].state = OrderState.Confirmed;
        orders[_orderID].confirmer = msg.sender;
        orders[_orderID].matchedOrder = _matchedOrderID;
        orders[_orderID].blockNumber = block.number;

        orders[_matchedOrderID].state = OrderState.Confirmed;
        orders[_matchedOrderID].confirmer = msg.sender;
        orders[_matchedOrderID].matchedOrder = _orderID;
        orders[_matchedOrderID].blockNumber = block.number;
    }