I am having been struggling to set up my first payment gateway in Merchello...

My client has gone for SagePay (Form Integration), and I have been attempting to work from the posts here and here (Thanks Barry, Chris & Gordon!) in order to get my checkout process complete.

Thus far I have:

• Installed Merchello.SagePay into my project and configured it in the backoffice with my test account details.
• Added a Payment button to my checkout page.
• Saved the payment method to my BasketSalePreparation.

After this, I attempt AuthorizePayment, and I receive an error message that says:

SagePay: request initialization error: Specified initialization vector (IV) does not match the block size for this algorithm.

I also am not sure how to validate the payment after the user has completed their checkout on SagePay. I presume there is a callback URL, we have to receive the Invoice Key back from the return call and mark it as paid? I can't find anything that shows how to do this...

Any help much appreciated! :)

Here is my controller code:

[HttpPost]
    public ActionResult ConfirmSagePayPayment()
    {
        var preparation = Basket.SalePreparation();
        preparation.RaiseCustomerEvents = false;

        // PAYMENT - Only one payment method so auto-select it (SagePay)
        var paymentGatewayMethod = MerchelloContext.Current.Gateways.Payment.GetPaymentGatewayMethods().FirstOrDefault();
        var paymentMethod = Payment.GetPaymentGatewayMethodByKey(paymentGatewayMethod.PaymentMethod.Key).PaymentMethod;

        preparation.SavePaymentMethod(paymentMethod);

        // AuthorizePayment will save the invoice with an Invoice Number.
        var attempt = PerformProcessPayment(preparation, paymentMethod);

        if (!attempt.Payment.Success)
        {
            return CurrentUmbracoPage();
        }

        // store the invoice key in the CustomerContext for use on the receipt page.
        CustomerContext.SetValue("invoiceKey", attempt.Invoice.Key.ToString());

        if (attempt.Payment.Result.ExtendedData != null && attempt.Payment.Result.ExtendedData.ContainsKey("SagePayPaymentUrl"))
            return new RedirectResult(attempt.Payment.Result.ExtendedData["SagePayPaymentUrl"]);


        var nullref = new NullReferenceException("SagePay payment URL not found in extended data");
        LogHelper.Error<CheckoutController>("SagePay payment URL not found in extended data", nullref);
        throw nullref;
    }

    protected IPaymentResult PerformProcessPayment(SalePreparationBase preparation, IPaymentMethod paymentMethod)
    {
        return preparation.AuthorizePayment(paymentMethod.Key);
    }