I am getting this error when I try and confirm an order within my shop.
Status: INVALID
Status Detail: 5030 : Unable to decrypt the request message. This might be caused by an incorrect password or invalid encoding.
I have spoken with SagePay and they have suggested that I change the Purchase URL to be the following, and submit a transaction to our 'showpost' address.
https://test.sagepay.com/showpost/showpost.asp
What template file within OpenCart do i need to change? - Are there any other files I need to change so i dont get this error?
Could someone please help me on this as I am getting nowhere fast and will have to re-do my shop uisng another eCommerce system

Many Thanks for any help