I want to pass a value say an amount to a payment Gateway. the gate way has given me functions to make the data secure and tamper proof. the problem is im taking this amount from a text box. By using fiddler and putting a breakpoint before the request the data can be changed even before it reaches the server and is encrypted by the functions.so now the tampered values is being sent to the payment gateway. Im using ASP.net The text box is used to take value that the customer wants to donate.
Anything that's on the client can be modified by the client. The fact that the value comes from text box does not matter.
HTTPS does not change the fact that the client can make any change to the data that it wants.
You either need to encrypt and authenticate the value on the server so that the client can't change it, or use server to server communication.