prepare signature using private key and certificate in C#.Net

Dear sir,
I have been developing an application in c#.net to integrate with Webmethod. My company (Insurance firm) need to connect to Bank (bank using webMethod ),Here bank provided https service (URL with parameter)
There are two requiment.
1 Account statement Module–for generating bank statement in MT940 format, I succeeded to generate ,it’s over
2 Payment module, everything working fine, but bank request me to add signature along with payment xml file. I need to prepare signature using private key and certificate.
but unforutnaly I don’t know to create a signature.

So kindly help me to provide the solution, I am looking for some source code in . Net or documentation.

Hoping you will consider my request

Regards
Shafeeque vc
shafeequevc@yahoo.com

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