"the way I see of using the MCU board is to make it more robust and secure the algorithm of data encryption."

Still don't see it - the PC has so much more raw horsepower that surely you could get far better encryption on a PC than an 8-bit micro?!

PCs routinely talk to modems and all sorts of other things using both secure and non-secure transfers - I can't see what adding an 8051 in the the path achieves that couldn't be done on the PC anyway?

But if your customer is determined, well - it's their money!!! ;-)