Build me a circuit for USB smart token (Digital Signature Certificate)

I am starting a manufacturing plant for USB smart token (also known as Digital Signature Certificate). It is a USB device where people have their secret encrypted keys that we can plugin to our laptop and can see certain encryption protected files. The device should be read only. Once I put the keys as asked by the customer, the usb device should never be modified, and can never be erased. People will use this usb device to store keys for very sensitive information. So that they can access those sensitive files using this usb device only. I have attached an image of the device. I need someone to tell me how to manufacture it, what machines would I require, what would be the circuit diagram of the device and how to program it with the given keys. If anyone has a prior experience, please help me out as I have no idea where to start.