digital signature

USB token driver is like a pen drive which stores the DSC. As and when you want to affix the DSC you will have to insert the USB token. The advantage of USB token is that it can be kept in safe custody and the chances of misuse are rare. It can easily be carried to other places and DSC affixed even in other machines / places.

As against this pfx file is stored in the computer itself. Its portability is a problem. Again there are chances of file damage due to virus in the computer.

As regards misuse by the agents, they are authorised by the Certifying Authority viz., TCS, Safecript, etc. and you can rely on them. However as a precautionary measure you change the password.