Q: USB Tokens
USB token is a hardware device to store DSC.
The device does not have any expiry like digital signature, means it can be reused.
USB token ensures that nobody can have unauthorized access of the DSC of a user, because when anybody wants to use the DSC, it asks for password to access the contents of the USB token.
The USB tokens are developed on the technology called FIPS (The Federal Information Processing Standard). FIPS level- 2 & 3 technologies token are used nowadays.