I am thinking about building a dongle that acts as a USB or PS/2 keyboard device and is capable of doing some cryptographic things for me, e.g. provide time-based authentication codes, store passwords and things like that. However, I would like to make this thing at least a little bit secure against power consumption analysis attacks.

One thing I have in mind is to put a small capacitor on the board and have two microcontrollers; microcontroller A talks to the computer and the user, microcontroller B does the crypto. When A requests a computation using a secret key, B would make sure that it has enough power in the capacitor and then disconnect itself from A using transistors before performing the actual computation.

Does this sound like a good idea? In other words, is it likely that this will make power consumption attacks much harder without significantly degrading the usability?

Edit: I'm assuming that the opponent is inside the PC but can't get hold of the board.