Techniques such as DPA and SPA can be used to find the secret keys stored in smart-cards. These techniques have caused concern for they can allow people to recharge their stored value smartcards (in effect printing money), or illegally use phone or digital TV services. We propose an addition to current processors which will counteract these techniques. By randomising register usage, we can hide the secret key stored in a smartcard. The extension we propose can be added to existing processors, and is transparent to the algorithm.