Changing the Odds Against Masked Logic

Random switching logic (RSL) has been proposed as an efficient countermeasure to mitigate power analyses. The logic style equalizes the output transition probabilities using a random mask-bit. This manuscript, however, will show a successful attack against RSL. The single mask-bit can only add one bit of entropy to the information content of the overall power consumption variations and can very easily be deduced from the power consumption. Once the mask-bit is known, the a posteriori probabilities of the output transitions are not equal anymore and power analyses can be mounted. A threshold filter suffices to remove the additional bit of information.