Power and electromagnetic analysis: Improved model, consequences and comparisons

Abstract

Since their publication in 1998 and 2001, respectively, Power and Electromagnetic Analysis (SPA, DPA, EMA) have been successfully used to retrieve secret information stored in cryptographic devices. Both attacks usually model the side-channel leakages using the so-called “Hamming weight” and “Hamming distance” models, i.e. they only consider the number of bit transitions in a device as an image of its leakage. In these models, the main difference between power and electromagnetic analysis is assumed to be the fact that the latter allows space localization (i.e. to observe the leakage of only a part of the cryptographic device). In this paper, we make use of a more accurate leakage model for CMOS devices and investigate its consequences. In particular, we show that it is practically feasible to distinguish between 0 → 1 and 1 → 0 bit transitions in certain implementations and that electromagnetic analysis is particularly efficient in this respect. We denote this model as the “switching distance” leakage model and show how it may be very helpful to defeat some commonly used countermeasures (e.g. data buses precharged with random values). Then, we compare the different models and stress their respective constraints/advantages regarding practical attacks.