Horizontal and Vertical Side-Channel Attacks against Secure RSA Implementations

Since the introduction of side-channel attacks in the nineties, RSA implementations have been a privileged target. A wide variety of countermeasures have been proposed and most of practical attacks are nowadays efficiently defeated by them. However, in a recent work published at ICICS 2010, Clavier et al. have pointed out that almost all the existing countermeasures were ineffective if the attacks are performed with a modus operandi called Horizontal. Such attacks, originally introduced by Colin Walter at CHES 2001, involve a single observation trace contrary to the classical attacks where several ones are required. To defeat Horizontal attacks, the authors of the ICICS paper have proposed a set of new countermeasures. In this paper, we introduce a general framework enabling to model both Horizontal and classical attacks (called Vertical) in a simple way. This framework enables to enlighten the similarities and the differences of those attack types. From this formalism, we show that even if Clavier et al.’s countermeasures thwart existing attacks, they do not fully solve the leakage issue. Actually, flaws are exhibited in this paper and efficient attacks are devised. We eventually propose a new countermeasure.