Abstract. Implementations of cryptographic algorithms are vulnerable to Side Channel Analysis (SCA). To counteract it, masking schemes are usually involved which randomize key-dependent data by the addition of one or several random value(s) (the masks). When dth-order masking is involved (i.e. when d masks are used per key-dependent variable), the complexity of performing an SCA grows exponentially with the order d. The design of generic dth-order masking schemes taking the order d as security parameter is therefore of great interest for the physical security of cryptographic implementations. This paper presents the first generic dth-order masking scheme for AES with a provable security and a reasonable software implementation overhead. Our scheme is based on the hardware-oriented masking scheme published by Ishai et al. at Crypto 2003. Compared to this scheme, our solution can be efficiently implemented in software on any general-purpose processor. This result is of importance considering the lack of solution for d 3.

Abstract. Masking is a well-known countermeasure to protect block cipher implementations against side-channel attacks. The principle is to randomly split every sensitive intermediate variable occurring in the computation into d + 1 shares, where d is called the masking order and plays the role of a security parameter. Although widely used in practice, masking is often considered as an empirical solution and its effectiveness is rarely proved. In this paper, we provide a formal security proof for masked implementations of block ciphers. Specifically, we prove that the information gained by observing the leakage from one execution can be made negligible (in the masking order). To obtain this bound, we assume that every elementary calculation in the implementation leaks a noisy function of its input, where the amount of noise can be chosen by the designer (yet linearly bounded). We further assume the existence of a leak-free component that can refresh the masks of shared variables. Our work can be viewed as an extension of the seminal work of Chari et al. published at CRYPTO in 1999 on the soundness of combining masking with noise to thwart side-channel attacks.

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Abstract. Mutual Information Analysis is a generic side-channel distinguisher that has been introduced at CHES 2008. It aims to allow successful attacks requiring minimum assumptions and knowledge of the target device by the adversary. In this paper, we compile recent contributions and applications of MIA in a comprehensive study. From a theoretical point of view, we carefully discuss its statistical properties and relationship with probability density estimation tools. From a practical point of view, we apply MIA in two of the most investigated contexts for side-channel attacks. Namely, we consider first order attacks against an unprotected implementation of the DES in a full custom IC and second order attacks against a masked implementation of the DES in an 8-bit microcontroller. These experiments allow to put forward the strengths and weaknesses of this new distinguisher and to compare it with standard power analysis attacks using the correlation coefficient.