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Abstract. Since Power Analysis on smart-cards was introduced by Paul Kocher [KJJ98], the validity of the model used for smart-cards has not been given much attention. In this paper, we first describe and analyze some different possible models. Then we apply these models to real components and clearly define what can be detected by power analysis (simple, differential, code reverse engineering...). We also study, from a statistical point of view, some new ideas to exploit these models to attack the card by power analysis. Finally we apply these ideas to set up real attacks on cryptographic algorithms or enhance existing ones.

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In order to protect a cryptographic algorithm against Power Analysis attacks, a well-known method consists in hiding all the internal data with randomly chosen masks. Following this idea, an AES implementation can be protected against Differential Power Analysis (DPA) by the "Transformed Masking Method", proposed by Akkar and Giraud at CHES'2001, requiring two distinct masks. At CHES'2002, Trichina, De Seta and Germani suggested the use of a single mask to improve the performances of the protected implementation. We show here that their countermeasure can still be defeated by usual first-order DPA techniques. In another direction, Akkar and Goubin introduced at FSE'2003 a new countermeasure for protecting secret-key cryptographic algorithms against high-order differential power analysis (HO-DPA). As particular case, the "Unique Masking Method" is particularly well suited to the protection of DES implementations. However, we prove in this paper that this method is not sufficient, by exhibiting a (first-order) enhanced differential power analysis attack. We also show how to avoid this new attack.

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