Abstract.A general method to secure cryptographic algorithms against side-channel attacks is the use of randomization techniques and, in particular, masking. Roughly speaking, using random values unknown to an adversary one masks the input to a cryptographic algorithm. As a result, the intermediate results in the algorithm computation are uncorrelated to the input and the adversary cannot obtain any useful information from the side-channel. Unfortunately, previous AES randomization techniques have based their security on heuristics and experiments. Thus, flaws have been found which make AES randomized implementations still vulnerable to side-channel cryptanalysis. In this paper, we provide a formal notion of security for randomized maskings of arbitrary cryptographic algorithms. Furthermore, we present an AES randomization technique that is provably secure against side-channel attacks if the adversary is able to access a single intermediate result. Our randomized masking technique is quite general and it can be applied to arbitrary algorithms using only arithmetic operations over some finite field. To our knowledge this is the first time that a randomization technique for the AES has been proven secure in a formal model.
Abstract. Cache based attacks (CBA) exploit the different access times of main memory and cache memory to determine information about internal states of cryptographic algorithms. CBAs turn out to be very powerful attacks even in practice. In this paper we present a general and strong model to analyze the security against CBAs. We introduce the notions of information leakage and resistance to analyze the security of several implementations of AES. Furthermore, we analyze how to use random permutations to protect against CBAs. By providing a successful attack on an AES implementation protected by random permutations we show that random permutations used in a straightforward manner are not enough to protect against CBAs. Hence, to improve upon the security provided by random permutations, we describe the property a permutation must have in order to prevent the leakage of some key bits through CBAs.
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