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The Decorrelation Technique

Last update: September 7th, 2000.

Content of this Document


Digital criminality is nowadays a big threat for the electronic marketplace. For this reason, cryptography provides various algorithms based on a heart cryptographic primitive: encryption. The Digital Encryption Standard (DES) has been developed by IBMTM for the US Department of Commerce in the seventies for this purpose, but its secret-key length (56 bits) provides no sufficient security at this time, so this standard is now over.

So far, real-life encryption algorithms used to have an empirical-based security: they were designed from an intricate substitution-permutation network and believed to be secure until someone published an attack on them. In parallel, research yielded several general attacks strategies, namely Biham and Shamir's "differential cryptanalysis", and Matsui's "linear cryptanalysis" (both are particular cases of the more general "iterated attacks of order 2"), which provided a better understanding on how to manage with security arguments.

The laboratory of computer sciences of the Ecole Normale Supérieure, associated with the Centre National de la Recherche Scientifique (CNRS), has recently developed a technique for making new encryption algorithms with a provable security against any iterated attacks of a fixed order (e.g. of order 2). Several properties of this technique - known as decorrelation - have been presented at international research conferences. Additionally, decorrelation has been used in order to propose a candidate for the "Advanced Encryption Standard" process of the US Department of Commerce.

Provable security is an important added value for cryptographic algorithms and is currently a hot topic in international conferences. The decorrelation technique is a part of this program.

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