Technical Report CS0816

TR#:CS0816
Class:CS
Title: HOW TO STRENGTHEN DES USING EXISTING HARDWARE.
Authors: E. Biham and A. Biryukov
PDFCS0816.pdf
PDF - RevisedCS0816.revised.pdf
Abstract:

Differential and linear attacks are capable of breaking DES faster than exhaustive search, but are usually impractical due to enormous amounts of data required. In [16] Wiener designed a million dollar special purpose computer capable of breaking DES in 3.5 hours in average by exhaustive search. In this paper we describe methods of strengthening DES against exhaustive search, differential attacks and linear attacks that can be applied on existing DES hardware. We use the fact that there are DES chips in the market that permit replacement of the S-boxes. We introduce the concept of key-dependent invariant S-box transformations. Differential and linear properties of the cipher are invariant under these transformations. Possible reorderings of S-boxes are discussed; we present orders of the original DES S-boxes which are slightly stronger than the standard order of S-boxes. Finally we suggest a concrete scheme to strengthen DES which uses the methods described above. This modified DES can be used with existing DES hardware and is much stronger than the standard DES.

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