Journal
JOURNAL OF CRYPTOLOGY
Volume 14, Issue 1, Pages 17-35Publisher
SPRINGER-VERLAG
DOI: 10.1007/s001450010015
Keywords
cryptanalysis; DES; DESX; export controls; key search
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The block cipher DESX is defined by DESXk.k1.k2(x) = k2 circle plus DESk (k1 circle plus x), where circle plus denotes bitwise exclusive-or. This construction was first suggested by Rivest as a computationally cheap way to protect DES against exhaustive key-search attacks. This paper proves, in a formal model, that the DESX construction is sound. We show that, when F is an idealized block cipher, FXk.k1.k2(x) = k2 circle plus F-k(k1 circle plus x) is substantially more resistant to key search than is F. In fact, our analysis says that FX has an effective key length of at least kappa + n - 1 - 1gm bits, where kappa is the key length of F, n is the block length, and m bounds the number of pairs the adversary can obtain.
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