Luby-Rackoff: Four Rounds is Not Enough

A four-round Luby-Rackoff cipher on 2n-bit messages has four round functions f_1, f_2, f_3, f_4, containing 4x2^n (n-bit) words of information. Thus, from an information-theoretic standpoint, we need at least 2x2^n pairs of plaintext and corresponding ciphertext in order to recover the contents of the round functions. In the present paper we show a computationally efficient attack, with a chosen-plaintext requirement very close to this lower bound: with O(2^n) chosen plaintexts we can recover most of the round functions f_i.

By: Don Coppersmith

Published in: RC20674 in 1996


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