Paper 2001/068

SQUARE Attacks on Reduced-Round PES and IDEA Block Ciphers

J.Nakahara Jr and P.S.L.M.Barreto and B.Preneel and J.Vandewalle and H.Y.Kim

Abstract

This paper reports on variants of the Square attack applied to reduced-round versions of the PES and IDEA block ciphers. Attacks on 2.5 rounds of IDEA require $3\cdot 2^{16}$ chosen-plaintexts and recover 78 key bits. A new kind of attack, the Square related-key attack, is applied on 2.5 rounds of IDEA and recovers 32 key bits, with 2 chosen-plaintexts and $2^{17}$ related keys. Similar results hold for 2.5 rounds of PES. Implementations of the attacks on 32-bit block mini-versions of both ciphers confirmed the expected computational complexity. Although our attacks do not improve on previous approaches, this report shows new variants of the Square attack on word-oriented block ciphers like IDEA and PES.

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