International Association for Cryptologic Research

International Association
for Cryptologic Research

IACR News item: 30 July 2015

Achiya Bar-On
ePrint Report ePrint Report
MISTY1 is a block cipher designed by Matsui in 1997. It is widely deployed in Japan, and is recognized internationally as a European

NESSIE-recommended cipher and an ISO standard. After almost 20 years of unsuccessful cryptanalytic attempts, a first attack on the full MISTY1 was presented at CRYPTO 2015 by Todo. The attack, using a new technique called {\\it division property}, requires almost the full codebook and has time complexity of 2^{107.3} encryptions.

In this paper we present a new attack on the full MISTY1. It is based on a modified variant of Todo\'s division property, along with a variety of refined key-recovery techniques. Our attack requires the full codebook, but allows to retrieve 49 bits of the secret key in time complexity of only 2^{64} encryptions, and the full key in time complexity of 2^{69.5} encryptions.

While our attack is clearly impractical due to its large data complexity, it shows that MISTY1 provides security of only 2^{70} --- significantly less than what was considered before.

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