IACR News item: 23 April 2015
Achiya Bar-On
ePrint Reportdeployed in Japan, and is recognized internationally as an European
NESSIE-recommended cipher and an ISO standard. Since its introduction,
MISTY1 was subjected to extensive cryptanalytic
efforts, yet no attack significantly faster than exhaustive key search is
known on its full version. The best currently
known attack is a higher-order differential attack presented by Tsunoo
et al. in 2012 which breaks a reduced variant of MISTY1 that contains
7 of the 8 rounds and 4 of the 5 $FL$ layers in $2^{49.7}$ data and $2^{116.4}$
time.
In this paper, we present improved higher-order differential attacks on
reduced-round MISTY1. Our attack on the variant considered by Tsunoo et al.
requires roughly the same amount of data and only $2^{100.4}$ time
(i.e., is $2^{16}$ times faster). Furthermore, we present the first attack
on a MISTY1 variant with 7 rounds and all 5 $FL$ layers, requiring
$2^{51.4}$ data and $2^{121}$ time. To achieve our results, we use a new
higher-order differential characteristic for 4-round MISTY1, as well as
enhanced key recovery algorithms based on the {\\it partial sums} technique.
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