[Resource Topic] 2022/1088: Tighter trail bounds for Xoodoo

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Title:
Tighter trail bounds for Xoodoo

Authors: Joan Daemen, Silvia Mella, Gilles Van Assche

Abstract:

Determining bounds on the differential probability of differential trails and
the squared correlation contribution of linear trails forms an important part of the
security evaluation of a permutation. For Xoodoo such bounds were proven with a
dedicated tool (XooTools), that scans the space of all r-round trails with weight
below a given threshold T_r. The search space grows exponentially with the value of
T_r and XooTools appeared to have reached its limit, requiring huge amounts of
CPU to push the bounds a little further. The bottleneck was the phase called trail
extension where short trails are extended to more rounds, especially in the backward
direction. In this work, we present a number of techniques that allowed us to make
extension much more efficient ant that allowed us to increase the bounds significantly.
Notably, we prove that the minimum weight of any 4-round trail is 80, the minimum
weight of any 6-round trail is at least 132 and the minimum weight of any 12-round
trail is at least 264, both for differential and linear trails.

ePrint: https://eprint.iacr.org/2022/1088

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