Improved Conditional Cube Attacks on Ascon AEADs in Nonce-Respecting Settings

with a Break-Fix Strategy

Abstract

The best-known distinguisher on 7-round Ascon-128 and Ascon-128a AEAD uses a 60-dimensional cube where the nonce bits are set to be equal in the third and fourth rows of the Ascon state during initialization (Rohit et al. ToSC 2021/1). It was not known how to use this distinguisher to mount key-recovery attacks. In this paper, we investigate this problem using a new strategy called break-fix for the conditional cube attack. The idea is to introduce slightly-modified cubes which increase the degrees of 7-round output bits to be more than 59 (break phase) and then find key conditions which can bring the degree back to 59 (fix phase). Using this idea, key-recovery attacks on 7-round Ascon-128, Ascon-128a and Ascon-80pq are proposed. The attacks have better time/memory complexities than the existing attacks, and in some cases improve the weak-key attacks as well.