Cryptology ePrint Archive: Report 2021/931

Practical Key Recovery Attacks on FlexAEAD

Orr Dunkelman and Maria Eichlseder and Daniel Kales and Nathan Keller and Gaëtan Leurent and Markus Schofnegger

Abstract: FlexAEAD is a block cipher candidate submitted to the NIST Lightweight Cryptography standardization project, based on repeated application of an Even-Mansour construction. In order to optimize performance, the designers chose a relatively small number of rounds, using properties of the mode and bounds on differential and linear characteristics to substantiate their security claims. Due to a forgery attack with complexity $2^{46}$, FlexAEAD was not selected to the second round of evaluation in the NIST project.

In this paper we present a practical key recovery attack on FlexAEAD, using clusters of differentials for the internal permutation and the interplay between different parts of the mode. Our attack, which was fully verified in practice, allows recovering the secret subkeys of FlexAEAD-64 with a time complexity of less than $2^{31}$ encryptions (with an experimental success rate of $75\,\%$). This is the first practical key recovery attack on a candidate of the NIST standardization project.

Category / Keywords: secret-key cryptography / Authenticated encryption, NIST LWC, practical key recovery, truncated differential

Date: received 8 Jul 2021

Contact author: orrd at cs haifa ac il, maria eichlseder at iaik tugraz at, daniel kales at iaik tugraz at, nkeller at math biu ac il, gaetan leurent at inria fr, markus schofnegger at iaik tugraz at

Available format(s): PDF | BibTeX Citation

Note: This paper is partially based on a paper presented at the IMACC 2019 workshop ( The main results of the paper are new.

Version: 20210709:180533 (All versions of this report)

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