Mathematical Methods of Cryptography

On properties of Flex-like block ciphers

M. A. Pudovkina, A. M. Smirnov


Abstract: The FlexAEAD block cipher is a participant in the NIST competition for a low-resource encryption standard. We introduce a parametric class of Flex-like block ciphers that is based on an iterative nonlinear transformation $s^{(r, \pi)}$ defined by the number of iterations $r$, S-box $\pi$ on $\mathbb{F}_{2^m}$ and Feistel network. The condition for imprimitivity of the group generated by permutations $s^{(0, \pi)},\ldots ,s^{(r, \pi)}$ and the key addition group has been obtained. We describe a distinguisher attack on the class of Flex-like block ciphers based on the yoyo game and structure of $s^{(r, \pi)}$. For two block cipher from the family, we have checked our attack by experiment.

Keywords: FlexAEAD, imprimitive group, yoyo game, distinguisher attack, Feistel network.

UDC: 519.7}\maketitle

DOI: 10.17223/2226308X/18/31