International Association
for Cryptologic Research

FIDES is a lightweight authenticated cipher, presented at CHES 2013.

The cipher has two version, providing either 80-bit or 96-bit

security. In this paper, we describe internal state-recovery attacks

on both versions of FIDES, and show that once we recover the internal

state, we can use it to immediately forge any message. Our attacks are

based on a guess-and-determine algorithm, exploiting the slow

diffusion of the internal linear transformation of FIDES. Our most

basic attacks have time complexities of 2^{75} and 2^{90} for FIDES-80

and FIDES-96, respectively, use a very small amount of memory, and

their most distinctive feature is their very low data complexity: the

attacks require at most 24 bytes of an arbitrary plaintext and its

corresponding ciphertext, in order to break the cipher with

probability 1. In addition to the basic attacks, we describe optimized

attacks which exploit additional data in order to reduce the time

complexities to 2^{73} and 2^{88} for FIDES-80 and FIDES-96,

respectively.