Abstract
Nonlinear stream ciphers have become a viable alternative to traditional cryptosystems in response to the growing need
for secure communication. These ciphers generate a keystream via feedback mechanisms and nonlinear functions, which
are then utilized for encryption. Geffe generator system is one of the most keystream generators. Also, these systems have
many benefits, like being fast, flexible, and able to create unpredictable and non-repeating keystreams, these systems are
susceptible to cryptanalysis attacks, which have the potential to compromise their security. This paper presents the first
study of applying chicken swarm optimization (CSO) algorithm in the field of cryptanalysis based on cipher only attack.
The standard CSO algorithm and an adaptive multi points CSO (AMPCSO) algorithm are proposed to cryptanalysis
nonlinear stream cipher based on Geffe keystream generator. Firstly, the traditional CSO is used to reveal the secret
initial values of the Geffe generator. Secondly, an adaptive multi points chicken swarm optimization (AMPCSO) has
been proposed to enhance the traditional CSO algorithm to attack Geffe generator systems. The AMPCSO is a new
idea to advance the CSO search abilities and improve the foraging behavior of hens and chicks by allowing hens to be
influenced by other individuals within the same or different groups and affected by the best individual in the population
and enable chicks to learn from four reference points rather than learn from their respective mothers only. Lastly, a new
criterion is used to estimate the value of fitness by utilizing a multi-objective fitness function (MOFF), which is grounded
on Pareto dominance. The experimental results showed that the CSO and AMPCSO are very effective tools in terms of
accuracy, information required, and CPU times when applied to the analysis of nonlinear stream cipher. The AMPCSO
required a few characters from ciphertext to attack systems with total LFSRs length up to 59 bits with an appropriate
CPU time.