New Most Efficient State Recovery Attacks on an eSTREAM Candidate F-FCSR-H v2 and F-FCSR-H Stream Ciphers

This paper presents three new most efficient state recovery attacks on FCSR-based ciphers. These attacks, named as Conventional Linearisation Attacks (CLAs), Fast Linearisation Attacks (FLAs) and Improved Linearisation Attacks (ILAs) are based on a new observation, which is also presented in this paper. The paper demonstrates these new attacks against F-FCSR-H and an eSTREAM candidate F-FCSR-H v2 ciphers by presenting state recovery algorithms based on CLAs, FLAs, ILAs and analyses the efficiency of these attacks in terms of data, time and memory complexities. The paper also presents a comparative analysis of new attacks and the previously known results of cryptanalysis on these ciphers. This comparative analysis shows that new attacks presented in this paper are the most efficient state recovery attacks known so far on these ciphers. Further more, this paper also presents a key recovery algorithm, which works in conjunction with the state recovery algorithms to recover the effective key used in these ciphers.