Guess and determine cryptanalysis with variable sampling and its applications
Samir Hodžić, Enes Pašalić, Yongzhuang Wei · IET Information Security · 2019
Non‐linear filtering generators, as a well‐known family of stream ciphers, employ a filtering function to process the secret state bits and thus outputs binary keystream blocks of length m . In this study, the authors extend the framework of a generic cryptanalytic method applicable to non‐linear filtering generators called generalised filter state guessing attacks (GFSGA), introduced as a generalisation of the filter state guessing attack method, by applying a variable sampling of the keystream bits in order to retrieve as much information about the secret state bits as possible. Two different modes that use a variable sampling of keystream blocks are presented and it is shown that in many cases these modes may outperform the standard GFSGA mode. They also demonstrate the possibility of employing GFSGA‐like attacks to other design strategies such as non‐linear feedback shift register‐based ciphers (Grain family for instance). It is also indicated that the tap positions of Grain‐128 are not chosen optimally with respect to this generic cryptanalytic method and provide a better selection of taps that gives higher resistance to GFSGA‐like attacks.