A New Approach in Cryptanalysis Through Combinatorial Equivalence of Cryptosystems

We propose a new approach in cryptanalysis based on an evolution of the concept of $\mathit{\text{Combinatorial}}\mathit{\text{Equivalence}}$. The aim is to rewrite a cryptosystem under a combinatorially equivalent form in order to make appear new properties that are more strongly discriminating the secret key used during encryption. We successfully applied this approach to the most secure stream ciphers category nowadays. We first define a concept cipher called Cipherbent6 that capture most of the difficulty of stream cipher cryptanalysis. We significantly outperformed all known cryptanalysis. We applied this approach to the Achterbahn cipher and we obtained again far better cryptanalysis results.